question
stringlengths 25
894
| answer
stringlengths 4
863
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|---|---|
I am currently reading the manpages abouttime.h. I got this far:
```
time_t now = time(0);
struct tm * local = localtime(&now);
```
Now I can work with the time, as far as good, but I do not find the information if its my duty tofree()the variablelocalor not.
|
Quoting theman page
The four functionsasctime(),ctime(),gmtime()andlocaltime()return a pointer to static dataand hence are not thread-safe. [...]
So, you need notfree()the returned pointer.
|
I want to know when anUPDATEquery reports 0 rows matches. I'm usinglibmysql.
Here's the code I'm using:
```
char query[300] = {0};
snprintf(query, 300, "UPDATE `my_table` SET name='%s' WHERE id=%d",
name, id);
if (mysql_query(db, query)) {
printf("Error!\n");
}
```
Essentially what I need to know is whether or not there is a match forid. I know I can check that by doing a select, but is there another way?
|
Checkmysql_affected_rows. This will return number of modified rows by the last query.
However, it may return just the rows actually modified. If you want to return matched row, you have to specifyCLIENT_FOUND_ROWSinmysql_real_connect. Check the same page for details.
|
I have a small Linux project written in C is it possible to analyze it with a SonarQube?
|
According tohttps://en.wikipedia.org/wiki/SonarQube, it support c/c++, and is cross-platform, so you can do it I think.
|
I came accross this statement:
```
time_t time = x / 1000LL;
```
So what does thisLLactually mean?
|
Copy-pasted fromthis question, which seems to be the exact same one with theULLsuffix :
From thegccmanual:ISO C99 supports data types for integers that are at least 64 bits wide, and as an extension GCC supports them in C90 mode and in C++.
Simply writelong long intfor a signed integer, orunsigned long
long intfor an unsigned integer. To make an integer constant of typelong long int, add the suffixLLto the integer. To make an
integer constant of typeunsigned long long int, add the suffixULLto the integer.
It, indeed, is a suffix for thelong long inttype.
|
```
#include <stdio.h>
int main()
{
char a[8];
printf("%d\n",a) ;
return 0;
}
```
For the above code the output was this :- 2686744
What is the reason behind this output?
I found that the output doesn't depend on the content of the array, but on the size of the array.I just want the explanation.
|
```
char a[8];
printf("%d\n",a);
```
This code has undefined behavior.
The array expressionais implicitly converted to achar*value, equivalent to&a[0]. That value is then passed toprintf-- but since the%dformat requires anintargument, the behavior is undefined.
Ifintandchar*happen to have the same size, and if they're passed as function arguments using the same mechanism, then it will likely print a decimal representation of the address of (the initial element of) the array.
But don't do that. If you want to print the address:
```
printf("%p\n", (void*)a);
```
|
Say I have this string:"this","is a, string"
I want to be able to check if a comma is inside a pair of quotation marks. The first comma is not, whereas the second comma is. How could I achieve this in C?
|
The way I would do it is :
initialize a boolean variable named likequotesOpento a falsy valuego char by char through the string until you find\0, and toggle the boolean if the char is"when you encounter a comma, check the state of your boolean to know if it is in quotes or not
|
Is there a simple lexer/parser for C language or a subset of it which is based on Flex/Bison?
I have found some open source parsers for C (TCC, LCC, ...) but none of them are based on bison.
|
There is aC11 grammarfor YACC (a predecessor of Bison), it should work with Bison (maybe some tweaks will be needed).
GCC used to be based on Bison a long ago. GCC 3.4 source code contains afilewith C grammar.
|
I am using codeblock in Windows 10 for C programs. I was writing this below program but instead of giving 12 it gives 24 as output. I have also checked it using online compiler but still it giving the same output.
```
#include<stdio.h>
int main()
{
int num[]={5,7,9,0,1,7};
printf("%d",sizeof(num));
return 0;
}
```
|
but instead of giving 12 it gives 24 as output
Not sure why you would expect the output to be12. You are probably assumingsizeof(int)to be2which is not necessarily true on all platforms.
sizeof(int)is platform dependent andsizeof(int)is4on your platform. Hence, the output is24.
Note: To print asize_t(which is whatsizeof()returns), use%zu:
```
printf("%zu",sizeof(num));
```
|
I'm trying to get the list of nodes in a cluster using only the C API.
More or less what the following shell command returns, but from C API
```
rabbitmqctl cluster_status -n rabbit@<remote hostname>
```
I'd like to avoid callingrabbitmqctlfrom the C withsystem()orpopoen()or whatever as I want to avoid having a dependency on the rabbitmq server package on the client side.
Any hint ?
|
Getting RabbitMQ cluster status is not something from AMQP standard and it's not something thatrabbitmq-csupports, at least at this time. There isManagement Pluginthat provides RabbitMQ HTTP API, which is the closest thing you want.
Have a look onRabbitMQ HTTP API client for Rubyreadme file, which covers getting cluster status from HTTP API.
|
Is it allowed to use same name structure with different definitions in 2 different c files in the same project. For eg.
File1.c
```
typedef struct
{
unsigned int unVar;
} abc;
```
File2.c
```
typedef struct
{
int var;
} abc;
```
abc is used in both the files. When i compile these file as part of same project there are no errors, but i want to understand whether this is correct usage.
|
6.7.2.1 Structure and union specifiersThe presence of a struct-declaration-list in a struct-or-union-specifier declares a new type,
within a translation unit.
Types are defined only within a translation unit, a .c file in this case.
There is no problem with defining two types with the same name in two different translation units.
However those two types are not compatible unless they follow the rules described in6.2.7., p1. The types you defined arenot compatible.
|
I have a defined raw data in my header file (which is generated automatically), like this:
```
#defined RAW_DATA 0x11, 0x20, 0x55, 0x00, x044
```
The aim is to check a specific parameter of RAW_DATA in compilation time and if it is wrong throw an #error.
For instance, During the compilation, the preprocessor should check if 2nd parameter of RAW_DATA (in this case 0x20) is really 0x20, if it is not throw an #error.
The main problem, how to access the specific parameter in defined RAW_DATA, is it somehow possible?
P.S. I am using Keil compiler in C.
|
```
#define RAW_DATA 0x11, 0x20, 0x55, 0x00, x044
#define X_GET_SECOND_PAR(par) GET_SECOND_PAR(par)
#define GET_SECOND_PAR(p1,p2,p3,p4,p5) p2
#if X_GET_SECOND_PAR(RAW_DATA) != 0x20
#error "2nd parameter shall be 0x20"
#endif
```
For specific parameter checking. It's not elegant.
|
I am currently reading the manpages abouttime.h. I got this far:
```
time_t now = time(0);
struct tm * local = localtime(&now);
```
Now I can work with the time, as far as good, but I do not find the information if its my duty tofree()the variablelocalor not.
|
Quoting theman page
The four functionsasctime(),ctime(),gmtime()andlocaltime()return a pointer to static dataand hence are not thread-safe. [...]
So, you need notfree()the returned pointer.
|
I want to know when anUPDATEquery reports 0 rows matches. I'm usinglibmysql.
Here's the code I'm using:
```
char query[300] = {0};
snprintf(query, 300, "UPDATE `my_table` SET name='%s' WHERE id=%d",
name, id);
if (mysql_query(db, query)) {
printf("Error!\n");
}
```
Essentially what I need to know is whether or not there is a match forid. I know I can check that by doing a select, but is there another way?
|
Checkmysql_affected_rows. This will return number of modified rows by the last query.
However, it may return just the rows actually modified. If you want to return matched row, you have to specifyCLIENT_FOUND_ROWSinmysql_real_connect. Check the same page for details.
|
I have a small Linux project written in C is it possible to analyze it with a SonarQube?
|
According tohttps://en.wikipedia.org/wiki/SonarQube, it support c/c++, and is cross-platform, so you can do it I think.
|
I am doing vectorization using AVX intrinsics, I want to fill constant floats like1.0into vector__m256. So that in one register I got a vector{1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0}Does anyone knows how to do it?
It is similar to this questionconstant float with SIMD
But I am using AVX not SSE
|
The simplest method is to use_mm256_set1_ps:
```
__m256 v = _mm256_set1_ps(1.0f);
```
|
This question already has answers here:C comma operator(4 answers)Closed7 years ago.
