Defined in header <stdlib.h>
int mblen( const char* s, size_t n );

Determines the size, in bytes, of the multibyte character whose first byte is pointed to by s.

If s is a null pointer, resets the global conversion state and (until C23) determined whether shift sequences are used.

This function is equivalent to the call mbtowc((wchar_t*)0, s, n), except that conversion state of mbtowc is unaffected.


s - pointer to the multibyte character
n - limit on the number of bytes in s that can be examined

Return value

If s is not a null pointer, returns the number of bytes that are contained in the multibyte character or -1 if the first bytes pointed to by s do not form a valid multibyte character or ​0​ if s is pointing at the null charcter '\0'.

If s is a null pointer, resets its internal conversion state to represent the initial shift state and (until C23) returns ​0​ if the current multibyte encoding is not state-dependent (does not use shift sequences) or a non-zero value if the current multibyte encoding is state-dependent (uses shift sequences).


Each call to mblen updates the internal global conversion state (a static object of type mbstate_t, only known to this function). If the multibyte encoding uses shift states, care must be taken to avoid backtracking or multiple scans. In any case, multiple threads should not call mblen without synchronization: mbrlen may be used instead.

(until C23)

mblen is not allowed to have an internal state.

(since C23)


#include <string.h>
#include <stdlib.h>
#include <locale.h>
#include <stdio.h>
// the number of characters in a multibyte string is the sum of mblen()'s
// note: the simpler approach is mbstowcs(NULL, str, sz)
size_t strlen_mb(const char* ptr)
    size_t result = 0;
    const char* end = ptr + strlen(ptr);
    mblen(NULL, 0); // reset the conversion state
    while(ptr < end) {
        int next = mblen(ptr, end - ptr);
        if(next == -1) {
        ptr += next;
    return result;
void dump_bytes(const char* str)
    const char* end = str + strlen(str);
    for (; str != end; ++str) {
        printf("%02X ", (unsigned char)str[0]);
int main(void)
    setlocale(LC_ALL, "en_US.utf8");
    const char* str = "z\u00df\u6c34\U0001f34c";
    printf("The string \"%s\" consists of %zu characters, but %zu bytes: ",
            str, strlen_mb(str), strlen(str));

Possible output:

The string "zß水🍌" consists of 4 characters, but 10 bytes: 7A C3 9F E6 B0 B4 F0 9F 8D 8C


  • C17 standard (ISO/IEC 9899:2018):
    • The mblen function (p: 260)
  • C11 standard (ISO/IEC 9899:2011):
    • The mblen function (p: 357)
  • C99 standard (ISO/IEC 9899:1999):
    • The mblen function (p: 321)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The mblen function

See also

converts the next multibyte character to wide character
returns the number of bytes in the next multibyte character, given state
C++ documentation for mblen

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