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FMEMOPEN(3)		   Linux Programmer's Manual		   FMEMOPEN(3)

       fmemopen, open_memstream, open_wmemstream -  open memory	as stream

       #include	<stdio.h>

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

       FILE *open_memstream(char **ptr,	size_t *sizeloc);

       #include	<wchar.h>

       FILE *open_wmemstream(wchar_t **ptr, size_t *sizeloc);

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       fmemopen(), open_memstream(), open_wmemstream():
	   Since glibc 2.10:
	       _XOPEN_SOURCE >=	700 || _POSIX_C_SOURCE >= 200809L
	   Before glibc	2.10:

       The  fmemopen()	function opens a stream	that permits the access	speci-
       fied by mode.  The stream allows	I/O to be performed on the  string  or
       memory  buffer  pointed	to  by buf.  This buffer must be at least size
       bytes long.

       The argument mode is the	same as	for fopen(3).  If  mode	 specifies  an
       append  mode,  then the initial file position is	set to the location of
       the first null byte ('\0') in the buffer; otherwise  the	 initial  file
       position	 is set	to the start of	the buffer.  Since glibc 2.9, the let-
       ter 'b' may be specified	as the second character	in  mode.   This  pro-
       vides  "binary"	mode:  writes  don't implicitly	add a terminating null
       byte, and fseek(3) SEEK_END is relative to the end of the buffer	(i.e.,
       the  value  specified  by  the  size argument), rather than the current
       string length.

       When a stream that has been opened for writing is  flushed  (fflush(3))
       or  closed (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 size	counts that byte) to allow for

       Attempts	to write more than size	bytes to the buffer result in  an  er-
       ror.  (By default, such errors will be visible only when	the stdio buf-
       fer is flushed.	Disabling buffering with setbuf(fp, NULL) may be  use-
       ful  to	detect	errors	at  the	time of	an output operation.  Alterna-
       tively, the caller can explicitly set buf as the	stdio  stream  buffer,
       at  the	same  time  informing  stdio  of the buffer's size, using set-
       buffer(fp, buf, size).)

       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  ad-
       vances size bytes past the start	of the buffer.

       If  buf	is  specified as NULL, then fmemopen() dynamically allocates a
       buffer 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 open_memstream() below).

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

       When the	stream is closed (fclose(3)) or	flushed	(fflush(3)), the loca-
       tions pointed to	by ptr and sizeloc are	updated	 to  contain,  respec-
       tively,	a  pointer  to	the buffer and the current size	of the buffer.
       These values remain valid only as long as the caller performs  no  fur-
       ther  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  not
       included	in the size value stored at sizeloc.

       The  stream's  file position can	be changed with	fseek(3) or fseeko(3).
       Moving the file position	past the end of	the data already written fills
       the intervening space with zeros.

       The  open_wmemstream()  is similar to open_memstream(), but operates on
       wide characters instead of bytes.

       Upon successful completion fmemopen(), open_memstream() and  open_wmem-
       stream()	 return	a FILE pointer.	 Otherwise, NULL is returned and errno
       is set to indicate the error.

       fmemopen() and open_memstream() were already available in glibc	1.0.x.
       open_wmemstream() is available since glibc 2.4.

       POSIX.1-2008.   These  functions	are not	specified in POSIX.1-2001, and
       are not widely available	on other systems.

       POSIX.1-2008 specifies that 'b' in mode	shall  be  ignored.   However,
       Technical  Corrigendum  1 adjusts the standard to allow implementation-
       specific	treatment for this case, thus permitting the  glibc  treatment
       of 'b'.

       There is	no file	descriptor associated with the file stream returned by
       these functions (i.e., fileno(3)	will return an error if	called on  the
       returned	stream).

       In  glibc  before version 2.7, seeking past the end of a	stream created
       by open_memstream() does	not enlarge the	buffer;	instead	 the  fseek(3)
       call fails, returning -1.

       If  size	 is specified as zero, fmemopen() fails	with the error EINVAL.
       It would	be more	consistent if this case	successfully created a	stream
       that  then  returned end	of file	on the first attempt at	reading.  Fur-
       thermore, POSIX.1-2008 does not specify a failure for this case.

       Specifying append mode ("a" or "a+") for	fmemopen()  sets  the  initial
       file  position to the first null	byte, but (if the file offset is reset
       to a location other than	the end	of the stream) does not	 force	subse-
       quent writes to append at the end of the	stream.

       If  the mode argument to	fmemopen() specifies append ("a" or "a+"), and
       the size	argument does not cover	a null byte in buf, then, according to
       POSIX.1-2008,  the initial file position	should be set to the next byte
       after the end of	the buffer.  However, in this case  the	 glibc	fmemo-
       pen() sets the file position to -1.

       To  specify binary mode for fmemopen() the 'b' must be the second char-
       acter in	mode.  Thus, for example, "wb+"	has the	 desired  effect,  but
       "w+b"  does  not.   This	 is inconsistent with the treatment of mode by

       The glibc 2.9 addition of "binary" mode for fmemopen() silently changed
       the ABI:	previously, fmemopen() ignored 'b' in mode.

       The  program  below  uses  fmemopen()  to  open	an  input  buffer, and
       open_memstream()	to open	a dynamically sized output buffer.   The  pro-
       gram  scans  its	 input string (taken from the program's	first command-
       line argument) reading integers,	and writes the squares of these	 inte-
       gers  to	 the output buffer.  An	example	of the output produced by this
       program is the following:

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

   Program source

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

       #define handle_error(msg) \
	   do {	perror(msg); exit(EXIT_FAILURE); } while (0)

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

	   if (argc != 2) {
	       fprintf(stderr, "Usage: %s <file>\n", argv[0]);

	   in =	fmemopen(argv[1], strlen(argv[1]), "r");
	   if (in == NULL)

	   out = open_memstream(&ptr, &size);
	   if (out == NULL)

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

	       s = fprintf(out,	"%d ", v * v);
	       if (s ==	-1)
	   printf("size=%zu; ptr=%s\n",	size, ptr);

       fopen(3), fopencookie(3)

       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at

GNU				  2014-04-06			   FMEMOPEN(3)


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