Defined in header <wchar.h>
size_t mbrlen( const char *s, size_t n, mbstate_t *ps );
(since C95)
size_t mbrlen( const char *restrict s, size_t n, mbstate_t *restrict ps );
(since C99)

Determines the size, in bytes, of the representation of a multibyte character.

This function is equivalent to the call mbrtowc(nullptr, s, n, ps?ps:&internal) for some hidden object internal of type mbstate_t, except that the expression ps is evaluated only once.


s - pointer to an element of a multibyte character string
n - limit on the number of bytes in s that can be examined
ps - pointer to the variable holding the conversion state

Return value

The first of the following that applies:

  • ​0​ if the next n or fewer bytes complete the null character or if s is a null pointer. Both cases reset the conversion state.
  • the number of bytes [1...n] that complete a valid multibyte character
  • (size_t)-2 if the next n bytes are part of a possibly valid multibyte character, which is still incomplete after examining all n bytes
  • (size_t)-1 if encoding error occurs. The value of errno is EILSEQ; the conversion state is unspecified.


#include <locale.h>
#include <string.h>
#include <stdio.h>
#include <wchar.h>
int main(void)
    // allow mbrlen() to work with UTF-8 multibyte encoding
    setlocale(LC_ALL, "en_US.utf8");
    // UTF-8 narrow multibyte encoding
    const char* str = u8"水";
    size_t sz = strlen(str);
    mbstate_t mb;
    memset(&mb, 0, sizeof mb);
    int len1 = mbrlen(str, 1, &mb);
    if(len1 == -2) 
        printf("The first 1 byte of %s is an incomplete multibyte char"
               " (mbrlen returns -2)\n", str);
    int len2 = mbrlen(str+1, sz-1, &mb);
    printf("The remaining %zu  bytes of %s hold %d bytes of the multibyte"
           " character\n", sz-1, str, len2);
    printf("Attempting to call mbrlen() in the middle of %s while in initial"
           " shift state returns %zd\n", str, mbrlen(str+1, sz-1, &mb));


The first 1 byte of 水 is an incomplete multibyte char (mbrlen returns -2)
The remaining 2  bytes of 水 hold 2 bytes of the multibyte character
Attempting to call mbrlen() in the middle of 水 while in initial shift state returns -1


  • C11 standard (ISO/IEC 9899:2011):
    • The mbrlen function (p: 442)
  • C99 standard (ISO/IEC 9899:1999):
    • The mbrlen function (p: 388)

See also

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

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