```
int a=3,1;
int b=(5,4);
```
I am a beginner in c and I noticed in a book this type of initialization . what does thisinitialisationmean?
|
int b = (5,4)will first evaluate 5 then 4. The last thing that is evaluated will be assigned to the variable. For example
```
int b = (5,4,3,2,1)
```
in this case the value of b will be 1.
|
I have so many includes like this in my application
```
#include "../../libs/helper.hpp"
```
I am glad to remove there../../libs/from every include. Is there any way to fix this problem so I can call the library this way?
```
#include "helper.hpp"
```
one possible way is to use-Iswitch in make file. But the problem is that then I should use angular brackets<and>for calling libraries. It is undesirable since I prefer to distinguish between my local libraries and the installed libraries. So I am looking for a way to call it with"around the name of the libraries.
Is there any way?
|
-iquotedirAdd the directorydirto the head of the list of directories to be searched for header files only for the case of#include "file"; they are not searched for#include <file>, otherwise just like-I.
https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Directory-Options.html#Directory-Options
|
```
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
uid_t ruid=-1, euid=-1, suid=-1;
int main() {
FILE *fh = fopen("file.txt", "r");
char c;
while ((c = fgetc(fh)) != EOF) {
printf("%c", c);
}
return 0;
```
}
So guys I have to open this file using c fopen command but I have to specify at which location the file actually is. That is for example the above file.txt is not at the location where is program is being executed but it is at a diffrent location, for example the above file.txt is at /home/my_user_name and the place where the program is being executed is /home/my_user/anyfolder.
So I want to know how to specify the location of the file in the program.
Thanks In Advance
|
You simply specify the path.....
fopen("/path/to/file.txt", "r")
|
Working with a double, I can set
```
double x = 216/22 //Output's 9.818181...
```
However, when I try:
```
double x = (216/22) //Outputs 9.00000...
double x = ((double)216/22) //Outputs 9.0000...
double x = (216/(double)22) //Outputs 9.818181...
```
Why does (216/22) return 9 and not 9.8181...?
|
Why does (216/22) return 9 and not 9.8181...?
Because both 216 and 22 areint- and the result is an int.
(216.0/22)would return double as expected.
|
If I have:
```
#include <stdio.h>
int main(int argc, char *argv[]) {
int length = strlen(argv[1]);
```
and argv[1] one is just a word, for example, "hello", how can I index through it backwards and print out letter by letter?
I tried using strrev, but apparently this isn't in linux and rather than include the function I'd rather just for loop through argv[1] backwards.
I tried:
```
int i;
for (i = length; i == 0; i--){
printf("%c", argv[1][i]);
}
```
but I knew this would be wrong before I even executed it.
|
how can I index through it backwards and print out letter by letter
```
int i;
for (i = strlen( argv[1] ) - 1 ; i >= 0; i--){
printf("%c", argv[1][i]);
}
```
Also, of course you need to#include <string.h>for string-related functions likestrlen.
|
I saw a call when reading PX4 code,
```
open(ACCEL_BASE_DEVICE_PATH "0", O_RDONLY)
```
where ,#define ACCEL_BASE_DEVICE_PATH "/dev/accel"
so,open("/dev/accel" "0", O_RDONLY)
is this even possible ?"/dev/accel" "0"
I'm expecting there may be pre processing macro
... But that will hard to find..
|
Consecutive string literals are concatenated by pre-processor.
It is legal.
"/dev/accel" "0"results in"/dev/accel0".
Note that space is irrelevant here (but more readable), it would be the same with"/dev/accel""0".
Note also that, in c++11, because of User-defined literals (UDL), space may be required sometime as for
```
#define ZERO "0"
"/dev/accel"ZERO // Here space is require since C++11. else there is an other meaning.
```
|
This question already has answers here:C comma operator(4 answers)Closed7 years ago.
```
int a=3,1;
int b=(5,4);
```
I am a beginner in c and I noticed in a book this type of initialization . what does thisinitialisationmean?
|
int b = (5,4)will first evaluate 5 then 4. The last thing that is evaluated will be assigned to the variable. For example
```
int b = (5,4,3,2,1)
```
in this case the value of b will be 1.
|
I have so many includes like this in my application
```
#include "../../libs/helper.hpp"
```
I am glad to remove there../../libs/from every include. Is there any way to fix this problem so I can call the library this way?
```
#include "helper.hpp"
```
one possible way is to use-Iswitch in make file. But the problem is that then I should use angular brackets<and>for calling libraries. It is undesirable since I prefer to distinguish between my local libraries and the installed libraries. So I am looking for a way to call it with"around the name of the libraries.
Is there any way?
|
-iquotedirAdd the directorydirto the head of the list of directories to be searched for header files only for the case of#include "file"; they are not searched for#include <file>, otherwise just like-I.
https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Directory-Options.html#Directory-Options
|
```
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
uid_t ruid=-1, euid=-1, suid=-1;
int main() {
FILE *fh = fopen("file.txt", "r");
char c;
while ((c = fgetc(fh)) != EOF) {
printf("%c", c);
}
return 0;
```
}
So guys I have to open this file using c fopen command but I have to specify at which location the file actually is. That is for example the above file.txt is not at the location where is program is being executed but it is at a diffrent location, for example the above file.txt is at /home/my_user_name and the place where the program is being executed is /home/my_user/anyfolder.
So I want to know how to specify the location of the file in the program.
Thanks In Advance
|
You simply specify the path.....
fopen("/path/to/file.txt", "r")
|
Working with a double, I can set
```
double x = 216/22 //Output's 9.818181...
```
However, when I try:
```
double x = (216/22) //Outputs 9.00000...
double x = ((double)216/22) //Outputs 9.0000...
double x = (216/(double)22) //Outputs 9.818181...
```
Why does (216/22) return 9 and not 9.8181...?
|
Why does (216/22) return 9 and not 9.8181...?
Because both 216 and 22 areint- and the result is an int.
(216.0/22)would return double as expected.
|
If I have:
```
#include <stdio.h>
int main(int argc, char *argv[]) {
int length = strlen(argv[1]);
```
and argv[1] one is just a word, for example, "hello", how can I index through it backwards and print out letter by letter?
I tried using strrev, but apparently this isn't in linux and rather than include the function I'd rather just for loop through argv[1] backwards.
I tried:
```
int i;
for (i = length; i == 0; i--){
printf("%c", argv[1][i]);
}
```
but I knew this would be wrong before I even executed it.
|
how can I index through it backwards and print out letter by letter
```
int i;
for (i = strlen( argv[1] ) - 1 ; i >= 0; i--){
printf("%c", argv[1][i]);
}
```
Also, of course you need to#include <string.h>for string-related functions likestrlen.
|
I saw a call when reading PX4 code,
```
open(ACCEL_BASE_DEVICE_PATH "0", O_RDONLY)
```
where ,#define ACCEL_BASE_DEVICE_PATH "/dev/accel"
so,open("/dev/accel" "0", O_RDONLY)
is this even possible ?"/dev/accel" "0"
I'm expecting there may be pre processing macro
... But that will hard to find..
|
Consecutive string literals are concatenated by pre-processor.
It is legal.
"/dev/accel" "0"results in"/dev/accel0".
Note that space is irrelevant here (but more readable), it would be the same with"/dev/accel""0".
Note also that, in c++11, because of User-defined literals (UDL), space may be required sometime as for
```
#define ZERO "0"
"/dev/accel"ZERO // Here space is require since C++11. else there is an other meaning.
```
|
I can't seem to google this because everything turns up as C++ or C# (Side note: any easy way to search for C specific?). All I'm trying to figure out is how to accept a console string input in such a way that I know it's length so I can return it in reverse order by indexing backwards through it with a for loop. I've had a little C++ experience in the past, but never really used console IO. Any help appreciated, thanks.
|
Read withfgets().Cope with a possible trailing\n.Find lengthprint in reverse.char buf[100];
if (fgets(buf, sizeof buf, stdin) == NULL) Handle_EOF();
buf[strcspn(buf, "\n")] = '\0'; // lop off potential trailing \n
size_t len = strlen(buf);
while (len) {
putc(buf[--len], stdout);
}
|
I can't seem to google this because everything turns up as C++ or C# (Side note: any easy way to search for C specific?). All I'm trying to figure out is how to accept a console string input in such a way that I know it's length so I can return it in reverse order by indexing backwards through it with a for loop. I've had a little C++ experience in the past, but never really used console IO. Any help appreciated, thanks.
|
Read withfgets().Cope with a possible trailing\n.Find lengthprint in reverse.char buf[100];
if (fgets(buf, sizeof buf, stdin) == NULL) Handle_EOF();
buf[strcspn(buf, "\n")] = '\0'; // lop off potential trailing \n
size_t len = strlen(buf);
while (len) {
putc(buf[--len], stdout);
}
|
I need to pass integer to kernel module, calculate sin there and return result.
