perror() in C

The perror() function is a built-in C function used to print a custom error message followed by the system-generated error description based on the value of the global errno variable.

• Displays a user-defined message along with the corresponding system error.
• Commonly used to identify the cause of errors after a function fails.

#include <stdio.h>

int main() {
    FILE *fp = fopen("missing.txt", "r");

    if (fp == NULL) {
        perror("Error");
        return 1;
    }

    fclose(fp);
    return 0;
}

Output

An error occurred: No such file or directory

Syntax

clearerr() is a standard library function defined in <stdio.h> file.

void perror(const char *message);

This function does not return any value.

Examples of perror()

The following examples demonstrate the use of perror() in our C programs:

Handle Memory Allocation Error

#include <stdio.h>
#include <stdlib.h>

int main() {
    size_t size = 1e18;
    void *ptr = malloc(size);

    if (ptr == NULL) {

        // Print memory allocation error using
        // perror()
        perror("Memory allocation failed");
    }
    else {
        free(ptr);
    }
    return 0;
}

Output

Memory allocation failed: Cannot allocate memory

Handle Insufficient Permission Error

#include <stdio.h>

int main() {
    FILE *file = fopen("/root/protected.txt", "w");

    if (file == NULL) {
        perror("Error creating file");
        return 1;
    }
    fclose(file);
    return 0;
}

Output

Error creating file: Permission denied

Common Functions Used with perror()

perror() is often used with functions that set errno when they fail.

• fopen(): File opening errors.
• malloc(): Memory allocation failures.
• remove(): File deletion errors.
• rename(): File renaming errors.

Advantages

The perror() function provides a simple way to display descriptive error messages, making debugging easier.

• Prints a meaningful system-generated error message.
• Helps quickly identify the cause of function failures.
• Easy to use with file handling and memory allocation functions.

Limitations

Although useful, perror() has some limitations that should be considered while handling errors.

• Depends on the value of the global errno variable.
• Prints the error message directly to stderr.
• Should be called immediately after the error occurs, before errno changes.

perror() Vs strerror()