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.

Practice

1. What does the perror function do in C?

easy

A. Prints a custom message followed by the system error message

B. Returns a string describing an error number

C. Terminates the program immediately

D. Clears the error state of the program

Solution

1. Step 1: Understand perror purpose

   perror prints a message you provide, then adds the system error message related to the last error.

2. Step 2: Compare with other options

   strerror returns a string but does not print; program termination or clearing errors are unrelated.

3. Final Answer:

   Prints a custom message followed by the system error message -> Option A

4. Quick Check:

   perror prints message + system error [OK]

Hint: perror prints errors; strerror returns error strings [OK]

Common Mistakes:

• Confusing perror with strerror
• Thinking perror returns a string
• Assuming perror stops the program

2. Which of the following is the correct syntax to print an error message using perror with the text "File error"?

easy

A. perror(File_error);

B. perror(File error);

C. perror("File error");

D. perror('File error');

Solution

1. Step 1: Check correct string syntax in C

   Strings must be in double quotes, so "File error" is correct.

2. Step 2: Validate perror usage

   perror takes a const char* string, so perror("File error") is correct syntax.

3. Final Answer:

   perror("File error"); -> Option C

4. Quick Check:

   Use double quotes for strings in perror [OK]

Hint: Use double quotes for strings in perror [OK]

Common Mistakes:

• Using single quotes instead of double quotes
• Passing variable names without quotes
• Missing parentheses in function call

3. What will be the output of the following C code snippet if errno is set to ENOENT (No such file or directory)?

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

int main() {
    printf("Error: %s\n", strerror(errno));
    return 0;
}

medium

A. Error: strerror function not found

B. Error: Permission denied

C. Error: Unknown error

D. Error: No such file or directory

Solution

1. Step 1: Understand strerror(errno)

   strerror returns a string describing the error number stored in errno.

2. Step 2: Match errno value to message

   ENOENT means "No such file or directory", so strerror(errno) returns that message.

3. Final Answer:

   Error: No such file or directory -> Option D

4. Quick Check:

   strerror(ENOENT) = "No such file or directory" [OK]

Hint: strerror(errno) shows error message for current error [OK]

Common Mistakes:

• Confusing error messages for different errno values
• Assuming strerror prints instead of returns string
• Not including header

4. Identify the error in this code snippet that tries to print an error message:

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

int main() {
    int errnum = ENOENT;
    printf("Error: %s\n", perror(errnum));
    return 0;
}

medium

A. Using perror inside printf is incorrect; perror prints directly and returns void

B. Missing header

C. Incorrect error number assigned to errnum

D. printf format specifier %s should be %d

Solution

1. Step 1: Check perror return type

   perror prints directly to stderr and returns void; it does not return a string.

2. Step 2: Analyze usage inside printf

   Passing perror(errnum) inside printf is invalid because perror returns void, not a string.

3. Final Answer:

   Using perror inside printf is incorrect; perror prints directly and returns void -> Option A

4. Quick Check:

   perror prints itself, don't use inside printf [OK]

Hint: perror prints error; don't use inside printf [OK]

Common Mistakes:

• Using perror inside printf expecting a string
• Confusing perror and strerror return types
• Ignoring compiler warnings about void return

5. You want to write a function that opens a file and prints a clear error message if it fails. Which approach correctly uses perror and strerror together to achieve this?

FILE *open_file(const char *filename) {
    FILE *f = fopen(filename, "r");
    if (!f) {
        // What to do here?
    }
    return f;
}

hard

A. Call perror(filename) without message, then return NULL

B. Call perror("Failed to open file") to print error, then return NULL

C. Print printf("Error: %s\n", strerror(errno)) only, then return NULL

D. Return NULL without printing any error message

Solution

1. Step 1: Understand error reporting with perror

   perror prints your message plus the system error, making it clear why fopen failed.

2. Step 2: Compare options for clarity

   Call perror("Failed to open file") to print error, then return NULL uses perror("Failed to open file"), which is clear and standard. Print printf("Error: %s\n", strerror(errno)) only, then return NULL prints only strerror message without context. Call perror(filename) without message, then return NULL prints filename only, which is less clear. Return NULL without printing any error message prints nothing.

3. Final Answer:

   Call perror("Failed to open file") to print error, then return NULL -> Option B

4. Quick Check:

   Use perror with custom message for clear errors [OK]

Hint: Use perror with custom text for clear error messages [OK]

Common Mistakes:

• Printing strerror without context message
• Using perror with filename only, confusing output
• Not printing any error message on failure