First of all, kernel has built-in sinfixp_t fixp_sin(unsigned int degrees)function, that returns fixed-point number (that can be negative).
So, my question is: if i pass integer using ioctl to kernel module, calculate sin, how i can return negative result back?
ioctl detects negative return values as errors.
|
I need to pass integer to kernel module, calculate sin there and
return result.
Instead of passing integer value toioctl(), structure can be passed. This structure can have inputs and output data fields. There is no need to mess withioctl()return values.
Sample ioctl() with passing struct to char driverhttps://github.com/jeyaramvrp/kernel-module-programming/tree/master/sample-char-dir
|
This question already has answers here:How does concatenation of two string literals work?(4 answers)Closed7 years ago.
I encountered the following type of string literal in an open source library, which I have not seen before. It turns out thataandbare the same. I'm confused why the syntax ofais correct? Does C preprocessor concatenate two strings automatically?
```
#include<stdio.h>
int main()
{
const char a[] =
"123\r\n"
"123\r\n";
const char b[] = "123\r\n123\r\n";
printf(a);
printf(b);
}
```
|
Fromsection 5.1.1.2.6 of the C99 standard:
Adjacent string literal tokens are concatenated.
So your assumption is correct. Anyplace you see string literals consecutively, the compiler implicitly concatenates them.
|
In our code base I found something similar to
```
static char foo[4] = "0";
```
Since the string is smaller than the array, is this still well-defined?Hypothetically, what if I had
```
static char bar[4] = "01234";
```
I assume the array would be initialized to "0123". But is this guaranteed by the standard or also another possible case of undefined behavior?
|
Since the string is smaller than the array, is this still well-defined?
Yes,
```
static char foo[4] = "0";
```
is well defined.foowill contain'0','\0','\0'and'\0'.
The snippet
```
static char bar[4] = "01234";
```
is constraint violation. While
```
static char bar[4] = "0123";
```
is well defined untilbaris not accessed as astring. In this casebaris achararray with 4 characters'0','1','2'and'3'. You may get a warning for the latter case that
```
Initializer-string for char array is too long
```
|
I need to include ZIMPL in a CMake project. In the source code the ZIMPL headers are included like this
```
#include zimpl/bool.h
```
The actual path within ZIMPL iszimpl/src/bool.h. How can I tell CMake (or the C linker) to look inzimpl/src/whenever it encounters azimpl/include?
I would prefer to avoid creating symlinks for the sake of platform independence.
|
You cannot#includefiles using relative path different from actual one.
The simples way to do with your case is to copy original headers into locations, which has needed path suffixes. It can be done with CMake viafile(COPY)orconfigure_file(.. COPY_ONLY):
```
file(COPY <source_include_dir>/zimpl/src/bool.h
DESTINATION ${CMAKE_BINARY_DIR}/include/zimple/
)
include_directories(${CMAKE_BINARY_DIR}/include)
```
|
I was looking into a code snippet and saw below statement. How will below statement evaluated?
```
x= 5|(high == 1 ? y : high == 0 ? z:0);
```
|
The expression
```
x= 5|(high == 1 ? y : high == 0 ? z:0);
```
is evaluated as
```
x= 5|( high == 1 ? y : (high == 0 ? z:0) );
```
It has similar effect as that of
```
if(high == 1)
x = 5|y;
else if(high == 0)
x = 5|z;
else
x = 5|0;
```
|
I have a function with something like
```
FILE *file1 = fopen("testing.txt", "r");
```
I can't modify this line. However, if I make a file named "testing.txt" in, say/tmp, would I be able to make the function load the file from/tmpinstead of it's own directory. (Maybe by modifying thePATHvariable?)
|
If the program doesn't change its own working directory, you couldcdinto/tmpand simply run the program from there.
```
$ cd /tmp
$ /absolute/path/to/my_program
```
|
I have a function with something like
```
FILE *file1 = fopen("testing.txt", "r");
```
I can't modify this line. However, if I make a file named "testing.txt" in, say/tmp, would I be able to make the function load the file from/tmpinstead of it's own directory. (Maybe by modifying thePATHvariable?)
|
If the program doesn't change its own working directory, you couldcdinto/tmpand simply run the program from there.
```
$ cd /tmp
$ /absolute/path/to/my_program
```
|
I am doing an exercise where I need to write Unicode on the terminal,
using onlywrite()in<unistd.h>.
I can't use :
putcharsetlocaleprintf (in fact the exercise is reproducingprintffunction)
Any "low level" advice on how to perform that?
|
As Chris wrote in the comments, you need a terminal (e.g. like xterm on Linux) that understands the Unicode and then you just write them. So by default xterm understands UTF8 and is set to a codepage such that this code will give you a UTF8 Smiley Face (☺).
```
#include <stdio.h>
#include <unistd.h>
char happy[] = { 0xe2, 0x98, 0xba }; /* U+263A */
int main()
{
write(1, happy, 3);
return 0;
}
```
|
I am trying to combine three bit operations in one line of C. For a 8 bit char, I have to set bits 2, 4, 6; clear bits 1, 3, 7 and toggle bit 0 and 5 all in one line code of C. I could do these in three line but I cannot combine these. Below is what I have done so far:
```
x= x & 0xD5;
x = x | 0x51;
x = x ^ 0x84;
```
They give the right answer for a given value of x. But I tried
```
x = (x & 0xD5) | (x | 0x51) | (x ^ 0x84)
```
And
```
x = x & 0xD5 | 0x51 ^ 0x84
```
Those do not work. Any suggestion would be appreciated.
|
It's simply this
```
x = (((x & 0xD5) | 0x51) ^ 0x84)
```
Your first try is wrong, because the values ofxare not updated so all the operations work on the same value, besides oring the values is not equivalent to assigning the result of the operation tox.
The second, is wrong because or operator precedence, so you just need parentheses.
|
This is best asked with a code example:
```
typedef struct {
... // Fields take up many bytes (>= 32 bytes)
} some_struct;
void alternative_1(some_struct arg);
void alternative_2(const some_struct *arg);
```
Assuming that both function alternatives are implemented inside an already compiled binary (and therefore cannot be inlined), is there a difference in the function call overhead?
|
Obviously yes:
When callingalternative_1, the whole struct data is pushed onto the stack prior to branching to the function (which may be quite a lot).
Whereasalternative_2only has a single pointer to the struct pushed onto the stack.
There is also a big difference when operating on the struct:
In the case ofalternative_1you work on a local copy.When modifying the struct inalternative_2you change the original data.
Its quite similar to a function receiving either anintegeror apointer to an integer.
|
p is pointer on a array
```
int array[] = {1,2,3,4,5};
int *p = array;
```
And when i want to printf the array entries:
is this the same and why:
*(p++)*p++
Hope you can help me!
|
++ has a higher precedence (meaning it binds to p tighter) than *
therefore*p++is equivalent to*(p++).
Something similar to this is the difference between*p[]and(*p)[].[]has a higher precedence than*therefore*p[]is equivalent to*(p[])which makes alist a pointersbut something like(*p)[]explicitly says to do whats in the brackets first making in this case apointer to a list(mind blowing)
|
This is the error I'm getting:
```
c:17:15: error: expected expression before ‘=’ token
suma + = mat[N][M];
```
And this is my code:
```
#include<stdio.h>
#define N 3
#define M 3
int mat[N][M]i,j,k,l,m,n;
int vector[N];
int suma;
int main (void){
for(i=0;i<N;i++){
for(j=0;j<M;j++){
printf("Enter a number: ");
scanf("%d", &mat[N][M]);
}
}
for(k=0;k<N;k++){
suma=0;
for(l=0;l<M;l++){
**suma+ = mat[N][M];
}
vector[i]=suma;
}
return 0;
}
```
|
```
+= != + =
```
Meaning: just write:suma += mat[N][M];............................................^ no space.
