Discussion on buffering of standard I/O library Post: 302501200

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Top Forums UNIX for Advanced & Expert Users Discussion on buffering of standard I/O library Post 302501200 by Edward114 on Thursday 3rd of March 2011 01:17:57 AM

03-03-2011

Registered User

I just found out the work of input buffer in glibc2.2.5.
At std input, like fgetc(), that finally call the __read() in fillbuf().

Code:

fillbuf (register FILE *fp)
{
......
  if (fp->__buffer == NULL)
    {
      /* We're unbuffered, so we want to read only one character.  */
      buffer = (char *) &c;
      to_read = 1;
    }
  else
    {
      /* We're buffered, so try to fill the buffer.  */
      buffer = fp->__buffer;
      to_read = fp->__bufsize;
    }
  while (!ferror (fp) && !feof (fp) && nread <= buffer_offset)
    {
      /* Try to fill the buffer.  */
      int count = (*fp->__io_funcs.__read) (fp->__cookie, buffer, to_read);//Edward. at this ,call __read,the buffer is  "buffer",buffer size is "to_read"    
...
    }
...
}

The following results:
fillbuf() check unbuffered, check fully buffered. but it doesn't chek line buffer.So
Unbuffered: read a character every time.
Fully buffered:__read() try to fill the buffer, except for encounter a EOF.
Line buffered:it doesn't affect the __read().If stream fp have the buffer,it behavior like Full buffer. If have no buffer, like Unbuffered.

For a stream, the terminal default use line buffer. And else default use fully buffered. If you want to use a nubuffered, you nee set the pf->__userbuf of FILE struct(use setvbuf()) before any other I/O operation is performed on the stream.

I have another question, When I view the source code for glibc2.2.5, many functions can be found in. But in the higher version, such as glibc2.9, many functions can't be found,Such as fgetc(),why? Is it in assembly?

Edward114

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libstdbuf

LIBSTDBUF(3)						   BSD Library Functions Manual 					      LIBSTDBUF(3)

NAME

     libstdbuf -- preloaded library to change standard streams initial buffering

DESCRIPTION

     The libstdbuf library is meant to be preloaded with the LD_PRELOAD environment variable to as to change the initial buffering of standard
     input, standard output and standard error streams.

     Although you may load and configure this library manually, an utility, stdbuf(1), can be used to run a command with the appropriate environ-
     ment variables.

ENVIRONMENT

     Each stream can be configured independently through the following environment variables (values are defined below):

	   _STDBUF_I
		 Initial buffering definition for the standard input stream

	   _STDBUF_O
		 Initial buffering definition for the standard output stream

	   _STDBUF_E
		 Initial buffering definition for the standard error stream

     Each variable may take one of the following values:

	   "0"	 unbuffered

	   "L"	 line buffered

	   "B"	 fully buffered with the default buffer size

	   size  fully buffered with a buffer of size bytes (suffixes 'k', 'M' and 'G' are accepted)

EXAMPLE

     In the following example, the stdout stream of the awk(1) command will be fully buffered by default because it does not refer to a terminal.
     libstdbuf is used to force it to be line-buffered so vmstat(8)'s output will not stall until the full buffer fills.

	   # vmstat 1 | LD_PRELOAD=/usr/lib/libstdbuf.so 
	       STDBUF_1=L awk '$2 > 1 || $3 > 1' | cat -n

     See also the manpage of stdbuf(1) for a simpler way to do this.

HISTORY

     The libstdbuf library first appeared in FreeBSD 8.4.

AUTHORS

     The original idea of the libstdbuf command comes from Padraig Brady who implemented it in the GNU coreutils.  Jeremie Le Hen implemented it
     on FreeBSD.

BSD
								  April 28, 2012							       BSD

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