Defined in header <uchar.h>
size_t mbrtoc32( char32_t restrict * pc32, const char * restrict s,
                 size_t n, mbstate_t * restrict ps );
(since C11)

Converts a single code point from its narrow multibyte character representation to its variable-length 32-bit wide character representation (but typically, UTF-32).

If s is not a null pointer, inspects at most n bytes of the multibyte character string, beginning with the byte pointed to by s to determine the number of bytes necessary to complete the next multibyte character (including any shift sequences, and taking into account the current multibyte conversion state *ps). If the function determines that the next multibyte character in s is complete and valid, converts it to the corresponding 32-bit wide character and stores it in *pc32 (if pc32 is not null).

If the multibyte character in *s corresponds to a multi-char32_t sequence (not possible with UTF-32), then after the first call to this function, *ps is updated in such a way that the next calls to mbrtoc32 will write out the additional char32_t, without considering *s.

If s is a null pointer, the values of n and pc32 are ignored and the call is equivalent to mbrtoc32(NULL, "", 1, ps).

If the wide character produced is the null character, the conversion state *ps represents the initial shift state.

If the macro __STDC_UTF_32__ is defined, the 32-bit encoding used by this function is UTF-32; otherwise, it is implementation-defined. In any case, the multibyte character encoding used by this function is specified by the currently active C locale.


pc32 - pointer to the location where the resulting 32-bit wide character will be written
s - pointer to the multibyte character string used as input
n - limit on the number of bytes in s that can be examined
ps - pointer to the conversion state object used when interpreting the multibyte string

Return value

The first of the following that applies:

  • ​0​ if the character converted from s (and stored in *pc32 if non-null) was the null character
  • the number of bytes [1...n] of the multibyte character successfully converted from s
  • (size_t)-3 if the next char32_t from a multi-char32_t character has now been written to *pc32. No bytes are processed from the input in this case.
  • (size_t)-2 if the next n bytes constitute an incomplete, but so far valid, multibyte character. Nothing is written to *pc32.
  • (size_t)-1 if encoding error occurs. Nothing is written to *pc32, the value EILSEQ is stored in errno and the value of *ps is unspecified.


#include <stdio.h>
#include <locale.h>
#include <string.h>
#include <uchar.h>
#include <assert.h>
int main(void)
    setlocale(LC_ALL, "en_US.utf8");
    char in[] = u8"zß水🍌"; // or "z\u00df\u6c34\U0001F34C"
    const size_t in_size = sizeof in / sizeof *in;
    printf("Processing %zu UTF-8 code units: [ ", in_size);
    for (size_t i = 0; i < in_size; ++i)
        printf("0x%02x ", (unsigned char)in[i]);
    char32_t out[in_size];
    char32_t *p_out = out;
    char *p_in = in, *end = in + in_size;
    mbstate_t state;
    memset(&state, 0, sizeof(state));
    size_t rc;
    while ((rc = mbrtoc32(p_out, p_in, end - p_in, &state)))
        assert(rc != (size_t)-3); // no surrogate pairs in UTF-32
        if (rc == (size_t)-1) break; // invalid input
        if (rc == (size_t)-2) break; // truncated input
        p_in += rc;
    size_t out_size = p_out+1 - out;
    printf("into %zu UTF-32 code units: [ ", out_size);
    for (size_t i = 0; i < out_size; ++i)
        printf("0x%08X ", out[i]);


Processing 11 UTF-8 code units: [ 0x7a 0xc3 0x9f 0xe6 0xb0 0xb4 0xf0 0x9f 0x8d 0x8c 0x00 ]
into 5 UTF-32 code units: [ 0x0000007A 0x000000DF 0x00006C34 0x0001F34C 0x00000000 ]


  • C17 standard (ISO/IEC 9899:2018):
    • The mbrtoc32 function (p: 293-294)
  • C11 standard (ISO/IEC 9899:2011):
    • The mbrtoc32 function (p: 400-401)

See also

converts a 32-bit wide character to narrow multibyte string
C++ documentation for mbrtoc32

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