---
name: "open_memstream"
section: "3"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/open_memstream"
---

# open\_memstream(3)

dynamic memory buffer stream open functions

## NAME

**open\_memstream**, **open\_wmemstream** – dynamic memory buffer stream open functions

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<stdio.h\>**

*FILE* *\**\

**open\_memstream**(*char \*\*bufp*, *size\_t \*sizep*);

**#include \<wchar.h\>**

*FILE* *\**\

**open\_wmemstream**(*wchar\_t \*\*bufp*, *size\_t \*sizep*);

## DESCRIPTION

The **open\_memstream**() and **open\_wmemstream**() functions create a write-only, seekable stream backed by a dynamically allocated memory buffer. The **open\_memstream**() function creates a byte-oriented stream, while the **open\_wmemstream**() function creates a wide-oriented stream.

Each stream maintains a current position and size. Initially, the position and size are set to zero. Each write begins at the current position and advances it the number of successfully written bytes for **open\_memstream**() or wide characters for **open\_wmemstream**(). If a write moves the current position beyond the length of the buffer, the length of the buffer is extended and a null character is appended to the buffer.

A stream's buffer always contains a null character at the end of the buffer that is not included in the current length.

If a stream's current position is moved beyond the current length via a seek operation and a write is performed, the characters between the current length and the current position are filled with null characters before the write is performed.

After a successful call to [fclose(3)](/fclose/3.md) or [fflush(3)](/fflush/3.md), the pointer referenced by *bufp* will contain the start of the memory buffer and the variable referenced by *sizep* will contain the smaller of the current position and the current buffer length.

After a successful call to [fflush(3)](/fflush/3.md), the pointer referenced by *bufp* and the variable referenced by *sizep* are only valid until the next write operation or a call to [fclose(3)](/fclose/3.md).

Once a stream is closed, the allocated buffer referenced by *bufp* should be released via a call to [free(3)](/free/3.md) when it is no longer needed.

## IMPLEMENTATION NOTES

Internally all I/O streams are effectively byte-oriented, so using wide-oriented operations to write to a stream opened via **open\_wmemstream**() results in wide characters being expanded to a stream of multibyte characters in stdio's internal buffers. These multibyte characters are then converted back to wide characters when written into the stream. As a result, the wide-oriented streams maintain an internal multibyte character conversion state that is cleared on any seek opertion that changes the current position. This should have no effect as long as wide-oriented output operations are used on a wide-oriented stream.

## RETURN VALUES

Upon successful completion, **open\_memstream**() and **open\_wmemstream**() return a FILE pointer. Otherwise, NULL is returned and the global variable *errno* is set to indicate the error.

## ERRORS

- \[EINVAL\]

  The *bufp* or *sizep* argument was NULL.

- \[ENOMEM\]

  Memory for the stream or buffer could not be allocated.

## SEE ALSO

[fclose(3)](/fclose/3.md), [fflush(3)](/fflush/3.md), [fopen(3)](/fopen/3.md), [free(3)](/free/3.md), [fseek(3)](/fseek/3.md), [stdio(3)](/stdio/3.md), [sbuf(9)](/sbuf/9.md)

## STANDARDS

The **open\_memstream**() and **open\_wmemstream**() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).