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    strlen, strnlen_s

    From cppreference.com
    < c | string | byte
     
    C
     
    Strings library
     
    Null-terminated byte strings
    Functions
    Character manipulation
    Conversions to and from numeric formats
    String manipulation
    String examination
    Memory manipulation
    Miscellaneous
    (C11)(C11)
     
    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 str is not a pointer to a null-terminated byte string and strsz is greater than the size of that character array.
    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 <stdio.h>
#include <string.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
}

    Possible output: 

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

    References

    • C23 standard (ISO/IEC 9899:2024):
    • 7.24.6.3 The strlen function (p: TBD)
    • K.3.7.4.4 The strnlen_s function (p: TBD)
    • C17 standard (ISO/IEC 9899:2018):
    • 7.24.6.3 The strlen function (p: TBD)
    • K.3.7.4.4 The strnlen_s function (p: TBD)
    • 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

    (C95)(C11)
    		 returns the length of a wide string
    (function) [edit]
    		returns the number of bytes in the next multibyte character
    (function) [edit]
    C++ documentation for strlen