The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group


mbstowcs - convert a character string to a wide-character string


#include <stdlib.h>

size_t mbstowcs(wchar_t *restrict
pwcs, const char *restrict s,


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2008 defers to the ISO C standard. [Option End]

The mbstowcs() function shall convert a sequence of characters that begins in the initial shift state from the array pointed to by s into a sequence of corresponding wide-character codes and shall store not more than n wide-character codes into the array pointed to by pwcs. No characters that follow a null byte (which is converted into a wide-character code with value 0) shall be examined or converted. Each character shall be converted as if by a call to mbtowc(), except that the shift state of mbtowc() is not affected.

No more than n elements shall be modified in the array pointed to by pwcs. If copying takes place between objects that overlap, the behavior is undefined.

The behavior of this function shall be affected by the LC_CTYPE category of the current locale. [XSI] [Option Start]  If pwcs is a null pointer, mbstowcs() shall return the length required to convert the entire array regardless of the value of n, but no values are stored. [Option End]


If an invalid character is encountered, mbstowcs() shall return (size_t)-1 [CX] [Option Start]  and shall set errno to indicate the error. [Option End]

Otherwise, mbstowcs() shall return the number of the array elements modified [XSI] [Option Start]  (or required if pwcs is null), [Option End] not including a terminating 0 code, if any. The array shall not be zero-terminated if the value returned is n.


The mbstowcs() function shall fail if:

[CX] [Option Start] An invalid character sequence is detected. In the POSIX locale an [EILSEQ] error cannot occur since all byte values are valid characters. [Option End]

The following sections are informative.










mblen, mbtowc, wctomb, wcstombs

XBD <stdlib.h>


First released in Issue 4. Aligned with the ISO C standard.

Issue 6

The mbstowcs() prototype is updated for alignment with the ISO/IEC 9899:1999 standard.

Extensions beyond the ISO C standard are marked.

Issue 7

Austin Group Interpretation 1003.1-2001 #170 is applied.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0371 [195] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0208 [663,674] is applied.

End of informative text.


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