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fmemopen - open a memory buffer stream


#include <stdio.h>

FILE *fmemopen(void *buf, size_t size, const char *mode);


The fmemopen() function associates the buffer given by the buf and size arguments with a stream. The buf argument must either be a null pointer or must point to a buffer that is at least size bytes long.

The argument mode points to a string beginning with one of the following sequences:

r or rb

Open the stream for reading.

w or wb

Open the stream for writing.

a or ab

Append; open the stream for writing at the first null byte.

r+ or rb+ or r+b

Open the stream for update (reading and writing).

w+ or wb+ or w+b

Open the stream for update (reading and writing). Truncate the buffer contents.

a+ or ab+ or a+b

Append; open the stream for update (reading and writing); the initial position is at the first null byte.

The character b has no effect.

If a null pointer is specified as the buf argument, fmemopen() allocates size bytes of memory as if by a call to malloc(). This buffer is automatically freed when the stream is closed.

The stream maintains a current position in the buffer. This position is initially set to either the beginning of the buffer (for r and w modes) or to the first null byte in the buffer (for a modes). If no null byte is found in append mode, the initial position is set to one byte after the end of the buffer.

If buf is a null pointer, the initial position is always set to the beginning of the buffer.

The stream also maintains the size of the current buffer contents. For modes r and r+ the size is set to the value given by the size argument. For modes w and w+ the initial size is zero and for modes a and a+ the initial size is either the position of the first null byte in the buffer or the value of the size argument if no null byte is found.

A read operation on the stream cannot advance the current buffer position beyond the current buffer size. Reaching the buffer size in a read operation counts as ’’end-of-file’’. Null bytes in the buffer have no special meaning for reads. The read operation starts at the current buffer position of the stream.

A write operation starts either at the current position of the stream (if mode has not specified ’a’ as the first character) or at the current size of the stream (if mode had ’a’ as the first character). If the current position at the end of the write is larger than the current buffer size, the current buffer size is set to the current position. A write operation on the stream cannot advance the current buffer size beyond the size given in the size argument.

When a stream open for writing is flushed or closed, a null byte is written at the current position or at the end of the buffer, depending on the size of the contents. If a stream open for update is flushed or closed and the last write has advanced the current buffer size, a null byte is written at the end of the buffer if it fits.

An attempt to seek a memory buffer stream to a negative position or to a position larger than the buffer size given if the size argument fails.


Upon successful completion, fmemopen() returns a pointer to the object controlling the stream. Otherwise, a null pointer is returned and errno is set to indicate the error.


The fmemopen() function will fail if:


The size argument specifies a buffer size of zero.


The value of the mode argument is not valid.


Insufficient storage space is available.


The mode of a stream opened by fmemopen() can be changed by a call to freopen(NULL, mode, stream).


See attributes(7) for descriptions of the following attributes:


fopen(3C), freopen(3C), malloc(3C), open_memstream(3C), attributes(7), standards(7)

solaris/fmemopen.3c.txt · Last modified: 2023/07/19 08:58 by A User Not Logged in