9.16. perror(), strerror()

Print an error as a human-readable string


#include <stdio.h>
#include <string.h>   // for strerror()

void perror(const char *s);
char *strerror(int errnum);


Since so many functions return -1 on error and set the value of the variable errno to be some number, it would sure be nice if you could easily print that in a form that made sense to you.

Mercifully, perror() does that. If you want more description to be printed before the error, you can point the parameter s to it (or you can leave s as NULL and nothing additional will be printed.)

In a nutshell, this function takes errno values, like ECONNRESET, and prints them nicely, like "Connection reset by peer."

The function strerror() is very similar to perror(), except it returns a pointer to the error message string for a given value (you usually pass in the variable errno.)

Return Value

strerror() returns a pointer to the error message string.


int s;

s = socket(PF_INET, SOCK_STREAM, 0);

if (s == -1) { // some error has occurred
    // prints "socket error: " + the error message:
    perror("socket error");

// similarly:
if (listen(s, 10) == -1) {
    // this prints "an error: " + the error message from errno:
    printf("an error: %s\n", strerror(errno));

See Also