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.