The Linux Programming Interface

1370 Chapter 63

Listing 63-8: Using pselect()

sigset_t emptyset, blockset;

struct sigaction sa;

sigemptyset(&blockset);

sigaddset(&blockset, SIGUSR1);

if (sigprocmask(SIG_BLOCK, &blockset, NULL) == -1)

errExit("sigprocmask");

sa.sa_sigaction = handler;

sigemptyset(&sa.sa_mask);

sa.sa_flags = SA_RESTART;

if (sigaction(SIGUSR1, &sa, NULL) == -1)

errExit("sigaction");

sigemptyset(&emptyset);

ready = pselect(nfds, &readfds, NULL, NULL, NULL, &emptyset);

if (ready == -1)

errExit("pselect");

The ppoll() and epoll_pwait() system calls

Linux 2.6.16 also added a new, nonstandard system call, ppoll(), whose relationship

to poll() is analogous to the relationship of pselect() to select(). Similarly, starting with

kernel 2.6.19, Linux also includes epoll_pwait(), providing an analogous extension

to epoll_wait(). See the ppoll(2) and epoll_pwait(2) manual pages for details.

63.5.2 The Self-Pipe Trick

Since pselect() is not widely implemented, portable applications must employ other

strategies to avoid race conditions when simultaneously waiting for signals and call-

ing select() on a set of file descriptors. One common solution is the following:

1. Create a pipe, and mark its read and write ends as nonblocking.


2. As well as monitoring all of the other file descriptors that are of interest,
   include the read end of the pipe in the readfds set given to select().


3. Install a handler for the signal that is of interest. When this signal handler is
   called, it writes a byte of data to the pipe. Note the following points about the
   signal handler:
   
   
   • The write end of the pipe was marked as nonblocking in the first step to
     prevent the possibility that signals arrive so rapidly that repeated invocations
     of the signal handler fill the pipe, with the result that the signal handler’s
     write() (and thus the process itself) is blocked. (It doesn’t matter if a write
     to a full pipe fails, since the previous writes will already have indicated the
     delivery of the signal.)