For the sake of completeness I will add what Sourav mentioned:
You need to reseti(i=0) after the firstforloop, to avoid accessing out of bounds memory and causing undefined behavior.
|
I get the following error :
Error:Neither MinGW nor Cygwin is selected
Do you have idea for resolve this issue?
|
To elaborate on the answer provided by "uta" you need either MinGW or Cygwin in order to compile your code. MinGW can be obtained from here:MinGW
Or Cygwin from here:Cygwin
For simplicity I will describe MinGW. Install MinGW then navigate like so in CLion:
File -> Settings -> Build, Execution, Deployment -> Toolchains
And select the directory you have MinGW installed. I believe by default on Windows it'sC:\MinGW. Once you've done this CLion will scan the directory and detect it.
If you already have them installed then you need only select the folders they're installed in in CLion.
Hope this helps.
|
I was shifting around the 'C' standard and I came across this:
$6.7.9.5:
If the declaration of an identifier has block scope, and the
identifier has externalor internal linkage, the declaration shall
have no initializer for the identifier.
So my question is on the title. I would also like some examples if possible.
|
```
static int i; // internal linkage
void f() {
extern int i; // block-scope declaration; refers to i in global scope
// still internal linkage - see 6.2.2/4
}
```
|
I have to use some char16-wide strings for uefi programming. How can I initialize them easily?
CHAR8 is easy:
```
CHAR8 *Str = "yehaw\0";
```
CHAR16 meanwhile is hard working that way, therefore I chose this initialization:
```
CHAR16 *Str;
Str = AllocatePool(6*2); //AllocatePool allocates bytewise
Str = ('y','e','h','a','w','\o');
```
So question is what would be the right and easiest way to initialize CHAR16 strings?
|
If you have the c standard library and are using a conformant C/C++ compiler, typically prefixing a string with L works for declared strings.As in :
```
CHAR16 *Str = L"yehaw";
```
works. However, why not use the ubiquitously accepted type of
```
wchar_t
```
?
|
I have this structure :
```
typedef struct s_hashmap
{
t_hashmap_elem **array_elements;
size_t array_size;
void *(*get)(struct s_hashmap **, void *);
void (*add)(struct s_hashmap **, t_hashmap_elem *);
} t_hashmap;
```
Can I access to array_elements when i'm in get function pointer without pass my variable as parameter like this :
h->get(&h, &key); // h is the t_hashmap variable.
if this is not possible please explain me an other way to do it.
thanks in advance.
|
No you can not. There are no methods of user-defined types in C. You have to pass a pointer (or pointer to pointer depending on the parameter declaration) to an object of the structure if you want to change it.
|
I am doing some logic tasks in C and while I evaluate by hand some of the results are different than the ones printed by CodeBlocks.
What is the decimal value of following expressions taking previously executed instructions into account?
Here're the numbers (I do not understand output of lines signed with "<<<<<<<<"):
```
int i, j, k, *p2i;
float fp;
p2i=&k;
i=0x22;
j=025;
k=25;
++k;
k++;
k|7&12; <<<<<
fp=10/20; <<<<<<
fp=(float)10/20; <<<<<<
```
|
```
k|7&12; ==> 27|7&12 ==> 27|4 ==> 31 (discarded value)
fp=10/20; ==> fp=0 ==> 0 (fp is now 0.0)
fp=(float)10/20; ==> fp=10.0f/20 ==> fp=0.5f (fp is 0.5)
```
|
I'm trying to read in an integer using getchar(). This is the code I'm using:
```
while (thisChar = getchar() != '\n') {
n = n * 10 + thisChar - '0';
}
```
int nis initialized to zero andthisCharis declared as anint
For single digit input, n is returning as -47 which means the character value forstart of headingis being read in first. Is there something incorrect about the way I am assigningthisCharin thewhileexpression?
|
It's missing parentheses:
```
int thisChar;
while ((thisChar = getchar()) != '\n') {
n = n * 10 + thisChar - '0';
}
```
Without these additional parentheses, you always assign tothisCharthe value of the comparisongetchar() != '\n', i.e. always1or0...
Note that you should also handleEOF:
```
int thisChar = getchar();
while (thisChar != EOF && thisChar != '\n') {
n = n * 10 + thisChar - '0';
thisChar = getchar();
}
```
|
In the following code
```
int main(){
int a=3;
printf("%d %d %d",++a,a,a++);
return 0;
}
```
As specified, From C99 appendix C:,
The following are the sequence points described in 5.1.2.3:
The call to a function, after the arguments have been evaluated
(6.5.2.2).The end of the first operand of the following operators: logical
AND && (6.5.13); logical OR || (6.5.14); conditional ? (6.5.15);
comma , (6.5.17)
The order in which the arguments to a function are evaluated are undefined as specified by the C standard.
However, in the function call for printf, we have arguments that are separated by commas which classify as sequence points. So why does this statement correspond to unspecified behavior?
|
Because thecommain the function call isnotthecomma operatorbut aseparator. So it doesn't introduce any sequence point(s).
|
I'm trying to run my program in valgrind 3.10.0, but it seems to hang inset_address_range_perms(). The last output I get is
```
--69447:1:aspacem allocated thread stack at 0x803c7c000 size 1064960
--69447:1:syswrap- run_a_thread_NORETURN(tid=2): pre-thread_wrapper
--69447:1:syswrap- thread_wrapper(tid=2): entry
```
Any idea how I can narrow down the problem?
|
The first thing to try is to use gdb+vgdb and connect to your program which is hanging: in another window, start gdb
and then in gdb, do target remote | vgdb
You can then use various gdb commands to see what is happening e.g. info threads; threads apply all bt full; ....
Seehttp://www.valgrind.org/docs/manual/manual-core-adv.html#manual-core-adv.gdbserverfor more information about valgrind+gdb.
If the valgrind gdbserver does not react, then you will have to attach directly to valgrind, and debug the valgrind internals.
|
I was shifting around the 'C' standard and I came across this:
$6.7.9.5:
If the declaration of an identifier has block scope, and the
identifier has externalor internal linkage, the declaration shall
have no initializer for the identifier.
So my question is on the title. I would also like some examples if possible.
|
```
static int i; // internal linkage
void f() {
extern int i; // block-scope declaration; refers to i in global scope
// still internal linkage - see 6.2.2/4
}
```
|
I have to use some char16-wide strings for uefi programming. How can I initialize them easily?
CHAR8 is easy:
```
CHAR8 *Str = "yehaw\0";
```
CHAR16 meanwhile is hard working that way, therefore I chose this initialization:
```
CHAR16 *Str;
Str = AllocatePool(6*2); //AllocatePool allocates bytewise
Str = ('y','e','h','a','w','\o');
```
So question is what would be the right and easiest way to initialize CHAR16 strings?
|
If you have the c standard library and are using a conformant C/C++ compiler, typically prefixing a string with L works for declared strings.As in :
```
CHAR16 *Str = L"yehaw";
```
works. However, why not use the ubiquitously accepted type of
```
wchar_t
```
?
|
I have this structure :
```
typedef struct s_hashmap
{
t_hashmap_elem **array_elements;
size_t array_size;
void *(*get)(struct s_hashmap **, void *);
void (*add)(struct s_hashmap **, t_hashmap_elem *);
} t_hashmap;
```
Can I access to array_elements when i'm in get function pointer without pass my variable as parameter like this :
h->get(&h, &key); // h is the t_hashmap variable.
if this is not possible please explain me an other way to do it.
thanks in advance.
|
No you can not. There are no methods of user-defined types in C. You have to pass a pointer (or pointer to pointer depending on the parameter declaration) to an object of the structure if you want to change it.
|
I am doing some logic tasks in C and while I evaluate by hand some of the results are different than the ones printed by CodeBlocks.
What is the decimal value of following expressions taking previously executed instructions into account?
