errno

Learn error codes with errno.

The errno global

C tracks failures with the errno global variable. You must include <errno.h>. errno holds an error code; successful calls don't reset it, so errno = 0; before a call is a good habit.

Example: if fopen returns NULL, errno tells you why. Values are constants like EINVAL, ERANGE, ENOENT. errno isn't an int but an expandable macro, so you can take its address with &errno.

Common errno constants

errno's values are standard constants. EINVAL means invalid argument, ERANGE result out of range, ENOENT no such file, ENOMEM out of memory.

You test these with if (errno == EINVAL). The numeric values vary by platform, so always compare by name. strerror and perror convert these codes to human-readable text; we'll see them in the next lesson.

Exercise Read and print errno

Include <errno.h> and <stdio.h>. In main, reset with errno = 0;, assign strtol("xyz", NULL, 10); to long v, then print errno with printf("%d\n", errno);. strtol doesn't change errno here; the goal is to show reading errno.

Show example solution

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

int main(void) {
    errno = 0;
    long v = strtol("xyz", NULL, 10);
    printf("%d\n", errno);
    return 0;
}

Test yourself

How do you reset errno?
```
errno = 0;
result = strtol("abc", NULL, 10);
```
- errno resets automatically
- Set errno = 0; after the call
- Set errno = 0; before the call
- errno never needs reset
Show answer

Set errno = 0; before the call — Successful calls don't clear errno; you reset it manually before the call.

Reset errno before the call

errno = ____;
long v = strtol("abc", NULL, 10);

Show answer

errno = 0;
long v = strtol("abc", NULL, 10);

Match errno constants to their meaning

EINVAL

Invalid argument
ERANGE

Result out of range
ENOENT

No such file
ENOMEM

Out of memory

Next lesson: perror

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies