Advanced Programming in the UNIX® Environment

ptg10805159

148 StandardI/O Library Chapter 5

5.5 OpeningaStream

Thefopen,freopen,andfdopenfunctions open a standardI/O stream.

#include <stdio.h>

FILE *fopen(const char *restrictpathname,const char *restricttype);

FILE *freopen(const char *restrictpathname,const char *restricttype,

FILE *restrictfp);

FILE *fdopen(intfd,const char *type);

All three return: file pointer if OK,NULLon error

The differences in these three functions are as follows:

1. Thefopenfunction opens a specified file.
   2Thefreopenfunction opens a specified file on a specified stream, closing the
   stream first if it is already open. If the stream previously had an orientation,
   freopenclears it. This function is typically used to open a specified file as one
   of the predefined streams: standardinput, standardoutput, or standarderror.


2. Thefdopenfunction takes an existing file descriptor,which we could obtain
   from theopen,dup,dup2,fcntl,pipe,socket,socketpair,oraccept
   functions, and associates a standardI/O stream with the descriptor.This
   function is often used with descriptors that arereturned by the functions that
   create pipes and network communication channels. Because these special types
   of files cannot be opened with the standardI/Ofopenfunction, we have to call
   the device-specific function to obtain a file descriptor,and then associate this
   descriptor with a standardI/O stream usingfdopen.

Bothfopenandfreopenarepart of ISO C;fdopenis part of POSIX.1, since ISO C doesn’t

deal with file descriptors.

type Description open( 2 )Flags

rorrb open for reading O_RDONLY

worwb truncate to 0 length or create for writing O_WRONLY|O_CREAT|O_TRUNC

aorab append; open for writing at end of file, or O_WRONLY|O_CREAT|O_APPEND

create for writing

r+orr+borrb+ open for reading and writing O_RDWR

w+orw+borwb+ truncate to 0 length or create for reading and O_RDWR|O_CREAT|O_TRUNC

writing

a+ora+borab+ open or create for reading and writing at O_RDWR|O_CREAT|O_APPEND

end of file

Figure 5.2Thetypeargument for opening a standardI/O stream

ISO C specifies 15 values for thetypeargument, shown in Figure5.3. Using the

characterbas part of thetypeallows the standardI/O system to differentiate between a