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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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mbstowcs(3) Library Functions Manual mbstowcs(3)
mbstowcs - convert a multibyte string to a wide-character string
Standard C library (libc, -lc)
#include <stdlib.h>
size_t mbstowcs(size_t dsize;
wchar_t dest[restrict dsize], const char *restrict src,
size_t dsize);
If dest is not NULL, convert the multibyte string src to a wide-
character string starting at dest. At most dsize wide characters
are written to dest. The sequence of characters in the string src
shall begin in the initial shift state. The conversion can stop
for three reasons:
• An invalid multibyte sequence has been encountered. In this
case, (size_t) -1 is returned.
• dsize non-L'\0' wide characters have been stored at dest. In
this case, the number of wide characters written to dest is
returned, but the shift state at this point is lost.
• The multibyte string has been completely converted, including
the terminating null character ('\0'). In this case, the
number of wide characters written to dest, excluding the
terminating null wide character, is returned.
If dest is NULL, dsize is ignored, and the conversion proceeds as
above, except that the converted wide characters are not written
out to memory, and that no length limit exists.
In order to avoid the case 2 above, the programmer should make
sure dsize is greater than or equal to mbstowcs(NULL,src,0)+1.
The programmer must ensure that there is room for at least dsize
wide characters at dest.
The number of wide characters that make up the converted part of
the wide-character string, not including the terminating null wide
character. If an invalid multibyte sequence was encountered,
(size_t) -1 is returned.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ mbstowcs() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
The function mbsrtowcs(3) provides a better interface to the same
functionality.
C11, POSIX.1-2008.
POSIX.1-2001, C99.
The behavior of mbstowcs() depends on the LC_CTYPE category of the
current locale.
The program below illustrates the use of mbstowcs(), as well as
some of the wide character classification functions. An example
run is the following:
$ ./t_mbstowcs de_DE.UTF-8 Grüße!
Length of source string (excluding terminator):
8 bytes
6 multibyte characters
Wide character string is: Grüße! (6 characters)
G alpha upper
r alpha lower
ü alpha lower
ß alpha lower
e alpha lower
! !alpha
Program source
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wchar.h>
#include <wctype.h>
int
main(int argc, char *argv[])
{
size_t mbslen; /* Number of multibyte characters in source */
wchar_t *wcs; /* Pointer to converted wide character string */
if (argc < 3) {
fprintf(stderr, "Usage: %s <locale> <string>\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Apply the specified locale. */
if (setlocale(LC_ALL, argv[1]) == NULL) {
perror("setlocale");
exit(EXIT_FAILURE);
}
/* Calculate the length required to hold argv[2] converted to
a wide character string. */
mbslen = mbstowcs(NULL, argv[2], 0);
if (mbslen == (size_t) -1) {
perror("mbstowcs");
exit(EXIT_FAILURE);
}
/* Describe the source string to the user. */
printf("Length of source string (excluding terminator):\n");
printf(" %zu bytes\n", strlen(argv[2]));
printf(" %zu multibyte characters\n\n", mbslen);
/* Allocate wide character string of the desired size. Add 1
to allow for terminating null wide character (L'\0'). */
wcs = calloc(mbslen + 1, sizeof(*wcs));
if (wcs == NULL) {
perror("calloc");
exit(EXIT_FAILURE);
}
/* Convert the multibyte character string in argv[2] to a
wide character string. */
if (mbstowcs(wcs, argv[2], mbslen + 1) == (size_t) -1) {
perror("mbstowcs");
exit(EXIT_FAILURE);
}
printf("Wide character string is: %ls (%zu characters)\n",
wcs, mbslen);
/* Now do some inspection of the classes of the characters in
the wide character string. */
for (wchar_t *wp = wcs; *wp != 0; wp++) {
printf(" %lc ", (wint_t) *wp);
if (!iswalpha(*wp))
printf("!");
printf("alpha ");
if (iswalpha(*wp)) {
if (iswupper(*wp))
printf("upper ");
if (iswlower(*wp))
printf("lower ");
}
putchar('\n');
}
exit(EXIT_SUCCESS);
}
mblen(3), mbsrtowcs(3), mbtowc(3), wcstombs(3), wctomb(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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man-pages@man7.org
Linux man-pages 6.15 2025-06-28 mbstowcs(3)
Pages that refer to this page: MB_CUR_MAX(3), mbsrtowcs(3), mbtowc(3), wcstombs(3), wctomb(3), wprintf(3), locale(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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