perror and strerror functions - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the perror function do in C?

perror prints a descriptive error message to stderr based on the current value of errno. It helps you understand what went wrong in your program.

beginner

How does strerror differ from perror?

strerror returns a pointer to a string describing the error code passed to it, instead of printing it. You can use this string in your own messages or logs.

beginner

What header file must you include to use perror and strerror?

You must include <stdio.h> for perror and <string.h> for strerror. Also, <errno.h> is needed to access errno.

beginner

What is errno in C?

errno is a global variable set by system calls and some library functions when an error occurs. It holds an error code that perror and strerror use to show error messages.

beginner

Show a simple example of using perror after a failed file open.

FILE *f = fopen("nofile.txt", "r");
if (!f) {
    perror("Error opening file");
}

This prints something like: Error opening file: No such file or directory

Which header file is required to use perror in C?

A <string.h>

B <stdio.h>

C <errno.h>

D <stdlib.h>

perror is declared in <stdio.h>. errno is in <errno.h>, but perror itself is in stdio.h.

What does strerror(errno) return?

A A string describing the last error

B An integer error code

C Prints error to stderr

D Sets errno to zero

strerror(errno) returns a pointer to a string describing the error code stored in errno.

What is the main difference between perror and strerror?

A <code>perror</code> returns error string; <code>strerror</code> prints error

B Both print error messages

C <code>perror</code> prints error; <code>strerror</code> returns error string

D Both return error codes

perror prints the error message directly, while strerror returns the error message as a string.

If fopen fails, which variable holds the error code?

A NULL

B perror

C strerror

D errno

errno is set by fopen when it fails, indicating the error reason.

Which function would you use to get a custom error message string for logging?

A strerror

B perror

C printf

D scanf

strerror returns the error message string, which you can use in logs or custom messages.

Explain how perror and strerror help in error handling in C.

Think about how you find out what went wrong in your program.

Write a short code snippet that opens a file and uses perror if the file cannot be opened.

Remember to include <stdio.h> and check the file pointer.