example/posix/c-error_handling.c

Shows basic error message formatting

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <error.h>

int main(int argc, const char* argv[])
{
    const char* badpath = "!#$%^&/";

    // this call will fail
    execlp(badpath, badpath, NULL);
    // perror, when called immediately after
    // a failed system call prints a message
    // describing the error that has occured
    perror("Exec failed");

    // another possibility
    // this will again fail
    // syscalls use a convention that an error
    // is indicated by returning -1
    // and the global errno variable is set
    if(open(badpath, 0) == -1)
    {
        // if the call failed remember the value of errno
        // which may be overwritten by subsequent syscalls
        int error = errno;
        // allocate a temporary buffer
        char buffer[1024] = {'\0'};
        // use the (reentrant version of the) strerror function
        // to get the error message associated with the error code
        const char* msg = strerror_r(error, buffer, 1024);
        // use the message
        printf("Exec failed with error code %d = '%s'\n", error, msg);
    }

    // yet another possibility
    // again creat returns -1 on failure and sets errno
    if(creat(badpath, O_CREAT) < 0)
    {
        int error = errno;
        error_at_line(
            0, // <- passing a nonzero here would cause the process to exit
            error,
            __FILE__,
            __LINE__,
            "Exec of '%s' failed",
            badpath
        );
    }

    return 0;
}