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String Length

You can get the length of a string using the strlen function. This function is declared in the header file string.h.

size_t strlen (const char *s) Function
The strlen function returns the length of the null-terminated string s in bytes. (In other words, it returns the offset of the terminating null character within the array.)

For example, 

strlen ("hello, world")
    => 12

When applied to a character array, the strlen function returns the length of the string stored there, not its allocated size. You can get the allocated size of the character array that holds a string using the sizeof operator: 

char string[32] = "hello, world";
sizeof (string)
    => 32
strlen (string)
    => 12

But beware, this will not work unless string is the character array itself, not a pointer to it. For example: 

char string[32] = "hello, world";
char *ptr = string;
sizeof (string)
    => 32
sizeof (ptr)
    => 4  /* (on a machine with 4 byte pointers) */

This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.

It must also be noted that for multibyte encoded strings the return value does not have to correspond to the number of characters in the string. To get this value the string can be converted to wide characters and wcslen can be used or something like the following code can be used: 

/* The input is in string.
   The length is expected in n.  */
{
  mbstate_t t;
  char *scopy = string;
  /* In initial state.  */
  memset (&t, '\0', sizeof (t));
  /* Determine number of characters.  */
  n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t);
}

This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters.

The wide character equivalent is declared in wchar.h.

size_t wcslen (const wchar_t *ws) Function
The wcslen function is the wide character equivalent to strlen. The return value is the number of wide characters in the wide character string pointed to by ws (this is also the offset of the terminating null wide character of ws).

Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters.

This function was introduced in Amendment 1 to ISO C90.

size_t strnlen (const char *s, size_t maxlen) Function
The strnlen function returns the length of the string s in bytes if this length is smaller than maxlen bytes. Otherwise it returns maxlen. Therefore this function is equivalent to (strlen (s) < n ? strlen (s) : maxlen) but it is more efficient and works even if the string s is not null-terminated. 
char string[32] = "hello, world";
strnlen (string, 32)
    => 12
strnlen (string, 5)
    => 5

This function is a GNU extension and is declared in string.h.

size_t wcsnlen (const wchar_t *ws, size_t maxlen) Function
wcsnlen is the wide character equivalent to strnlen. The maxlen parameter specifies the maximum number of wide characters.

This function is a GNU extension and is declared in wchar.h.