strlen, strnlen_s

Defined in header `<string.h>`
size_t strlen( const char *str );	(1)
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.

Parameters

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.

Notes

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.

Example

Run this code

#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));
#endif
}

Output:

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

References

C11 standard (ISO/IEC 9899:2011):

7.24.6.3 The strlen function (p: 372)

K.3.7.4.4 The strnlen_s function (p: 623)

C99 standard (ISO/IEC 9899:1999):

7.21.6.3 The strlen function (p: 334)

C89/C90 standard (ISO/IEC 9899:1990):

4.11.6.3 The strlen function

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