/tmp>cat error.c
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char ** argv)
{
        int fd;
        char buf[100];
        ssize_t n;

        fd = open("foo", O_RDONLY);
        if (fd == -1) {
                perror("failed to open foo"); /* evaluates errno */
                return 1;
        }

        n = read(fd, buf, sizeof(buf));
        if (n == -1) {
                perror("failed to read from foo");
                return 2;
        }
        if (n != sizeof(buf)) {
                fprintf(stderr, "Short read from foo (got %z, expected %z)\n", n, sizeof(buf));
                return 3;
        }

        if (close(fd) == -1) {
                perror("failed to close foo");
                return 4;
        }

        // and so on ...
}

/tmp>make error
cc     error.c   -o error

/tmp>./error
failed to open foo: No such file or directory

/tmp>