strlen, strnlen_s

< c‎ | string‎ | byte
 
C
 
Strings library
 
Null-terminated byte strings
Functions
Character manipulation
Conversions to numeric formats
String manipulation
(C11)
(C11)
(C11)
(C11)
String examination
strlenstrnlen_s
(C11)
Memory manipulation
(C11)
(C11)
(C11)
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 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

(C95)(C11)
 returns the length of a wide string
(function)
returns the number of bytes in the next multibyte character
(function)