Here're the numbers (I do not understand output of lines signed with "<<<<<<<<"):
```
int i, j, k, *p2i;
float fp;
p2i=&k;
i=0x22;
j=025;
k=25;
++k;
k++;
k|7&12; <<<<<
fp=10/20; <<<<<<
fp=(float)10/20; <<<<<<
```
|
```
k|7&12; ==> 27|7&12 ==> 27|4 ==> 31 (discarded value)
fp=10/20; ==> fp=0 ==> 0 (fp is now 0.0)
fp=(float)10/20; ==> fp=10.0f/20 ==> fp=0.5f (fp is 0.5)
```
|
I'm trying to read in an integer using getchar(). This is the code I'm using:
```
while (thisChar = getchar() != '\n') {
n = n * 10 + thisChar - '0';
}
```
int nis initialized to zero andthisCharis declared as anint
For single digit input, n is returning as -47 which means the character value forstart of headingis being read in first. Is there something incorrect about the way I am assigningthisCharin thewhileexpression?
|
It's missing parentheses:
```
int thisChar;
while ((thisChar = getchar()) != '\n') {
n = n * 10 + thisChar - '0';
}
```
Without these additional parentheses, you always assign tothisCharthe value of the comparisongetchar() != '\n', i.e. always1or0...
Note that you should also handleEOF:
```
int thisChar = getchar();
while (thisChar != EOF && thisChar != '\n') {
n = n * 10 + thisChar - '0';
thisChar = getchar();
}
```
|
In the following code
```
int main(){
int a=3;
printf("%d %d %d",++a,a,a++);
return 0;
}
```
As specified, From C99 appendix C:,
The following are the sequence points described in 5.1.2.3:
The call to a function, after the arguments have been evaluated
(6.5.2.2).The end of the first operand of the following operators: logical
AND && (6.5.13); logical OR || (6.5.14); conditional ? (6.5.15);
comma , (6.5.17)
The order in which the arguments to a function are evaluated are undefined as specified by the C standard.
However, in the function call for printf, we have arguments that are separated by commas which classify as sequence points. So why does this statement correspond to unspecified behavior?
|
Because thecommain the function call isnotthecomma operatorbut aseparator. So it doesn't introduce any sequence point(s).
|
I'm trying to run my program in valgrind 3.10.0, but it seems to hang inset_address_range_perms(). The last output I get is
```
--69447:1:aspacem allocated thread stack at 0x803c7c000 size 1064960
--69447:1:syswrap- run_a_thread_NORETURN(tid=2): pre-thread_wrapper
--69447:1:syswrap- thread_wrapper(tid=2): entry
```
Any idea how I can narrow down the problem?
|
The first thing to try is to use gdb+vgdb and connect to your program which is hanging: in another window, start gdb
and then in gdb, do target remote | vgdb
You can then use various gdb commands to see what is happening e.g. info threads; threads apply all bt full; ....
Seehttp://www.valgrind.org/docs/manual/manual-core-adv.html#manual-core-adv.gdbserverfor more information about valgrind+gdb.
If the valgrind gdbserver does not react, then you will have to attach directly to valgrind, and debug the valgrind internals.
|
I am working on an cufft implementation and can't find any reference to the cufftcomplex functions. I found cucomplex.h through google, though, but that doesn't help me. Specifically i want to know, how to read out the imaginary part and the real part of the cufftcomplex struct.
|
The typescufftComplexandcuComplexare actually same. It is documented in thecuFFT documentation. Incufft.hyou will find the typedef:
```
typedef cuComplex cufftComplex;
```
IncuComplex.hyou will find thatcuComplexis indeed afloat2, i.e. you can read out the real value withc.xand the imaginary value withc.y. Or better use the functionscuCrealf()andcuCimagf()which are provided incuComplex.h.
|
I have two arrays of char: a,b. How can i create with a loop "for" the new array vet, which is the union of the two alternating array a, b?
```
#include <stdio.h>
int main(void) {
char a[] = "BNSIO";
char b[] = "EISM\a";
char vet[sizeof(a) + sizeof(b)];
for (int i = 0; i < (sizeof(a) + sizeof(b)); i++) {
}
for (int i = 0; i < (sizeof(a) + sizeof(b)); i++){
printf("%c", vet[i]);
}
}
```
|
You can try this:
```
for (int i = 0,j=0,k=0; k < (sizeof(a) + sizeof(b) -2);)
{
if(i+1<sizeof(a))
{
vet[k] = a[i];
i++;k++;
}
if(j+1<sizeof(b))
{
vet[k] = b[j];
j++;k++;
}
}
for (int i = 0; i < (sizeof(a) + sizeof(b)-2); i++){
printf("%c", vet[i]);
}
```
You need to subtract 1 for the null terminating character
|
This question already has answers here:What is the behavior of printing NULL with printf's %s specifier?(4 answers)Closed7 years ago.
```
#include<stdio.h>
#include<string.h>
int main() {
char *ptr = NULL;
printf("%s", ptr);//The output is null
// printf("%s\n", ptr); //addition of **\n** give segmentation fault
return 0;
}
```
the firstprintfoutputs:(null). But why the secondprintf's output is:Segmentation fault (core dumped)just on adding:\n?
|
printf("%s", ptr);
Hereprintfexpects a valid pointer, which points to null terminated string, you can't pass NULL to it. If you do so, you will trigger undefined behaviour, and can't reason about output of the program.
PS. I have found an answer which might have slightly more details about the thing you might be interested in. Seehere. Actually your question seems to be a duplicate of that one.
|
I would like to set the pointers*nameand*agein Student struct to NULL. Im not sure how this could be done from the main().
```
typedef struct Student Student;
typedef struct Student{
char *name;
int *age;
} Student;
```
I would like to set the pointers mentioned above to NULL in my main.
My declaration of Student in main is:
```
Student Tom;
```
I am not sure how to set tom.name and tom.age pointers to NULL.
|
You can do it for example the following way
```
Student Tom = { NULL, NULL };
```
or
```
Student Tom = { .name = NULL, .age = NULL };
```
or
```
Student Tom;
Tom.name = NULL;
Tom.age = NULL;
```
|
I have two arrays of char: a,b. How can i create with a loop "for" the new array vet, which is the union of the two alternating array a, b?
```
#include <stdio.h>
int main(void) {
char a[] = "BNSIO";
char b[] = "EISM\a";
char vet[sizeof(a) + sizeof(b)];
for (int i = 0; i < (sizeof(a) + sizeof(b)); i++) {
}
for (int i = 0; i < (sizeof(a) + sizeof(b)); i++){
printf("%c", vet[i]);
}
}
```
|
You can try this:
```
for (int i = 0,j=0,k=0; k < (sizeof(a) + sizeof(b) -2);)
{
if(i+1<sizeof(a))
{
vet[k] = a[i];
i++;k++;
}
if(j+1<sizeof(b))
{
vet[k] = b[j];
j++;k++;
}
}
for (int i = 0; i < (sizeof(a) + sizeof(b)-2); i++){
printf("%c", vet[i]);
}
```
You need to subtract 1 for the null terminating character
|
This question already has answers here:What is the behavior of printing NULL with printf's %s specifier?(4 answers)Closed7 years ago.
```
#include<stdio.h>
#include<string.h>
int main() {
char *ptr = NULL;
printf("%s", ptr);//The output is null
// printf("%s\n", ptr); //addition of **\n** give segmentation fault
return 0;
}
```
the firstprintfoutputs:(null). But why the secondprintf's output is:Segmentation fault (core dumped)just on adding:\n?
|
printf("%s", ptr);
Hereprintfexpects a valid pointer, which points to null terminated string, you can't pass NULL to it. If you do so, you will trigger undefined behaviour, and can't reason about output of the program.
PS. I have found an answer which might have slightly more details about the thing you might be interested in. Seehere. Actually your question seems to be a duplicate of that one.
|
I would like to set the pointers*nameand*agein Student struct to NULL. Im not sure how this could be done from the main().
```
typedef struct Student Student;
typedef struct Student{
char *name;
int *age;
} Student;
```
I would like to set the pointers mentioned above to NULL in my main.
