strerror

Learn to get error messages with strerror.

The strerror function

strerror converts an errno code to a human-readable string. Usage: char *msg = strerror(errno);. You print the returned string with printf("%s\n", msg);. You need <string.h>.

perror combines your prefix with the message and writes to stderr; strerror returns just the message. So strerror is more flexible: you can write it to a file, log it or concatenate with another string. You choose the output stream.

strerror and fprintf

You can write strerror's string to stderr with fprintf(stderr, "error: %s\n", strerror(errno));. This works like perror but you fully control the format.

The string strerror returns lives in static memory; don't free it. A later strerror call in the same thread overwrites this buffer. So use the string at once or copy it with strncpy. In multi-threaded programs prefer strerror_r, the thread-safe version.

Exercise Print an error with strerror

Include <stdio.h> and <string.h>. In main, assign errno = ENOENT;, then get the message with char *msg = strerror(errno); and print it with printf("%s\n", msg);. The typical output for ENOENT is No such file or directory.

Show example solution

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

int main(void) {
    errno = ENOENT;
    char *msg = strerror(errno);
    printf("%s\n", msg);
    return 0;
}

Test yourself

Match perror vs strerror

perror

Writes to stderr
strerror

Returns a string
Prefix

perror parameter
Flexibility

strerror advantage

Complete the strerror call

char *msg = ____(errno);
printf("%s\n", msg);

Show answer

char *msg = strerror(errno);
printf("%s\n", msg);

What does strerror(errno) return?
- char * string
- FILE * stream
- void
- int error code
Show answer

char * string — strerror returns the error string as char *.

Next lesson: assert

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