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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 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
       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.  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.

RETURN VALUE
       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.

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 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)

       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)
 

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Gedruckt am: 16.12.2017 04:05 GMT+0100 (2017-12-16T04:05:38+01:00)