My declaration of Student in main is:
```
Student Tom;
```
I am not sure how to set tom.name and tom.age pointers to NULL.
|
You can do it for example the following way
```
Student Tom = { NULL, NULL };
```
or
```
Student Tom = { .name = NULL, .age = NULL };
```
or
```
Student Tom;
Tom.name = NULL;
Tom.age = NULL;
```
|
ok, so i'm trying to compile my code using makefile, i've got only 2 .c file and 1 .h file, i used "sqrt()" function from math.h (only in main), here is my makefile:
```
a.out: GBST.o main.o
gcc GBST.o main.o
GBST.o: GBST.c GBST.h
gcc -c GBST.c
main.o: main.c
gcc -c main.c -lm
```
still, I get main.c:(.text+0x170): undefined reference to `sqrt' error, what can it be? (btw, i wrote -lm in GBST line before and it did not help, so i have deleted it)
|
You need to use-lmin the link line, not in the compile line.
```
a.out: GBST.o main.o
gcc GBST.o main.o -lm
# ^^^^ Need it here
GBST.o: GBST.c GBST.h
gcc -c GBST.c
main.o: main.c
gcc -c main.c
# ^^^^ Don't need it here
```
|
Regular expression in C.
Can I declare a pointer likevoid *{a,b}which meansvoid *a, *b;
Is it possible, are regex expressions a standard in ANSI C
|
C does not have support for regular expressions in the code itself. Libraries exist to perform regular expressions on text, but not on the code.
Variable declarations must be done separately. For pointers in particular, a*next to a declared variable applies only to that variable. For example:
```
int *a, b, *c;
```
This declaresaandcasint *andbasint. Another way of looking at is that*a,b, and*care all of typeint.
|
I tried using all these flagsldliandluwithsscanfbut none worked. The following gets4294967295no matter what number i put inbuf:
```
long unsigned data;
char buf[40] = "data 2349872764943587";
if (sscanf (buf, "data %lu", &data) == 1)
{
printf ("%s\n", buf);
printf ("data:\t%lu\n", data);
}
```
Output:
```
data 2349872764943587
data: 4294967295
```
Am I using the wrong conversion flag or is there something I missed?
|
OP'sunsigned longappears to be a 32-bit and so cannot represent the 51-bit2349872764943587.
If stuck using a compiler that lacks 64-bit integers, code could usedouble, which typically handles 53-bit whole number values without loss of precision.
```
double data;
char buf[40] = "data 2349872764943587";
if (sscanf (buf, "data %lf", &data) == 1)
{
printf ("%s\n", buf);
printf ("data:\t%.0f\n", data);
}
```
Output
```
data: 2349872764943587
```
|
How to useCopyFilefor copying specific type of files from one folder to a backup file (backup.bkp)
Example:
```
C:\HHH
abc.jpeg
def.txt
ghi.jpeg
```
I want to copy only jpeg files to thebackup.bkpfile
I tried the below syntax but it is not working,
```
CopyFile( _T("C:\\HHH\*.jpeg"),_T("C:\\Backup.bak", FALSE);
```
Can anyone suggest a solution for this?
|
CopyFile doesn't accept wild cards or copy multiple files. It can copy a single file from one fully specified place to another.
To achieve what you want you need to enumerate over the directory usingFindFirstFile/FindNextFileand copy the files one by one using CopyFile. These functions take wildcards or you can ask for all files, and do the filtering yourself.
|
I know if I am inputting a string into a pointer variable I would malloc that based on the size of the input received.
But what if I am using a pointer variable for the use instrchr? For example if I want the pointer to point to the the character "z" in a string.
```
char *pointer;
pointer=strchr(string,'z');
```
Would I need to malloc pointer? Or is it not needed?
|
You don't have to allocate any memory here . Strchr function returns a pointer to the first occurence of a char in a string if it exists. If a given string deos not contain a char then a null will be returned.
You need to allocate memory only if you want to create something new, that has not been created yet. In this case the input string exists and a pointer will point to a position within this string.
|
I'm writing a C program usingnftw()to walk a filesystem and retrieve file modification times for each file.
nftw()calls a supplied function pointer and provides astruct statas an argument.
man stat(2)states that the time modification fields are:
```
struct timespec st_atim; /* time of last access */
struct timespec st_mtim; /* time of last modification */
struct timespec st_ctim; /* time of last status change */
```
However, whilstman stat(2)provides an example of how to print the time fields, it doesn't tell me how to find information aboutstruct timespec, nor how to query/manipulate the time modification fields.
How should I go about finding that information on my computer alone, without resorting to Google?
|
Typically one of the man pages describes what these structures contain. If you tell us your platform I can give further details. Otherwise, open up the header/usr/include/time.hto see whatstruct timespecis defined as.
|
I tried using all these flagsldliandluwithsscanfbut none worked. The following gets4294967295no matter what number i put inbuf:
```
long unsigned data;
char buf[40] = "data 2349872764943587";
if (sscanf (buf, "data %lu", &data) == 1)
{
printf ("%s\n", buf);
printf ("data:\t%lu\n", data);
}
```
Output:
```
data 2349872764943587
data: 4294967295
```
Am I using the wrong conversion flag or is there something I missed?
|
OP'sunsigned longappears to be a 32-bit and so cannot represent the 51-bit2349872764943587.
If stuck using a compiler that lacks 64-bit integers, code could usedouble, which typically handles 53-bit whole number values without loss of precision.
```
double data;
char buf[40] = "data 2349872764943587";
if (sscanf (buf, "data %lf", &data) == 1)
{
printf ("%s\n", buf);
printf ("data:\t%.0f\n", data);
}
```
Output
```
data: 2349872764943587
```
|
How to useCopyFilefor copying specific type of files from one folder to a backup file (backup.bkp)
Example:
```
C:\HHH
abc.jpeg
def.txt
ghi.jpeg
```
I want to copy only jpeg files to thebackup.bkpfile
I tried the below syntax but it is not working,
```
CopyFile( _T("C:\\HHH\*.jpeg"),_T("C:\\Backup.bak", FALSE);
```
Can anyone suggest a solution for this?
|
CopyFile doesn't accept wild cards or copy multiple files. It can copy a single file from one fully specified place to another.
To achieve what you want you need to enumerate over the directory usingFindFirstFile/FindNextFileand copy the files one by one using CopyFile. These functions take wildcards or you can ask for all files, and do the filtering yourself.
|
I know if I am inputting a string into a pointer variable I would malloc that based on the size of the input received.
But what if I am using a pointer variable for the use instrchr? For example if I want the pointer to point to the the character "z" in a string.
```
char *pointer;
pointer=strchr(string,'z');
```
Would I need to malloc pointer? Or is it not needed?
|
You don't have to allocate any memory here . Strchr function returns a pointer to the first occurence of a char in a string if it exists. If a given string deos not contain a char then a null will be returned.
You need to allocate memory only if you want to create something new, that has not been created yet. In this case the input string exists and a pointer will point to a position within this string.
|
I'm writing a C program usingnftw()to walk a filesystem and retrieve file modification times for each file.
nftw()calls a supplied function pointer and provides astruct statas an argument.
man stat(2)states that the time modification fields are:
```
struct timespec st_atim; /* time of last access */
struct timespec st_mtim; /* time of last modification */
struct timespec st_ctim; /* time of last status change */
```
However, whilstman stat(2)provides an example of how to print the time fields, it doesn't tell me how to find information aboutstruct timespec, nor how to query/manipulate the time modification fields.
How should I go about finding that information on my computer alone, without resorting to Google?
|
Typically one of the man pages describes what these structures contain. If you tell us your platform I can give further details. Otherwise, open up the header/usr/include/time.hto see whatstruct timespecis defined as.
|
Very simple script throw this error :
passing argument 1 of ‘fprintf’ makes pointer from integer without a
cast
Why is that ? The code is :
```
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[])
{
FILE *FP;
if((FP = fopen("file.txt", "r+")) == NULL) {
printf("File won't open\n");
return(1);
}
foo(FP);
return(0);
}
int foo(FP) {
char name[31];
printf( "Please enter a name (within 30 char) : \n");
gets(name);
fprintf(FP, "%s\n", name);
return(0);
}
```
|
Change
```
int foo(FP) {
```
to
```
int foo(FILE * FP) {
```
You must put a prototype offoofunction at the top of your code.
|
This question already has answers here:Compiler error: memset was not declared in this scope(2 answers)Closed7 years ago.
