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select/poll and Signal Safety

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Old 01-27-2006
select/poll and Signal Safety
Hi

I am struggling to understand why one should use pselect()/ppoll() instead of wrapping an ordinary select() or poll() call around sigprocmask(). The linux man page talks about “race conditions”, but how would such dangers occur?

I plan to use poll() for an application (since ppoll() isn't widely supported) but wish to know how (if possible) it can be made signal safe? please see preliminary code scenario below.

Code:
void foofunc() {
	int retval;

	sigprocmask(...); // block certain signals

	...

	while(1) {
		sigprocmask(...); // set empty mask

		// <- can something go wrong here?

		retval = poll(...); // wait indefinitely

		// <- and what about here?

		sigprocmask(...); // restore old mask

		if (retval != -1) {
			... // handle file descriptor events
		} else (errno == EINTR) {
			// just signal interruptions, don't panic
		} else {
			... // handle error
		}
	}
}

Any help is greatly appreciated.
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LEARN ABOUT DEBIAN

pcap_get_selectable_fd

PCAP_GET_SELECTABLE_FD(3PCAP)											     PCAP_GET_SELECTABLE_FD(3PCAP)

NAME

       pcap_get_selectable_fd - get a file descriptor on which a select() can be done for a live capture

SYNOPSIS

       #include <pcap/pcap.h>

       int pcap_get_selectable_fd(pcap_t *p);

DESCRIPTION

       pcap_get_selectable_fd()  returns, on UNIX, a file descriptor number for a file descriptor on which one can do a select() or poll() to wait
       for it to be possible to read packets without blocking, if such a descriptor exists, or -1, if no such  descriptor  exists.   Some  network
       devices	opened	with  pcap_create() and pcap_activate(), or with pcap_open_live(), do not support select() or poll() (for example, regular
       network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so -1 is returned for those devices.

       Note that in:

	      FreeBSD prior to FreeBSD 4.6;

	      NetBSD prior to NetBSD 3.0;

	      OpenBSD prior to OpenBSD 2.4;

	      Mac OS X prior to Mac OS X 10.7;

       select() and poll() do not work correctly on BPF devices; pcap_get_selectable_fd() will return a file descriptor on most of those  versions
       (the  exceptions being FreeBSD 4.3 and 4.4), but a simple select() or poll() will not indicate that the descriptor is readable until a full
       buffer's worth of packets is received, even if the read timeout expires before then.   To  work	around	this,  an  application	that  uses
       select()  or  poll()  to  wait for packets to arrive must put the pcap_t in non-blocking mode, and must arrange that the select() or poll()
       have a timeout less than or equal to the read timeout, and must try to read packets after  that	timeout  expires,  regardless  of  whether
       select() or poll() indicated that the file descriptor for the pcap_t is ready to be read or not.  (That workaround will not work in FreeBSD
       4.3 and later; however, in FreeBSD 4.6 and later, select() and poll() work correctly on BPF devices, so	the  workaround  isn't	necessary,
       although it does no harm.)

       Note  also that poll() doesn't work on character special files, including BPF devices, in Mac OS X 10.4 and 10.5, so, while select() can be
       used on the descriptor returned by pcap_get_selectable_fd(), poll() cannot be used on it those versions of Mac OS X.   Kqueues  also  don't
       work  on  that descriptor.  poll(), but not kqueues, work on that descriptor in Mac OS X releases prior to 10.4; poll() and kqueues work on
       that descriptor in Mac OS X 10.6 and later.

       pcap_get_selectable_fd() is not available on Windows.

RETURN VALUE

       A selectable file descriptor is returned if one exists; otherwise, -1 is returned.

SEE ALSO

       pcap(3PCAP), select(2), poll(2)

								   5 April 2008 				     PCAP_GET_SELECTABLE_FD(3PCAP)