<errno.h> provides errno, a thread-local variable that functions set to indicate why they failed. Common codes: ENOENT (no file), EACCES (permission denied), EINVAL (invalid argument).

Only check errno immediately after a function that failed; successful calls may overwrite it, giving you meaningless noise.

This example checks errno to distinguish different error conditions after failed file operations.

// compile: gcc -o errnoexample errnoexample.c
// run: ./errnoexample
// description: interpret errno after system call failures
 
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
 
int main() {
    FILE* f = fopen("/nonexistent/file.txt", "r");
    if (!f) {
        printf("fopen failed: %s (errno %d)\n", strerror(errno), errno);
    }
 
    if (access("/etc/shadow", R_OK) < 0) {
        printf("access denied: %s\n", strerror(errno));
    }
 
    return 0;
}