C Strlen Example

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

#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

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