<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;
}

Error reporting:

• errno: thread-local variable holding last error code (set by library functions on error)
• strerror(code): convert errno value to human-readable string
• perror(prefix): print prefix: error message to stderr (from <stdio.h>)

Common error codes (POSIX):

• ENOENT: no such file or directory
• EACCES: permission denied
• EINVAL: invalid argument
• ENOMEM: out of memory (malloc, etc.)
• EEXIST: file exists
• EISDIR: is a directory (expected file)
• ENOTDIR: not a directory (expected directory)
• EPERM: operation not permitted
• EAGAIN, EWOULDBLOCK: resource temporarily unavailable
• EINTR: interrupted system call
• EBADF: bad file descriptor
• ERANGE: result out of range
• EIO: input/output error
• ENOSPC: no space left on device

Usage notes:

• Check errno immediately after suspect function (other functions may overwrite it)
• errno is meaningful only when function signals error (check return value first)
• Always #include <errno.h> before using errno
• errno is never set to 0 by functions (only by your code)