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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
       error.	(By  default,  such errors will be visible only when the stdio
       buffer is flushed.  Disabling buffering with  setbuf(fp, NULL)  may  be
       useful  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
       advances 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 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

GNU				  2014-04-06			   FMEMOPEN(3)

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