Advanced Programming in the UNIX® Environment

ptg10805159

Section 15.5 FIFOs 553

#include <sys/stat.h>

int mkfifo(const char *path,mode_tmode);

int mkfifoat(intfd,const char *path,mode_t mode);

Both return: 0 if OK,−1 on error

The specification of the mode argument is the same as for the open function

(Section 3.3). The rules for the user and group ownership of the new FIFO arethe same

as we described in Section 4.6.

Themkfifoatfunction is similar to themkfifofunction, except that it can be used

to create a FIFO in a location relative to the directory represented by thefd file

descriptor argument. Like the other*atfunctions, thereare three cases:

1. If thepathparameter specifies an absolute pathname, then thefdparameter is
   ignored and themkfifoatfunction behaves like themkfifofunction.


2. If thepathparameter specifies a relative pathname and thefdparameter is a
   valid file descriptor for an open directory,the pathname is evaluated relative to
   this directory.


3. If thepathparameter specifies a relative pathname and thefdparameter has the
   special valueAT_FDCWD,the pathname is evaluated starting in the current
   working directory,andmkfifoatbehaves likemkfifo.
   Once we have usedmkfifoormkfifoatto create a FIFO, we open it usingopen.
   Indeed, the normal file I/O functions (e.g.,close,read,write,unlink)all work
   with FIFOs.
   Applications can create FIFOs with themknodandmknodatfunctions. Because POSIX.1
   originally didn’t includemknod,themkfifofunction was invented specifically for POSIX.1.
   Themknodandmknodatfunctions areincluded in the XSI option in POSIX.1.
   POSIX.1 also includes support for themkfifo( 1 )command. All four platforms discussed in
   this text provide this command. As a result, we can create a FIFO using a shell command and
   then access it with the normal shell I/O redirection.

When weopenaFIFO, the nonblocking flag (O_NONBLOCK)affects what happens.

•Inthe normal case (withoutO_NONBLOCK), anopenfor read-only blocks until

some other process opens the FIFO for writing. Similarly,anopenfor write-

only blocks until some other process opens the FIFO for reading.

•IfO_NONBLOCKis specified, anopenfor read-only returns immediately.But an

openfor write-only returns−1witherrnoset toENXIOif no process has the

FIFO open for reading.

As with a pipe, if wewriteto a FIFO that no process has open for reading, the signal

SIGPIPEis generated. When the last writer for a FIFO closes the FIFO, an end of file is

generated for the reader of the FIFO.

It is common to have multiple writers for a given FIFO. This means that we have to

worry about atomic writes if we don’t want the writes from multiple processes to be