Error Handling – `errno` & `perror`

errno (declared in <errno.h>) holds the error code from the last library call that set it. Functions like strerror translate it to a readable string.

FILE *fp = fopen("missing.txt", "r");
if (!fp) {
    perror("fopen"); // prints: fopen: No such file or directory
    // errno now contains ENOENT
}

Never rely on errno after a successful call — only check it when a function signals an error (often returns -1 or NULL).

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📚 Example: Using strerror

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main(void) {
    FILE *f = fopen("/root/secret", "r");
    if (!f) {
        printf("Error (%d): %s\n", errno, strerror(errno));
    }
    return 0;
}