man-pages/man3/fmemopen.3

.\" Copyright 2005 walter harms (walter.harms@informatik.uni-oldenburg.de),
.\" and Copyright 2005 Michael Kerrisk <mtk-manpages@gmx.net>
.\" Distributed under the GPL.
.\"
.TH FMEMOPEN 3 2005-12-08 "GNU" "glibc function"
.SH NAME
fmemopen, open_memstream \-  open memory as stream
.SH SYNOPSIS
.B #define _GNU_SOURCE
.br
.B #include <stdio.h>
.sp
.BI "FILE *fmemopen(void *"buf ", size_t "size ","
.BI "const char *" mode ");"
.sp
.BI "FILE *open_memstream(char **" ptr ", size_t *" sizeloc )
.SH DESCRIPTION
The
.BR fmemopen ()
function opens a stream that permits the access specified by
.IR mode .
The stream allows I/O to be performed on the string or memory buffer 
pointed to by
.IR buf .  
This buffer must be at least
.I size
bytes long.
.PP
The argument
.I mode
is the same as for
.BR fopen (3) .
If
.I mode
specifies an append mode, then the initial file position is set to 
location of the first null byte ('\\0') in the buffer;
otherwise the initial file position is set to the start of the buffer.
.PP
When a stream that has been opened for writing is flushed  
.RB ( fflush (3))
or closed
.RB ( fclose (3)),
a null byte is written at the end of the buffer if there is space.
The caller should ensure that an extra byte is available in the 
buffer
(and that
.IR size
counts that byte)
to allow for this.  

Attempts to write more than
.I size
bytes to the buffer result in an error.
(By default, such errors will only be visible when the 
.I stdio
buffer is flushed.
Disabling buffering with 
.I setbuf(fp,\ NULL)
may be useful to detect errors at the time of an output operation.
Alternatively, the caller can explicitly set 
.I buf
as the stdio stream buffer, at the same time informing stdio
of the buffer's size, using 
.IR "setbuffer(fp, buf, size)" .)
.\" See http://sourceware.org/bugzilla/show_bug.cgi?id=1995
.\" and
.\" http://sources.redhat.com/ml/libc-alpha/2006-04/msg00064.html
.PP
In a stream opened for reading, 
null bytes ('\\0') in the buffer do not cause read 
operations to return an end-of-file indication.
A read from the buffer will only indicate end-of-file
when the file pointer advances 
.I size
bytes past the start of the buffer.
.PP
If 
.I buf 
is specified as NULL, then
.BR  fmemopen ()
dynamically allocates a buffer
.I size
bytes long. 
This is useful for an application that wants to write data to
a temporary buffer and then read it back again.
The buffer is automatically freed when the stream is closed.
Note that the caller has no way to obtain a pointer to the
temporary buffer allocated by this call (but see
.BR open_memstream ()
below).

The
.BR open_memstream ()
opens a stream for writing to a buffer.  
The buffer
is dynamically allocated (as with 
.BR malloc (3)),
and automatically grows as required.
After closing the stream, the caller should
.BR free (3)
this buffer.

When the stream is closed 
.RB ( fclose (3))
or flushed 
.RB ( fflush (3)),
the locations pointed to by
.I ptr 
and 
.I sizeloc
are updated to contain, respectively, a pointer to the buffer and the
current size of the buffer.  
These values remain valid only as long as the caller 
performs no further output on the stream.  
If further output is performed, then the stream 
must again be flushed before trying to access these variables.

A null byte is maintained at the end of the buffer.  
This byte is 
.I not 
included in the size value stored at 
.IR sizeloc .
.\" 
.\" FIXME The glibc info doc has text like the following, but it appears
.\" not to be true: http://sourceware.org/bugzilla/show_bug.cgi?id=1996
.\" The stream's file position can be changed with
.\" .BR fseek (3) 
.\" or 
.\" .BR fseeko (3).
.\" Moving the file position past the end
.\" of the data already written fills the intervening space with
.\" zeroes.
.SH "RETURN VALUE"
Upon successful completion  
.BR fmemopen () 
and
.BR open_memstream ()
return a
.B FILE 
pointer.  
Otherwise, NULL is  returned  and  the  global  variable
.I errno 
is set to indicate the error.
.SH "EXAMPLE"
The program below uses 
.BR fmemopen ()
to open an input buffer, and
.BR open_memstream ()
to open a dynamically sized output buffer.
The program scans its input string (taken from the program's
first command-line argument) reading integers,
and writes the squares of these integers to the output buffer.
An example of the output produced by this program is the following:
.nf

$ ./a.out "1 23 43"
size=11; ptr=1 529 1849

#define _GNU_SOURCE
#include <assert.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

int 
main(int argc, char *argv[])
{
    FILE *out, *in;
    int v, s;
    size_t size;
    char *ptr;

    assert(argc == 2);

    in = fmemopen(argv[1], strlen(argv[1]), "r");
    if (in == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}

    out = open_memstream(&ptr, &size);
    if (out == NULL) { perror("fmemopen"); exit(EXIT_FAILURE);}

    for (;;) {
        s = fscanf(in, "%d", &v);
        if (s <= 0)
            break;

        s = fprintf(out, "%d ", v * v);
        if (s == -1) { perror("fprintf"); exit(EXIT_FAILURE); }
    }
    fclose(in);
    fclose(out);
    printf("size=%ld; ptr=%s\\n", (long) size, ptr);
    free(ptr);
    exit(EXIT_SUCCESS);
}
.fi
.SH "CONFORMING TO"
These functions are GNU extensions.
.\" Jan 06: But they appear to be going up for standardization by
.\" POSIX/PASC/IEEE.
.SH "SEE ALSO"
.BR open (3),
.BR feature_test_macros (7)