mbtowc

Defined in header `<stdlib.h>`
int mbtowc( wchar_t* pwc, const char* s, size_t n )		(until C99)
int mbtowc( wchar_t* restrict pwc, const char* restrict s, size_t n )		(since C99)

Converts a multibyte character whose first byte is pointed to by s to a wide character, written to *pwc if pwc is not null.

If s is a null pointer, resets the global conversion state and determines whether shift sequences are used.

Notes

Each call to mbtowc updates the internal global conversion state (a static object of type mbstate_t, known only 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 mbtowc without synchronization: mbrtowc may be used instead.

Parameters

pwc	-	pointer to the wide character for output
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 character '\0'.

If s is a null pointer, resets its internal conversion state to represent the initial shift state and 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).

Example

Run this code

#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wchar.h>
 
// print multibyte string to wide-oriented stdout
// equivalent to wprintf(L"%s\n", ptr);
void print_mb(const char* ptr)
{
    mbtowc(NULL, NULL, 0); // reset the conversion state
    const char* end = ptr + strlen(ptr);
    int ret = 0;
    for (wchar_t wc; (ret = mbtowc(&wc, ptr, end - ptr)) > 0; ptr += ret)
        wprintf(L"%lc", wc);
    wprintf(L"\n");
}
 
int main(void)
{
    setlocale(LC_ALL, "en_US.utf8");
    // UTF-8 narrow multibyte encoding
    print_mb("z\u00df\u6c34\U0001F34C"); // or "zß水🍌"
}

Output:

zß水🍌

References

C23 standard (ISO/IEC 9899:2024):

7.24.7.2 The mbtowc function (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.22.7.2 The mbtowc function (p: 260)

C11 standard (ISO/IEC 9899:2011):

7.22.7.2 The mbtowc function (p: 358)

C99 standard (ISO/IEC 9899:1999):

7.20.7.2 The mbtowc function (p: 322)

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

4.10.7.2 The mbtowc function

Compiler support
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Concurrency support (C11)
Technical Specifications
Symbol index

mbrtowc (C95)	converts the next multibyte character to wide character, given state (function)
mblen	returns the number of bytes in the next multibyte character (function)
C++ documentation for mbtowc