C strerror() Function

Table of Contents

1. Introduction
2. strerror() Function Syntax
3. Understanding strerror() Function
4. Examples
   • Basic Usage of strerror()
   • Using strerror() with File Operations
5. Real-World Use Case
6. Conclusion

Introduction

The strerror() function in C is a standard library function that returns a pointer to the textual representation of the current errno value. It is part of the C standard library (string.h). This function is useful for obtaining a human-readable string that describes an error code.

strerror() Function Syntax

The syntax for the strerror() function is as follows:

char *strerror(int errnum);

Parameters:

• errnum: The error code to be converted into a textual representation.

Returns:

• The function returns a pointer to a string that describes the error code specified by errnum.

Understanding strerror() Function

The strerror() function takes an error code as input and returns a pointer to a string that contains a human-readable description of the error. The error code is typically obtained from the errno variable, which is set by system calls and library functions when an error occurs.

Examples

Basic Usage of strerror()

To demonstrate how to use strerror() to get a human-readable error message, we will write a simple program.

Example

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

int main() {
    int errnum = EINVAL;

    // Get the error message using strerror
    char *error_message = strerror(errnum);

    // Print the error message
    printf("Error message for EINVAL: %s\n", error_message);

    return 0;
}

Output:

Error message for EINVAL: Invalid argument

Using strerror() with File Operations

This example shows how to use strerror() to display error messages for file operations.

Example

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

int main() {
    FILE *file = fopen("nonexistentfile.txt", "r");

    if (file == NULL) {
        // Get the error message using strerror
        char *error_message = strerror(errno);

        // Print the error message
        printf("Error opening file: %s\n", error_message);
    } else {
        // Close the file if it was opened successfully
        fclose(file);
    }

    return 0;
}

Output:

Error opening file: No such file or directory

Real-World Use Case

Handling Network Errors

In real-world applications, the strerror() function can be used to handle network errors by converting error codes into meaningful error messages.

Example: Handling Network Errors

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <arpa/inet.h>

int main() {
    int sock = socket(AF_INET, SOCK_STREAM, 0);

    if (sock == -1) {
        // Get the error message using strerror
        char *error_message = strerror(errno);

        // Print the error message
        printf("Error creating socket: %s\n", error_message);
        return 1;
    }

    // Setup the socket address structure
    struct sockaddr_in server;
    server.sin_family = AF_INET;
    server.sin_port = htons(80);
    server.sin_addr.s_addr = inet_addr("192.168.1.1");

    // Try to connect to the server
    if (connect(sock, (struct sockaddr *)&server, sizeof(server)) < 0) {
        // Get the error message using strerror
        char *error_message = strerror(errno);

        // Print the error message
        printf("Error connecting to server: %s\n", error_message);
        return 1;
    }

    // Close the socket if connected successfully
    close(sock);

    return 0;
}

Output (example network error):

Error connecting to server: Connection refused

Conclusion

The strerror() function is used for converting error codes into human-readable error messages in C. By understanding and using this function correctly, you can provide meaningful error messages to users, which can help in debugging and improving the user experience. This is particularly helpful in applications that involve file operations, network communications, and other system-level interactions where errors may occur.