perror and strerror functions

Introduction

These functions help show error messages when something goes wrong in your program. They make it easier to understand what caused the problem.

When a file cannot be opened and you want to tell the user why.

When a network connection fails and you want to explain the error.

When a system call returns an error and you want to print a readable message.

When debugging your program to find out what caused an error.

When logging errors to a file for later review.

Syntax

C

void perror(const char *s);

char *strerror(int errnum);

perror prints your message s followed by a colon and the error description for the current errno.

strerror returns a string describing the error number you give it.

Examples

This prints: "File open error: " to standard error.

C

perror("File open error");

This prints the error message string for the current errno value.

C

printf("Error: %s\n", strerror(errno));

This prints the message for the error number ENOENT (usually "No such file or directory").

C

int err = ENOENT;
printf("Error message: %s\n", strerror(err));

Sample Program

This program tries to open a file that does not exist. It uses perror to print the error message, then prints the same message using strerror.

C

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

int main() {
    FILE *f = fopen("nonexistent.txt", "r");
    if (!f) {
        perror("Failed to open file");
        printf("Using strerror: %s\n", strerror(errno));
        return 1;
    }
    fclose(f);
    return 0;
}

Important Notes

errno is a global variable set by system calls and some library functions when an error happens.

Always include <errno.h> and <string.h> to use these functions.

perror prints directly to standard error, while strerror returns a string you can use anywhere.

Summary

perror prints an error message with your text and the system error.

strerror returns a string describing an error number.

Use these to make your program's errors clear and easy to understand.