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Linux 2.6 - man page for open_wmemstream (linux section 3)

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

NAME
       fmemopen, open_memstream, open_wmemstream -  open memory as stream

SYNOPSIS
       #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:
	       _GNU_SOURCE

DESCRIPTION
       The  fmemopen()	function  opens  a stream that permits the access specified 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 letter 'b' may be specified as the second character in mode.  This  provides  "binary"
       mode:  writes don't implicitly add a terminating null byte, and fseek(3) SEEK_END is rela-
       tive 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 call-
       er  should ensure that an extra byte is available in the buffer (and that size counts that
       byte) to allow for this.

       Attempts to write more than size bytes to the buffer result in  an  error.   (By  default,
       such  errors  will  only be visible 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.
       Alternatively,  the  caller can explicitly set buf as the stdio stream buffer, at the same
       time informing stdio of the buffer's size, using setbuffer(fp, buf, size).)

       In a stream opened for reading, null bytes ('\0') in the buffer do not cause  read  opera-
       tions 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 automatically 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 locations pointed to  by
       ptr and sizeloc are updated to contain, respectively, a pointer to the buffer and the cur-
       rent 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 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.

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

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

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

NOTES
       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).

BUGS
       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() call fails, returning -1.

EXAMPLE
       The program below uses fmemopen() to open an input buffer, and open_memstream() to open	a
       dynamically  sized output buffer.  The program scans its input string (taken from the pro-
       gram'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:

	   $ ./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)

       int
       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]);
	    exit(EXIT_FAILURE);
	   }

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

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

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

	       s = fprintf(out, "%d ", v * v);
	       if (s == -1)
		   handle_error("fprintf");
	   }
	   fclose(in);
	   fclose(out);
	   printf("size=%ld; ptr=%s\n", (long) size, ptr);
	   free(ptr);
	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       fopen(3), fopencookie(3), feature_test_macros(7)

COLOPHON
       This page is part of release 3.27 of the Linux man-pages project.  A  description  of  the
       project,   and	information  about  reporting  bugs,  can  be  found  at  http://www.ker-
       nel.org/doc/man-pages/.

GNU					    2010-09-15				      FMEMOPEN(3)


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