strlen man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

strlen, strnlen — get length of fixed size string


#include <string.h>

size_t strlen(const char *s);
size_t strnlen(const char *s, size_t maxlen);


For strlen(): The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1‐2008 defers to the ISO C standard.

The strlen() function shall compute the number of bytes in the string to which s points, not including the terminating NUL character.

The strnlen() function shall compute the smaller of the number of bytes in the array to which s points, not including the terminating NUL character, or the value of the maxlen argument. The strnlen() function shall never examine more than maxlen bytes of the array pointed to by s.

Return Value

The strlen() function shall return the length of s; no return value shall be reserved to indicate an error.

The strnlen() function shall return an integer containing the smaller of either the length of the string pointed to by s or maxlen.


No errors are defined.

The following sections are informative.


Getting String Lengths

The following example sets the maximum length of key and data by using strlen() to get the lengths of those strings.

#include <string.h>
struct element {
    char *key;
    char *data;
char *key, *data;
int len;

*keylength = *datalength = 0;
if ((len = strlen(key)) > *keylength)
    *keylength = len;
if ((len = strlen(data)) > *datalength)
    *datalength = len;

See Also


The Base Definitions volume of POSIX.1‐2008, <string.h>