I have the following c code:
```
#include<stdio.h>
int main(void)
{
char buff[10];
memset(buff,0,sizeof(buff));
gets(buff);
printf("\n The buffer entered is [%s]\n",buff);
return 0;
}
```
When I run the code, I get the following warning:
```
warning: implicit declaration of function ‘memset’ [-Wimplicit-function-declaration]
```
How should I solve the problem?
Thanks
|
Add
```
#include <string.h>
```
at the top of file.
This because is the header file where thememsetprototype can be found by compiler.
Avoid using gets function... Usescanforfgetsinstead.
Take a lookHERE
|
I wrote a simple lex file to identify C keywords. My rules looks like:
```
keyword do|while|char|if
%%
{keyword} { printf("Keyword %s found.", yytext); }
```
The problem is the rule correctly identifiescharin source code, but it also identifies things likeputcharas keywordchar. How can I force the rule to only identify the keywordcharand not when it's present in other words?
|
You need to put keywords before identifiers. That's all. Lex is searching for regular expressions sequentially.
```
%%
IF|ELSE|etc {action for keywords }
[a-zA-Z_][a-zA-Z0-9]* {action for identifiers}
%%
```
|
I have recently started programming in C and was wondering about a more concise way to write a simple function that returns 1 or -1 depending on the equality of two int values. What I wrote:
```
int valueCompare(int i, int j) {
if (i != j) {
return -1;
}
else {
return 1;
}
}
```
It's readable but seems inefficient. I have seen return statements that utilizes a colon and question mark however am not familiar with that style. Does anyone have any recommendations on how to write a more efficient function for this without a loss of readability?
|
You can use the ternary operator:
```
int valueCompare(int i, int j)
{
return (i == j) ? 1 : -1;
}
```
|
If I am looping through an array, should I useintoruint8_t?
```
for (int i = 0; i < 100; i++) {}
```
or
```
for (uint8_t i = 0; i < 100; i++) {}
```
Which is better when working with embedded systems?
|
The first choice is much better:
```
for (int i = 0; i < 100; i++) { }
```
The alternative is sometimes encouraged by local coding rules, it is a bad recommendation:
it tends to generate more code if you useiin the loop body as an index or in an expression.it can also lead to more bugs especially if you later modify your code and the upper boundary exceeds 255 and is read from aunsignedvariable.a good embedded compiler will take advantage of the small range if it is pertinent anyway.
Embedded systems have plenty of resources nowadays, but if you are targeting a very limited chipset, you may want to check both alternatives and make an informed decision.
|
My objective is to call a void function "sum" which is passed two integers where they will be added together. I then want the first integer variable to be modified so that it is pointing at the sum of the two integers.
When I compile I get "error: indirection requires pointer operand ('int' invalid)."
|
This should work:
```
void sum(int *x, int y){
*x += y; // *x means "contents of what is pointed to by pointer x"
}
void call_sum() {
int x = 1, y = 2;
sum(&x,y); // &x because you're passing the address
printf("%d\n", x); // this should print 3
}
```
|
This is the if statement I have:
```
if (fgetc(fileName) != EOF) {
}
```
I know if I runfgetc()when not within theifstatement it will remove a character and I would have to doungetcto return it. Will this happen when thefgetcis within theifstatement?
|
Yes it will, and since you did not store the byte that was read, you lost it. Use this instead:
```
int c;
if ((c = fgetc(fp)) != EOF) {
ungetc(c, fp); /* put the byte back into the input stream */
/* not at end of file, keep reading */
...
} else {
/* at end of file: no byte was read, nothing to put back */
}
```
Also note that you should pass aFILE*tofgetc, not a filename, andungetcdoes notreturnthe character, but puts it back into the input stream so it can be read by the nextfgetc()orfgets()... You may not be able toungetcmore than one byte at a time before reading again from the stream.
|
I just read in a string using the following statement:
```
fgets(string, 100, file);
```
This string that was just read in was the last line. If I callfeof()now will it return TRUE? Is it the same as callingfeof()right at the start before reading in any lines?
|
No, don't usefeof()to detect the end of the file. Instead check for a read failure, for examplefgets()will returnNULLif it attempts to read past the end of the file whereasfeof()will return0until some function attempts to read past the end of the file, only after that it returns non-zero.
|
I was wondering if (and how) one can print only the sign of an array entry. For example I'd have something like
```
{1, -1, -1, 1}
```
and I would like the output to look something like
```
+ - - +
```
I'm pretty new to C and the only solution I can come up with is some sort ofif (... < 0)contdition that results in either a+or-char. But that seems quite unhandy.
This is merely meant to pretty up large "patterns" I am drawing with+1and-1. I'd really be glad if someone could help.
|
The way you have mentioned is effective for this purpose. If you want to do same thing by some other way, you can use ternary operator as:
```
a[i] < 0 ? printf("-"): printf("+");
```
|
I tried to implementstrcmp:
```
int strCmp(char string1[], char string2[])
{
int i = 0, flag = 0;
while (flag == 0) {
if (string1[i] > string2[i]) {
flag = 1;
} else
if (string1[i] < string2[i]) {
flag = -1;
} else {
i++;
}
}
return flag;
}
```
but I'm stuck with the case that the user will input the same strings, because the function works with1and-1, but it's doesn't return0. Can anyone help? And please without pointers!
|
Uhm.. way too complicated. Go for this one:
```
int strCmp(const char* s1, const char* s2)
{
while(*s1 && (*s1 == *s2))
{
s1++;
s2++;
}
return *(const unsigned char*)s1 - *(const unsigned char*)s2;
}
```
It returns <0, 0 or >0 as expected
You can't do it without pointers. In C, indexing an arrayisusing pointers.
Maybe you want to avoid using the*operator? :-)
|
I have a server application that creates a UNIX Domain Socket in a specific path with a name andbind()s to it.
I need to delete the socket only when I close/stop the application intentionally, from within the the application code; otherwise it needs to be open. How do I do this?
Thanks!
Edit:Consider that I start and run my application from inside a terminal.
|
You're making this harder than it needs to be. Put theunlink()right before thebind(). That's how everybody else does it. (Example:BSD syslogd, one of the classic unix-domain-socket-based services)
|
I am a bit at a loss as to why the compiler doesn't throw any kind of warning.
```
int32_t CALLBACK WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int32_t nCmdShow)
{
//..
}
```
Above is my Windows specific main function. Note thatlpCmdLineparameter has typeLPSTRwhich atypedeffor achar *.
Now, I have defined a function with the following prototype:
```
void COM_ParseCmdLine(uint8_t *lpCmdLine);
```
When I pass mylpCmdLinevariable into this function, it operates as expected and gives the correct results, because command line characters are 8 bits and positive in decimal value.
However, I am somewhat surprised that the compiler doesn't warn against the fact that we are potentially losing the sign if we deference auint8_t *vs achar *.
|
Setting the compilation warning level to/Wallwill produce the mentioned warnings.
|
For example if the file contains:
```
12345
-3445654
1245646
```
I want to read the first line into a string usingfgets(). Then I want to read the second line in too check if there is a'-'in the first spot. If there is one, I will read the second line andstrcatit to the first line.
Then I want to read the thrid line usingfgets()again. This time when there is no'-'I just want to make the file go back to the beginning of the third line so that the next time I callfgets()it will read the same third line again.
Is there a way I can do this?
|
Use fgetc to read the first character on the next line, and if it's not a '-' use ungetc to put it back.
|
I'm trying to debug a setup.py file for a package that builds a C extension with
```
from distutils.core import Extension
ext = Extension(...
```
I'd like to see what compiler / linker commands are actually getting executed. How do I print them when running
```
pip install -e ./
```
or
```
python setup.py install
```
|
The problem seems to be that it wasn't rebuilding the extension, but rather using a cached version. If I run
```
python setup.py clean --all
python setup.py develop
```
it rebuilds and shows all the compile/link commands.
|
I wrote two little programs in C,cryptanddecrypt.
I can call from the terminal:
```
./crypt some_argument it works.
```
But I want to pass to decrypt the output ofcrypt.
I already tried:
```
./decrypt $(./crypt hello) does not work
./crypt hello | ./decrypt does not work
```
Source code:
Crypt:
```
#include <stdio.h>
#include <stdlib.h>
#define CRYPT_KEY 5
char *crypto(char *str) {
char *tmp_adr = str;
while (*str != 0) {
*str += CRYPT_KEY;
str++;
}
return tmp_adr;
}
int main(int argc, char *argv[]) {
printf("Crypted message: %s\n", crypto(argv[1]));
}
```
Decypt is the same, but with a minus here:
```
*str -= CRYPT_KEY;
```
Someone know how to do that?
|
The output contains spaces. You need to quote it if you want everything inargv[1].
i.e../decrypt "$(./crypt some_argument)"
|
if you have a simple function e.gint open(const char *path, int oflags);you could pass a string directly as*pathe.g"filename.txt. You also could pass a&foo(which is an address, it might have no sense in this case). You can even put a normal pointer in it.
So my question is, is actually anaddressexpected when you have a pointer as parameter or is a just some data expected which fits to the parameter (in this caseconst char). I wondering because you can pass ascii directly without problems.
Thx.
|
It expects a pointer. A string constant, e.g."filename.txt", actually has a pointer value: An array is created to hold the string, and the value of the expression is the address of that array, i.e. a pointer to it.
|
This question already has answers here:What is the difference between %*c%c and %c as a format specifier to scanf?(3 answers)Closed7 years ago.
So I stumbled across this code and I haven't been able to figure out what the purpose of it is, or how it works:
```
int word_count;
scanf("%d%*c", &word_count);
```
My first thought was that%*dwas referencing acharpointer or disallowingword_countfrom takingcharvariables.
Can someone please shed some light on this?
|
*cmeans, that a char will be read but won't be assigned, for example for the input "30a" it will assign 30 toword_count, but 'a' will be ignored.
|
I was wondering if (and how) one can print only the sign of an array entry. For example I'd have something like
```
{1, -1, -1, 1}
```
and I would like the output to look something like
```
+ - - +
```
I'm pretty new to C and the only solution I can come up with is some sort ofif (... < 0)contdition that results in either a+or-char. But that seems quite unhandy.
This is merely meant to pretty up large "patterns" I am drawing with+1and-1. I'd really be glad if someone could help.
|
The way you have mentioned is effective for this purpose. If you want to do same thing by some other way, you can use ternary operator as:
```
a[i] < 0 ? printf("-"): printf("+");
```
|
I tried to implementstrcmp:
```
int strCmp(char string1[], char string2[])
{
int i = 0, flag = 0;
while (flag == 0) {
if (string1[i] > string2[i]) {
flag = 1;
} else
if (string1[i] < string2[i]) {
flag = -1;
} else {
i++;
}
}
return flag;
}
```
but I'm stuck with the case that the user will input the same strings, because the function works with1and-1, but it's doesn't return0. Can anyone help? And please without pointers!
|
Uhm.. way too complicated. Go for this one:
```
int strCmp(const char* s1, const char* s2)
{
while(*s1 && (*s1 == *s2))
{
s1++;
s2++;
}
return *(const unsigned char*)s1 - *(const unsigned char*)s2;
}
```
It returns <0, 0 or >0 as expected
You can't do it without pointers. In C, indexing an arrayisusing pointers.
Maybe you want to avoid using the*operator? :-)
|
I have a server application that creates a UNIX Domain Socket in a specific path with a name andbind()s to it.
I need to delete the socket only when I close/stop the application intentionally, from within the the application code; otherwise it needs to be open. How do I do this?
Thanks!
Edit:Consider that I start and run my application from inside a terminal.
|
You're making this harder than it needs to be. Put theunlink()right before thebind(). That's how everybody else does it. (Example:BSD syslogd, one of the classic unix-domain-socket-based services)
|
I am a bit at a loss as to why the compiler doesn't throw any kind of warning.
```
int32_t CALLBACK WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int32_t nCmdShow)
{
//..
}
```
Above is my Windows specific main function. Note thatlpCmdLineparameter has typeLPSTRwhich atypedeffor achar *.
Now, I have defined a function with the following prototype:
```
void COM_ParseCmdLine(uint8_t *lpCmdLine);
```
When I pass mylpCmdLinevariable into this function, it operates as expected and gives the correct results, because command line characters are 8 bits and positive in decimal value.
However, I am somewhat surprised that the compiler doesn't warn against the fact that we are potentially losing the sign if we deference auint8_t *vs achar *.
|
Setting the compilation warning level to/Wallwill produce the mentioned warnings.
|
For example:
"16" should print out the decimal values: 49,54"24" should print out the decimal values: 50,52
How do I achieve this?
|
It's really simple, you don't need to convert anything it's a matter of representation, example
```
const char *sixteen = "16";
const char *twentyfour = "24";
const char *number = "1345461";
printf("%d,%d\n", sixteen[0], sixteen[1]);
printf("%d,%d\n", twentyfour[0], twentyfour[1]);
// ^ ^ use the `%d' specifier to see the decimal value
// of the corresponding ascii.
for (int i = 0 ; number[i] != '\0' ; ++i)
printf("%d,", number[i]);
```
|
I have a program that make the sum of a and b(readed from file) and write it in a file.The problem come when i compile the code.
I have no idea why i get this error because on windows the same code works fine.Thank You.
|
Most likely your use of relative paths is incorrect — Xcode doesn't launch the executable in the folder where the source is (even if it tried to: what should it do when the source is in multiple folders?), soinisNULLand therefore thefscanfhas undefined behaviour.
In general, assuming you know the current working directory when launching from an IDE or from a GUI is dodgy.
|
I am usingVexRobotCand have a function:setTouchLEDRGB(portx, R,G,B);which sets the RGB colour of a touch LED.
I have 9 TouchLEDs and want to change the colour of them all at once, now annoyingly this is 9 lines of code at a time, i hope to create a function with an iteration such as:
```
for (int i = 0, i < 9, i++)
{
setTouchLEDRGB(port[i], R, G, B);
}
```
Is there a way to accomplish this?
|
```
setTouchLEDRGB(portx, R,G,B);
```
Not sure about the platform, but you could create an array containg the ports:
```
#define NUM_PORTS 9
// 'int' should be the type of your port parameter
int ports[NUM_PORTS] = {PORTA, PORTB, etc};
for (int i = 0; i < NUM_PORTS; ++i) {
setTouchLEDRGB(ports[i], R, G, B);
}
```
|
I'm porting some Arduino code to a mcu in pure c.
The data typebyteis used in the arduino code which is not supported by the compiler. I'm wondering which data byte should I useuint8_torunsigned char? Which is more correct? I'm relatively new to coding.
I gather it depends what the intent of the variable.
The variable will store hex data from this camera (screen shot of output).
sample output
```
byte incomingbyte;
void loop()
{
byte a[32];
int ii;
while(Serial2.available()>0)
{
incomingbyte=Serial2.read();
}
}
```
Many thanks
|
According tothis linkan arduino byte is an unsigned 8 bit value. I personally would use the more expressiveuint8_t, but anunsigned charwould work as well.
What type is used on the new platform to return a byte received from the serial port?
Might also want to check out thisquestion.
|
Closed. This question needsdetails or clarity. It is not currently accepting answers.Want to improve this question?Add details and clarify the problem byediting this post.Closed7 years ago.Improve this question
Basically which one of the Codes will return 0, faster ?
// if the answer is "the same" because the code is simple than imagine a more complex code variant written in the same way but with more code.
Code 1:
```
int a = 0;
if(a == 1){
return 1;
}else{
return 0;
}
```
Code 2:
```
int a = 0;
if(a == 1){
return 1;
}
return 0;
```
|
There will be no difference in the compiler generated code. Theelseis unnecessary but may be written for clarity.
|
I've encountered to this following code during my C learning. I've paste it into CodeBlocks therefore I know the output but (here's my question) how it's done? Can you explain it to me?
```
#include <stdio.h>
int main()
{
unsigned char t[10] = {0,1,2,3,4,5,6,7,8,9};
unsigned short *w;
w = (unsigned int*) &t;
printf("%d\n",*w++);
printf("%d\n",*w++);
printf("%d\n",*w++);
return 0;
}
```
|
Beware that this code, as is, contains errors: bad cast, undefined behavior and platform dependent results...
This is pointer arithmetic.winitially points to the content at the address of the first element oft(this is whatw = (unsigned short *)&tmeans). Then, hereafter, you access this memory as containing successivesshorts. At the same time the pointer is moved from ashortto its successor.
|
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