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### 4.42 `open_memstream`, `open_wmemstream`—open a write stream around an arbitrary-length string

Synopsis

         #include <stdio.h>
         FILE *open_memstream(char **restrict buf,
             size_t *restrict size);

         #include <wchar.h>
         FILE *open_wmemstream(wchar_t **restrict buf,
             size_t *restrict size);

Description
`open_memstream` creates a seekable, byte-oriented `FILE` stream that wraps an arbitrary-length buffer, created as if by `malloc`. The current contents of *buf are ignored; this implementation uses *size as a hint of the maximum size expected, but does not fail if the hint was wrong. The parameters buf and size are later stored through following any call to `fflush` or `fclose`, set to the current address and usable size of the allocated string; although after fflush, the pointer is only valid until another stream operation that results in a write. Behavior is undefined if the user alters either *buf or *size prior to `fclose`.

`open_wmemstream` is like `open_memstream` just with the associated stream being wide-oriented. The size set in size in subsequent operations is the number of wide characters.

The stream is write-only, since the user can directly read *buf after a flush; see `fmemopen` for a way to wrap a string with a readable stream. The user is responsible for calling `free` on the final *buf after `fclose`.

Any time the stream is flushed, a NUL byte is written at the current position (but is not counted in the buffer length), so that the string is always NUL-terminated after at most *size bytes (or wide characters in case of `open_wmemstream`). However, data previously written beyond the current stream offset is not lost, and the NUL value written during a flush is restored to its previous value when seeking elsewhere in the string.

Returns
The return value is an open FILE pointer on success. On error, `NULL` is returned, and `errno` will be set to EINVAL if buf or size is NULL, ENOMEM if memory could not be allocated, or EMFILE if too many streams are already open.

Portability
POSIX.1-2008

Supporting OS subroutines required: `sbrk`.