Example of strerror_r - c

Question

I'm a newbie in error handling; in my code I need to test the returned value of a function and to print the error's description if an error happens.
In order to keep the code thread-safe I have to use strerror_r, but I have some difficult to use it. In the following code the error number 22 happens (ret_setschedparam is 22). How can I print the description of the error number 22, i.e. "Invalid argument", by using the strerror_r?
I think that this prototype should be the right strerror_r I need:
char *strerror_r(int errnum, char *buf, size_t buflen);
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <sched.h>
#include <errno.h>
#include <string.h>
void *task();
int main()
{
pthread_attr_t attr;
struct sched_param prio;
pthread_t tid;
int ret_create;
int ret_setschedparam;
int ret_getschedparam;
int ret_join;
char *buf_setschedparam;
size_t size_setschedparam = 1024;
pthread_attr_init(&attr);
prio.sched_priority = 12;
ret_setschedparam = pthread_attr_setschedparam(&attr, &prio);
if (ret_setschedparam != 0) {
printf("Errore numero (pthread_attr_setschedparam): %s\n", strerror_r(errno, buf_setschedparam, size_setschedparam));
exit(EXIT_FAILURE);
}
ret_create = pthread_create(&tid, &attr, task, NULL);
printf("%d %d\n", ret_create, EPERM);
if (ret_create != 0) {
printf("Errore numero (pthread_create): %d\n", ret_create);
exit(EXIT_FAILURE);
}
ret_getschedparam = pthread_attr_getschedparam(&attr, &prio);
if (ret_getschedparam != 0) {
printf("Errore numero (pthread_attr_getschedparam): %d\n", ret_getschedparam);
exit(EXIT_FAILURE);
}
printf("Livello di priorità del thread: %d\n", prio.sched_priority);
ret_join = pthread_join(tid, NULL);
if (ret_join != 0) {
printf("Errore numero (pthread_join): %d\n", ret_join);
exit(EXIT_FAILURE);
}
return(0);
}
void *task()
{
printf("I am a simple thread.\n");
pthread_exit(NULL);
}
The compiler gives me an error: it said that the output of strerror_r is an int, not a char.

Answer 1

I think that this prototype should be the right strerro_r I need:
Note that this isn't the standard strerror_r interface, but a GNU extension.
You probably want to build your program with -D_GNU_SOURCE or add #define _GNU_SOURCE 1 to the top of your file to get this prototype instead of the standard one.
You are also not calling strerror_r correctly. This call:
char *buf_setschedparam;
size_t size_setschedparam = 1024;
... strerror_r(errno, buf_setschedparam, size_setschedparam)
promises to strerror_r that buf_setscheparam points to a buffer of size 1024. In fact that pointer is uninitialized, so once you get your program to build, it will promptly crash.
In addition, pthread_* functions do not set errno, they return the error code directly.
You want:
const size_t size_setschedparam = 1024;
char buf_setschedparam[size_setschedparam];
... sterror_r(ret_setschedparam, buf_setschedparam, size_setschedparam);
or even better:
char buf[1024];
... sterror_r(ret_setschedparam, buf, sizeof(buf));