C strerror() Function

Beginner
5 min read
Updated: Nov 2026
3 examples
string.h
Decorative image with the word STRERROR

Definition

In C, strerror() turns an error number into a short message you can print. You usually pass errno right after a library call fails.

The error number is required. The result is a pointer to text that the library owns, so do not free it or change it. Read errno before another call replaces it. Include <string.h> and <errno.h>.

Syntax

Pass the error number. That is usually errno just after a call fails.

C
strerror(errno);
CallWhat it does
strerror(errno)Looks up a short message for the current errno value.

Include <string.h> and <errno.h>. Exact wording can differ by OS and locale.

Library declarations
C
char *strerror(int errnum);

Parameters

ParameterDescription
errnumRequired. The error number, usually errno right after the call that failed.

Return Value

Return ValueDescription
Message pointerPoints to library-owned text. Do not free it or change it.

C Examples

Example 1 — Look Up a Known Code

Pass an errno.h constant directly to see its message.

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

int main(void) {
    printf("%s\n", strerror(ENOENT));
    return 0;
}

How It Works

  1. Include errno.h. ENOENT is a named error constant.
  2. Call strerror. The library maps that number to a message string.
  3. Print the text. You get a readable phrase instead of a raw integer.
  4. Wording may vary. Different systems can phrase the same error slightly differently.

Example 2 — Report a Failed fopen

After a failed open, print strerror(errno) so the user knows why it failed.

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

int main(void) {
    FILE *fp = fopen("missing.txt", "r");

    if (fp == NULL) {
        printf("open failed: %s\n", strerror(errno));
        return 1;
    }

    fclose(fp);
    return 0;
}

How It Works

  1. fopen fails. Opening a missing file returns NULL.
  2. errno is set. The library stores a code such as ENOENT.
  3. strerror translates it. You print a clear message with your own prefix.
  4. Exit with status 1. Signals failure to the shell or caller.

Example 3 — Save errno Before Other Calls

Other library functions can change errno. Save it first, then format the message.

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

int main(void) {
    FILE *fp = fopen("missing.txt", "r");

    if (fp == NULL) {
        int err = errno;
        printf("code = %d\n", err);
        printf("text = %s\n", strerror(err));
        return 1;
    }

    fclose(fp);
    return 0;
}

How It Works

  1. Capture errno. err = errno freezes the failure code.
  2. Print the number. Useful for logs and debugging (exact value can vary).
  3. Translate with strerror. Pass the saved err, not a later errno.
  4. Safe habit. This pattern stays correct even if you add more calls before printing.

Frequently Asked Questions

strerror() turns an error number into a short message you can print.
errnum is required. It is usually errno right after a failing call.
A pointer to library-owned text. Do not free it or change it.
Include <string.h> and <errno.h>.
Read it immediately after the failing call, before another library call replaces it.

Next: strpbrk()

Locate the first character in a string that belongs to a given set.

strpbrk() tutorial →

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

9 people found this page helpful

C strdup() Function C strpbrk() Function