open_memstream (3p) - Linux Manuals

open_memstream: open a dynamic memory buffer stream


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


open_memstream, open_wmemstream --- open a dynamic memory buffer stream


#include <stdio.h>

FILE *open_memstream(char **bufp, size_t *sizep);

#include <wchar.h>

FILE *open_wmemstream(wchar_t **bufp, size_t *sizep);


The open_memstream() and open_wmemstream() functions shall create an I/O stream associated with a dynamically allocated memory buffer. The stream shall be opened for writing and shall be seekable.

The stream associated with a call to open_memstream() shall be byte-oriented.

The stream associated with a call to open_wmemstream() shall be wide-oriented.

The stream shall maintain a current position in the allocated buffer and a current buffer length. The position shall be initially set to zero (the start of the buffer). Each write to the stream shall start at the current position and move this position by the number of successfully written bytes for open_memstream() or the number of successfully written wide characters for open_wmemstream(). The length shall be initially set to zero. If a write moves the position to a value larger than the current length, the current length shall be set to this position. In this case a null character for open_memstream() or a null wide character for open_wmemstream() shall be appended to the current buffer. For both functions the terminating null is not included in the calculation of the buffer length.

After a successful fflush() or fclose(), the pointer referenced by bufp shall contain the address of the buffer, and the variable pointed to by sizep shall contain the smaller of the current buffer length and the number of bytes for open_memstream(), or the number of wide characters for open_wmemstream(), between the beginning of the buffer and the current file position indicator.

After a successful fflush() the pointer referenced by bufp and the variable referenced by sizep remain valid only until the next write operation on the stream or a call to fclose().


Upon successful completion, these functions shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, and errno shall be set to indicate the error.


These functions may fail if:
bufp or sizep are NULL.
{FOPEN_MAX} streams are currently open in the calling process.
Memory for the stream or the buffer could not be allocated.

The following sections are informative.


#include <stdio.h>
#include <stdlib.h>

main (void)
    FILE *stream;
    char *buf;
    size_t len;
    off_t eob;

    stream = open_memstream (&buf, &len);
    if (stream == NULL)
        /* handle error */ ;
    fprintf (stream, "hello my world");
    fflush (stream);
    printf ("buf=%s, len=%zu\n", buf, len);
    eob = ftello(stream);
    fseeko (stream, 0, SEEK_SET);
    fprintf (stream, "good-bye");
    fseeko (stream, eob, SEEK_SET);
    fclose (stream);
    printf ("buf=%s, len=%zu\n", buf, len);
    free (buf);
    return 0;

This program produces the following output:

buf=hello my world, len=14
buf=good-bye world, len=14


The buffer created by these functions should be freed by the application after closing the stream, by means of a call to free().


These functions are similar to fmemopen() except that the memory is always allocated dynamically by the function, and the stream is opened only for output.




Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see .


fclose(), fdopen(), fflush(), fmemopen(), fopen(), free(), freopen()

The Base Definitions volume of POSIX.1-2008, <stdio.h>, <wchar.h>