What is errno?
errno is a global variable indicating the error occurred during any function call and it is defined inside <errno.h> header file.
When a function is called in C, a variable named errno is automatically assigned a code (value) which can be used to identify the type of error that has been encountered. Different codes (values) for errno mean different types of errors.
Below is a list of a few different errno values and their corresponding meaning:
| errno value | Error |
|
1 |
Operation not permitted |
|
2 |
No such file or directory |
|
3 |
No such process |
|
4 |
Interrupted system call |
|
5 |
I/O error |
|
6 |
No such device or address |
|
7 |
The argument list is too long |
|
8 |
Exec format error |
|
9 |
Bad file number |
|
10 |
No child processes |
|
11 |
Try again |
|
12 |
Out of memory |
|
13 |
Permission denied |
Example of errno
C
#include <errno.h> #include <stdio.h> int main() { // If a file is opened which does not exist, // then it will be an error and corresponding // errno value will be set FILE* fp; // opening a file which does not exist fp = fopen("w3wiki.txt", "r"); printf("Value of errno: %d\n", errno); return 0; } |
Value of errno: 2
Note: Here the errno is set to 2 which means “No such file or directory”. It may give Errno 13 in online IDE, which says permission denied.
Error Handling in C
Although C does not provide direct support to error handling (or exception handling), there are ways through which error handling can be done in C. A programmer has to prevent errors in the first place and test return values from the functions.
A lot of C function calls return -1 or NULL or set an in case of an error code as the global variable errno, so quick tests on these values are easily done with an instance of ‘if statement’.