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strlen - Man Page

get length of fixed size string


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.


#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.1-2017 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 any 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 the number of bytes preceding the first null byte in the array to which s points, if s contains a null byte within the first maxlen bytes; otherwise, it shall return 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;

Application Usage




Future Directions


See Also


The Base Definitions volume of POSIX.1-2017, <string.h>

Referenced By

localeconv(3p), string.h(0p), wcslen(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual