strlen, strnlen_s

Defined in header <string.h>
size_t strlen( const char *str );
size_t strnlen_s( const char *str, size_t strsz );
(2) (since C11)
1) Returns the length of the given null-terminated byte string, that is, the number of characters in a character array whose first element is pointed to by str up to and not including the first null character.
The behavior is undefined if str is not a pointer to a null-terminated byte string.
2) Same as (1), except that the function returns zero if str is a null pointer and returns strsz if the null character was not found in the first strsz bytes of str.
The behavior is undefined if both str points to a character array which lacks the null character and the size of that character array < strsz; in other words, an erroneous value of strsz does not expose the impending buffer overflow. As with all bounds-checked functions, strnlen_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <string.h>.


str - pointer to the null-terminated byte string to be examined
strsz - maximum number of characters to examine

Return value

1) The length of the null-terminated byte string str.
2) The length of the null-terminated byte string str on success, zero if str is a null pointer, strsz if the null character was not found.


strnlen_s and wcsnlen_s are the only bounds-checked functions that do not invoke the runtime constraints handler. They are pure utility functions used to provide limited support for non-null terminated strings.


#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
#include <stdio.h>
int main(void)
    const char str[] = "How many characters does this string contain?";
    printf("without null character: %zu\n", strlen(str));
    printf("with null character:    %zu\n", sizeof str);
#ifdef __STDC_LIB_EXT1__
    printf("without null character: %zu\n", strnlen_s(str, sizeof str));

Possible output:

without null character: 45
with null character:    46
without null character: 45


  • C11 standard (ISO/IEC 9899:2011):
    • The strlen function (p: 372)
    • K. The strnlen_s function (p: 623)
  • C99 standard (ISO/IEC 9899:1999):
    • The strlen function (p: 334)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The strlen function

See also

returns the length of a wide string
returns the number of bytes in the next multibyte character
C++ documentation for strlen

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