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Showing content from https://cplusplus.com/reference/cstdlib/mblen/ below:

function

mblen

int mblen (const char* pmb, size_t max);

Get length of multibyte character

Returns the size of the multibyte character pointed by pmb, examining at most max bytes.

mblen has its own internal shift state, which is altered as necessary only by calls to this function. A call to the function with a null pointer as pmb resets the state (and returns whether multibyte characters are state-dependent).

The behavior of this function depends on the LC_CTYPE category of the selected C locale.

Parameters

pmb: Pointer to the first byte of a multibyte character.
Alternativelly, the function may be called with a null pointer, in which case the function resets its internal shift state to the initial value and returns whether multibyte characters use a state-dependent encoding.
max: Maximum number of bytes of pmb to consider for the multibyte character.
No more than MB_CUR_MAX characters are examined in any case.
size_t is an unsigned integral type.

Return Value If the argument passed as pmb is not a null pointer, the size in bytes of the character pointed by pmb is returned when it forms a valid multibyte character and is not the terminating null character. If it is the terminating null character, the function returns zero, and in the case they do not form a valid multibyte character, -1 is returned.

If the argument passed as pmb is a null pointer, the function returns a nonzero value if multibyte character encodings are state-dependent, and zero otherwise.

Example

/* mblen example */
#include <stdio.h>      /* printf */
#include <stdlib.h>     /* mblen, mbtowc, wchar_t(C) */

void printbuffer (const char* pt, size_t max)
{
  int length;
  wchar_t dest;

  mblen (NULL, 0);         /* reset mblen */
  mbtowc (NULL, NULL, 0);  /* reset mbtowc */

  while (max>0) {
    length = mblen (pt, max);
    if (length<1) break;
    mbtowc(&dest,pt,length);
    printf ("[%lc]",dest);
    pt+=length; max-=length;
  }
}

int main()
{
  const char str [] = "test string";

  printbuffer (str,sizeof(str));

  return 0;
}

printbuffer prints a multibyte string character by character.

The example uses a trivial string using the "C" locale, but locales that interpret multibyte string are supported by the function.

Output:

[t][e][s][t][ ][s][t][r][i][n][g]

Data races The function accesses the array pointed by pmb.
The function also accesses and modifies an internal state object, which may cause data races on concurrent calls to this function (see mbrlen for an alternative that may use an external state object).
Concurrently changing locale settings may also introduce data races.

Exceptions (C++)No-throw guarantee: this function throws no exceptions.

If pmb is neither a null pointer nor a pointer to an array long enough (as described above), it causes undefined behavior.