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



NAME
       fmemopen, open_memstream, open_wmemstream -  open memory as stream

SYNOPSIS
       #define _GNU_SOURCE
       #include <stdio.h>

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

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

       #define _GNU_SOURCE
       #include <wchar.h>

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

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 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 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 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  mal-
       loc(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 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 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  buf-
       fer.   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 fol-
       lowing:

           $ ./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 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



GNU                                                        2010-06-10                                                FMEMOPEN(3)