ptg10805159164 StandardI/O Library Chapter 5
5.12 Implementation Details
As we’ve mentioned, under the UNIX System, the standardI/O library ends up calling
the I/O routines that we described in Chapter 3. Each standardI/O stream has an
associated file descriptor,and we can obtain the descriptor for a stream by calling
fileno.
Note thatfilenois not part of the ISO C standard, but rather an extension supported by
POSIX.1.#include <stdio.h>
int fileno(FILE *fp);
Returns: the file descriptor associated with the stream
We need this function if we want to call theduporfcntlfunctions, for example.
To look at the implementation of the standardI/O library on your system, start
with the header<stdio.h>.This will show how theFILEobject is defined, the
definitions of the per-stream flags, and any standardI/O routines, such asgetc,that
aredefined as macros. Section 8.5 of Kernighan and Ritchie[ 1988 ] has a sample
implementation that shows the flavor of many implementations on UNIX systems.
Chapter 12 of Plauger[ 1992 ]provides the complete source code for an implementation
of the standardI/O library.The implementation of the GNU standardI/O library is
also publicly available.Example
The program in Figure5.11prints the buffering for the three standardstreams and for a
stream that is associated with a regular file.
#include "apue.h"
void pr_stdio(const char *, FILE *);
int is_unbuffered(FILE *);
int is_linebuffered(FILE *);
int buffer_size(FILE *);
int
main(void)
{
FILE *fp;fputs("enter any character\n", stdout);
if (getchar() == EOF)
err_sys("getchar error");
fputs("one line to standard error\n", stderr);
pr_stdio("stdin", stdin);
pr_stdio("stdout", stdout);
pr_stdio("stderr", stderr);