# strlen, strnlen_s

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< c‎ | string‎ | byte

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

### See also

wcslen wcsnlen_s (C95)(C11) | returns the length of a wide string (function)
mblen | returns the number of bytes in the next multibyte character (function)
C++ documentation for strlen

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