mblen

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.

Parameters

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).

Notes

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)

Example

Run this code

#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.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) {
           perror("strlen_mb");
           break;
        }
        ptr += next;
        ++result;
    }
    return result;
}
 
void dump_bytes(const char* str)
{
    for (const char* end = str + strlen(str); str != end; ++str)
        printf("%02X ", (unsigned char)str[0]);
    printf("\n");
}
 
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));
    dump_bytes(str);
}

Possible output:

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

References

C17 standard (ISO/IEC 9899:2018):

7.22.7.1 The mblen function (p: 260)

C11 standard (ISO/IEC 9899:2011):

7.22.7.1 The mblen function (p: 357)

C99 standard (ISO/IEC 9899:1999):

7.20.7.1 The mblen function (p: 321)

C89/C90 standard (ISO/IEC 9899:1990):

4.10.7.1 The mblen function

Language
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Error handling
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Technical Specifications
Symbol index

mbtowc	converts the next multibyte character to wide character (function)
mbrlen (C95)	returns the number of bytes in the next multibyte character, given state (function)