Using errno in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is errno in C programming?

errno is a global variable set by system calls and some library functions in C to indicate what error occurred during the last operation.

beginner

How do you include errno in your C program?

You include the header <errno.h> which declares errno and error codes.

intermediate

Why should you set errno to 0 before a function call?

Setting errno to 0 before a function call helps to check if the function actually changed errno to indicate an error.

beginner

What function can you use to get a human-readable string for the current errno value?

You can use strerror(errno) to get a readable error message describing the error.

intermediate

Is errno reset to 0 automatically after a successful function call?

No, errno is not reset automatically. It keeps its value until changed by a function that sets it.

Which header file must you include to use errno in C?

A <errno.h>

B <stdio.h>

C <stdlib.h>

D <string.h>

The <errno.h> header declares errno and error codes.

What type of variable is errno?

A Static double variable

B Local float variable

C Pointer to char

D Global integer variable

errno is a global integer variable used to store error codes.

What does strerror(errno) return?

A The numeric value of <code>errno</code>

B The last function's return value

C A string describing the error code stored in <code>errno</code>

D The current system time

strerror(errno) returns a human-readable string describing the error.

Why should you set errno = 0 before calling a function that might fail?

A To clear the function's return value

B To detect if the function changed <code>errno</code> indicating an error

C To reset the program counter

D To initialize a local variable

Setting errno to 0 helps confirm if the function sets it on error.

If a function succeeds, what happens to errno?

A It remains unchanged

B It resets to 0 automatically

C It becomes -1

D It is set to the function's return value

errno is not reset automatically after success; it keeps its previous value.

Explain how to use errno to detect errors after calling a system function in C.

Think about the steps to safely check errors using errno.

Describe why errno is useful and how it differs from a function's return value.

Consider the difference between error signaling and error details.

Practice

1. What is the primary purpose of errno in C programming?

easy

A. To store error codes set by system or library functions when they fail

B. To count the number of errors in a program

C. To reset the program state after an error

D. To display error messages automatically

Solution

1. Step 1: Understand what errno stores

   errno is a global variable that stores error codes set by system or library functions when they fail.

2. Step 2: Differentiate from other options

   It does not count errors, reset program state, or display messages automatically.

3. Final Answer:

   To store error codes set by system or library functions when they fail -> Option A

4. Quick Check:

   errno stores error codes = C [OK]

Hint: Remember: errno holds error codes after failure [OK]

Common Mistakes:

• Thinking errno counts errors
• Assuming errno resets program state
• Believing errno prints messages automatically

2. Which of the following is the correct way to include errno in a C program?

easy

A. #include

B. #include

C. #include

D. #include

Solution

1. Step 1: Identify the standard header for errno

   The standard header file to use errno is <errno.h>.

2. Step 2: Verify other options

   <error.h>, <err.h>, and <errors.h> are not standard headers for errno.

3. Final Answer:

   #include <errno.h> -> Option B

4. Quick Check:

   errno header = <errno.h> [OK]

Hint: Use #include to access errno [OK]

Common Mistakes:

• Using wrong header files
• Forgetting to include errno.h
• Confusing with similar-sounding headers

3. What will the following C code print if the file nonexistent.txt does not exist?

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

int main() {
    FILE *f = fopen("nonexistent.txt", "r");
    if (!f) {
        printf("Error: %s\n", strerror(errno));
    }
    return 0;
}

medium

A. Error: No such file or directory

B. Error: Permission denied

C. Error: File exists

D. Error: Unknown error

Solution

1. Step 1: Understand fopen failure and errno

   When fopen fails to open a non-existing file for reading, it sets errno to indicate the error, usually ENOENT meaning "No such file or directory".

2. Step 2: Use strerror to get error message

   strerror(errno) converts the error code to a readable string, which will be "No such file or directory".

3. Final Answer:

   Error: No such file or directory -> Option A

4. Quick Check:

   fopen missing file error = "No such file or directory" [OK]

Hint: fopen missing file sets errno to ENOENT message [OK]

Common Mistakes:

• Assuming errno is zero without checking fopen result
• Confusing error messages for different errors
• Not including string.h for strerror

4. Identify the error in this code snippet that uses errno:

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

int main() {
    errno = 0;
    FILE *f = fopen("file.txt", "r");
    if (f == NULL) {
        perror("File open error");
    }
    if (errno != 0) {
        printf("Error code: %d\n", errno);
    }
    return 0;
}

medium

A. Using perror instead of strerror is incorrect

B. Setting errno to 0 before fopen is unnecessary

C. Checking errno after fopen without checking fopen result first

D. Not closing the file pointer after fopen

Solution

1. Step 1: Analyze errno usage after fopen

   errno should only be checked if the function indicates failure. Here, errno is checked regardless of fopen result, which can be misleading.

2. Step 2: Confirm correct error checking pattern

   The correct pattern is to check if fopen returned NULL before reading errno. This code checks errno even if fopen succeeded.

3. Final Answer:

   Checking errno after fopen without checking fopen result first -> Option C

4. Quick Check:

   Check function result before errno [OK]

Hint: Always check function result before reading errno [OK]

Common Mistakes:

• Reading errno without function failure
• Confusing perror and strerror usage
• Ignoring file close after fopen

5. You want to open a file for reading and print a custom error message if it fails. Which code snippet correctly uses errno and strerror to do this?

A) FILE *f = fopen("data.txt", "r");
   if (!f) {
       printf("Error: %s\n", strerror(errno));
   }

B) errno = 0;
   FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       printf("Error: %s\n", strerror(errno));
   }

C) FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       errno = 0;
       printf("Error: %s\n", strerror(errno));
   }

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

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Check correct error handling with errno

   Option D uses perror, which is the standard and simplest way to print a custom error message along with the error string corresponding to errno. It automatically handles errno correctly.

2. Step 2: Identify issues in other options

   Option D does not reset errno before fopen, which is less safe. Option D resets errno before fopen and checks failure, but it's generally unnecessary to reset errno before calls that set it on failure. Option D resets errno after failure, clearing the error code before printing, which is incorrect.

3. Final Answer:

   Option D -> Option D

4. Quick Check:

   Use perror for simple and correct error messages [OK]

Hint: Use perror for automatic error message printing [OK]

Common Mistakes:

• Resetting errno unnecessarily
• Resetting errno after failure
• Using strerror without valid errno