The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group


mbsnrtowcs, mbsrtowcs - convert a character string to a wide-character string (restartable)


#include <wchar.h>

[CX] [Option Start] size_t mbsnrtowcs(wchar_t *restrict dst, const char **restrict src,
nmc, size_t len, mbstate_t *restrict ps); [Option End]

size_t mbsrtowcs(wchar_t *restrict
dst, const char **restrict src,
len, mbstate_t *restrict ps);


For mbsrtowcs(): [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 mbsrtowcs() function shall convert a sequence of characters, beginning in the conversion state described by the object pointed to by ps, from the array indirectly pointed to by src into a sequence of corresponding wide characters. If dst is not a null pointer, the converted characters shall be stored into the array pointed to by dst. Conversion continues up to and including a terminating null character, which shall also be stored. Conversion shall stop early in either of the following cases:

Each conversion shall take place as if by a call to the mbrtowc() function.

If dst is not a null pointer, the pointer object pointed to by src shall be assigned either a null pointer (if conversion stopped due to reaching a terminating null character) or the address just past the last character converted (if any). If conversion stopped due to reaching a terminating null character, and if dst is not a null pointer, the resulting state described shall be the initial conversion state.

If ps is a null pointer, the mbsrtowcs() function shall use its own internal mbstate_t object, which is initialized at program start-up to the initial conversion state. Otherwise, the mbstate_t object pointed to by ps shall be used to completely describe the current conversion state of the associated character sequence.

[CX] [Option Start] The mbsnrtowcs() function shall be equivalent to the mbsrtowcs() function, except that the conversion of characters pointed to by src is limited to at most nmc bytes (the size of the input buffer). [Option End]

The behavior of these functions shall be affected by the LC_CTYPE category of the current locale.

The implementation shall behave as if no function defined in this volume of POSIX.1-2008 calls these functions.

[CX] [Option Start] The mbsnrtowcs() and mbsrtowcs() functions need not be thread-safe if called with a NULL ps argument. [Option End]

The mbsrtowcs() function shall not change the setting of errno if successful.


If the input conversion encounters a sequence of bytes that do not form a valid character, an encoding error occurs. In this case, these functions shall store the value of the macro [EILSEQ] in errno and shall return (size_t)-1; the conversion state is undefined. Otherwise, these functions shall return the number of characters successfully converted, not including the terminating null (if any).


These functions shall fail if:

An invalid character sequence is detected.

These functions may fail if:

[CX] [Option Start] ps points to an object that contains an invalid conversion state. [Option End]

The following sections are informative.










iconv, mbrtowc, mbsinit

XBD <wchar.h>


First released in Issue 5. Included for alignment with ISO/IEC 9899:1990/Amendment 1:1995 (E).

Issue 6

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

The [EINVAL] error condition is marked CX.

Issue 7

Austin Group Interpretation 1003.1-2001 #170 is applied.

The mbsnrtowcs() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 1.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0370 [109,105] is applied.

End of informative text.


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