Multibyte conversion
- C course
- Advanced
- Lesson 64/83
- 5 min
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.
Practice 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);.
Starter code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <locale.h>
// Call setlocale to 'C'
// For "Merhaba" print strlen and mbstowcs counts
int main(void) {
return 0;
}Sample 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;
}Expected output
mb: 7 wcs: 7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does mbstowcs(NULL, s, 0) do?
- Returns an error
- Allocates memory
- Copies the string
- Counts wide characters Correct answer
Explanation: When the third parameter is 0, it only counts.
2 Match conversion functions
| Term | Meaning |
|---|---|
mbstowcs |
Multibyte→wide |
wcstombs |
Wide→multibyte |
setlocale |
Sets locale |
strlen |
Counts bytes |
3 mbstowcs call
size_t n = mbstowcs(NULL, s, ____);size_t n = mbstowcs(NULL, s, 0);Explanation: A third parameter of 0 is the counting mode.