(libc.info.gz) String Streams

Info Catalog

(libc.info.gz) Other Kinds of Streams

(libc.info.gz) Custom Streams

 
 12.21.1 String Streams
 ----------------------
 
 The 'fmemopen' and 'open_memstream' functions allow you to do I/O to a
 string or memory buffer.  These facilities are declared in 'stdio.h'.
 
  -- Function: FILE * fmemopen (void *BUF, size_t SIZE, const char
           *OPENTYPE)
      Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem lock |
       POSIX Safety Concepts.
 
      This function opens a stream that allows the access specified by
      the OPENTYPE argument, that reads from or writes to the buffer
      specified by the argument BUF.  This array must be at least SIZE
      bytes long.
 
      If you specify a null pointer as the BUF argument, 'fmemopen'
      dynamically allocates an array SIZE bytes long (as with 'malloc';
       Unconstrained Allocation).  This is really only useful if
      you are going to write things to the buffer and then read them back
      in again, because you have no way of actually getting a pointer to
      the buffer (for this, try 'open_memstream', below).  The buffer is
      freed when the stream is closed.
 
      The argument OPENTYPE is the same as in 'fopen' ( Opening
      Streams).  If the OPENTYPE specifies append mode, then the
      initial file position is set to the first null character in the
      buffer.  Otherwise the initial file position is at the beginning of
      the buffer.
 
      When a stream open for writing is flushed or closed, a null
      character (zero byte) is written at the end of the buffer if it
      fits.  You should add an extra byte to the SIZE argument to account
      for this.  Attempts to write more than SIZE bytes to the buffer
      result in an error.
 
      For a stream open for reading, null characters (zero bytes) in the
      buffer do not count as "end of file".  Read operations indicate end
      of file only when the file position advances past SIZE bytes.  So,
      if you want to read characters from a null-terminated string, you
      should supply the length of the string as the SIZE argument.
 
    Here is an example of using 'fmemopen' to create a stream for reading
 from a string:
 
 
      #include <stdio.h>
 
      static char buffer[] = "foobar";
 
      int
      main (void)
      {
        int ch;
        FILE *stream;
 
        stream = fmemopen (buffer, strlen (buffer), "r");
        while ((ch = fgetc (stream)) != EOF)
          printf ("Got %c\n", ch);
        fclose (stream);
 
        return 0;
      }
 
    This program produces the following output:
 
      Got f
      Got o
      Got o
      Got b
      Got a
      Got r
 
  -- Function: FILE * open_memstream (char **PTR, size_t *SIZELOC)
      Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem | 
      POSIX Safety Concepts.
 
      This function opens a stream for writing to a buffer.  The buffer
      is allocated dynamically and grown as necessary, using 'malloc'.
      After you've closed the stream, this buffer is your responsibility
      to clean up using 'free' or 'realloc'.   Unconstrained
      Allocation.
 
      When the stream is closed with 'fclose' or flushed with 'fflush',
      the locations PTR and SIZELOC are updated to contain the pointer to
      the buffer and its size.  The values thus stored remain valid only
      as long as no further output on the stream takes place.  If you do
      more output, you must flush the stream again to store new values
      before you use them again.
 
      A null character is written at the end of the buffer.  This null
      character is _not_ included in the size value stored at SIZELOC.
 
      You can move the stream's file position with 'fseek' or 'fseeko'
      ( File Positioning).  Moving the file position past the end
      of the data already written fills the intervening space with
      zeroes.
 
    Here is an example of using 'open_memstream':
 
 
      #include <stdio.h>
 
      int
      main (void)
      {
        char *bp;
        size_t size;
        FILE *stream;
 
        stream = open_memstream (&bp, &size);
        fprintf (stream, "hello");
        fflush (stream);
        printf ("buf = `%s', size = %d\n", bp, size);
        fprintf (stream, ", world");
        fclose (stream);
        printf ("buf = `%s', size = %d\n", bp, size);
 
        return 0;
      }
 
    This program produces the following output:
 
      buf = `hello', size = 5
      buf = `hello, world', size = 12
 

Info Catalog

(libc.info.gz) Other Kinds of Streams

(libc.info.gz) Custom Streams