Multibyte conversion

Learn multibyte and wide character conversion.

mbstowcs and wcstombs

mbstowcs converts a multibyte string to a wide character array; wcstombs does the reverse. Both are in <stdlib.h>.

mbstowcs(NULL, s, 0) only counts the resulting wide characters; it does not allocate memory. mbstowcs(wbuf, s, n) writes to wbuf. For an ASCII string the byte count and wide character count are equal: both are 7 for "Merhaba". You must call setlocale(LC_ALL, "") or "C" before conversion; otherwise the behavior is undefined.

ASCII conversion

For the ASCII string "Merhaba", strlen returns 7 and mbstowcs(NULL, s, 0) also returns 7. One byte per character maps to one wide character.

If the string were "çağ", strlen would return 5 (two letters are two bytes each) and mbstowcs would return 3. In this lesson we stick to ASCII; we begin with setlocale(LC_ALL, "C") so the output is deterministic. After conversion we print the counts with printf instead of wprintf.

Exercise Byte and wide character count

First call setlocale(LC_ALL, "C");. Declare const char *s = "Merhaba";. Compute size_t mb = strlen(s); and size_t wcs = mbstowcs(NULL, s, 0);. Print printf("mb: %zu\n", mb); and printf("wcs: %zu\n", wcs);.

Show example solution

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <locale.h>

int main(void) {
    setlocale(LC_ALL, "C");
    const char *s = "Merhaba";
    size_t mb = strlen(s);
    size_t wcs = mbstowcs(NULL, s, 0);
    printf("mb: %zu\n", mb);
    printf("wcs: %zu\n", wcs);
    return 0;
}

Test yourself

What does mbstowcs(NULL, s, 0) do?
- Returns an error
- Allocates memory
- Copies the string
- Counts wide characters
Show answer

Counts wide characters — When the third parameter is 0, it only counts.
Match conversion functions

mbstowcs

Multibyte→wide
wcstombs

Wide→multibyte
setlocale

Sets locale
strlen

Counts bytes

mbstowcs call

size_t n = mbstowcs(NULL, s, ____);

Show answer

size_t n = mbstowcs(NULL, s, 0);

Next lesson: UTF-8 handling

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