SELECT_TUT(2) Linux Programmer's Manual SELECT_TUT(2)
NAME
select(2,7,2 select_tut), pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
multiplexing
SYNOPSIS
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(2,7,2 select_tut)(int nfds, fd_set *readfds, fd_set *writefds, fd_set
*exceptfds, struct timeval *utimeout);
int pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set
*exceptfds, const struct timespec *ntimeout, sigset_t *sigmask);
FD_CLR(int fd, fd_set *set(7,n,1 builtins));
FD_ISSET(int fd, fd_set *set(7,n,1 builtins));
FD_SET(int fd, fd_set *set(7,n,1 builtins));
FD_ZERO(fd_set *set(7,n,1 builtins));
DESCRIPTION
select(2,7,2 select_tut) (or pselect) is the pivot function of most C programs that han-
dle more than one simultaneous file(1,n) descriptor (or socket(2,7,n) handle) in(1,8) an
efficient manner. Its principal arguments are three arrays of file(1,n)
descriptors: readfds, writefds, and exceptfds. The way that select(2,7,2 select_tut) is
usually used is to block while waiting for a "change of status" on one
or more of the file(1,n) descriptors. A "change of status" is when more
characters become available from the file(1,n) descriptor, or when space
becomes available within the kernel's internal buffers for more to be
written to the file(1,n) descriptor, or when a file(1,n) descriptor goes into
error(8,n) (in(1,8) the case of a socket(2,7,n) or pipe(2,8) this is when the other end of
the connection is closed).
In summary, select(2,7,2 select_tut) just watches multiple file(1,n) descriptors, and is the
standard Unix call to do so.
The arrays of file(1,n) descriptors are called file(1,n) descriptor sets. Each
set(7,n,1 builtins) is declared as type fd_set, and its contents can be altered with
the macros FD_CLR, FD_ISSET, FD_SET, and FD_ZERO. FD_ZERO is usually
the first function to be used on a newly declared set. Thereafter, the
individual file(1,n) descriptors that you are interested in(1,8) can be added one
by one with FD_SET. select(2,7,2 select_tut) modifies the contents of the sets according
to the rules described below; after calling select(2,7,2 select_tut) you can test if(3,n) your
file(1,n) descriptor is still present in(1,8) the set(7,n,1 builtins) with the FD_ISSET macro.
FD_ISSET returns non-zero if(3,n) the descriptor is present and zero if(3,n) it
is not. FD_CLR removes a file(1,n) descriptor from the set(7,n,1 builtins) although I can't
see the use for it in(1,8) a clean program.
ARGUMENTS
readfds
This set(7,n,1 builtins) is watched to see if(3,n) data is available for reading from
any of its file(1,n) descriptors. After select(2,7,2 select_tut) has returned, readfds
will be cleared of all file(1,n) descriptors except for those file(1,n)
descriptors that are immediately available for reading with a
recv() (for sockets) or read(2,n,1 builtins)() (for pipes, files, and sockets)
call.
writefds
This set(7,n,1 builtins) is watched to see if(3,n) there is space to write(1,2) data to
any of its file(1,n) descriptor. After select(2,7,2 select_tut) has returned, writefds
will be cleared of all file(1,n) descriptors except for those file(1,n)
descriptors that are immediately available for writing with a
send(2,n)() (for sockets) or write(1,2)() (for pipes, files, and sockets)
call.
exceptfds
This set(7,n,1 builtins) is watched for exceptions or errors on any of the file(1,n)
descriptors. However, that is actually just a rumor. How you use
exceptfds is to watch for out-of-band (OOB) data. OOB data is
data sent on a socket(2,7,n) using the MSG_OOB flag, and hence
exceptfds only really applies to sockets. See recv(2) and
send(2,n)(2) about this. After select(2,7,2 select_tut) has returned, exceptfds will be
cleared of all file(1,n) descriptors except for those descriptors
that are available for reading OOB data. You can only ever read(2,n,1 builtins)
one byte of OOB data though (which is done with recv()), and
writing OOB data (done with send(2,n)) can be done at any time(1,2,n) and
will not block. Hence there is no need for a fourth set(7,n,1 builtins) to check
if(3,n) a socket(2,7,n) is available for writing OOB data.
nfds This is an integer one more than the maximum of any file(1,n)
descriptor in(1,8) any of the sets. In other words, while you are
busy adding file(1,n) descriptors to your sets, you must calculate
the maximum integer value of all of them, then increment this
value by one, and then pass this as nfds to select(2,7,2 select_tut).
utimeout
This is the longest time(1,2,n) select(2,7,2 select_tut) must wait before returning, even
if(3,n) nothing interesting happened. If this value is passed as
NULL, then select(2,7,2 select_tut) blocks indefinitely waiting for an event.
utimeout can be set(7,n,1 builtins) to zero seconds, which causes select(2,7,2 select_tut) to
return immediately. The structure struct timeval is defined as,
struct timeval {
time_t tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
ntimeout
This argument has the same meaning as utimeout but struct time-
spec has nanosecond precision as follows,
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
sigmask
This argument holds a set(7,n,1 builtins) of signals to allow while performing a
pselect call (see sigaddset(3) and sigprocmask(2)). It can be
passed as NULL, in(1,8) which case it does not modify the set(7,n,1 builtins) of
allowed signals on entry and exit(3,n,1 builtins) to the function. It will then
behave just like select(2,7,2 select_tut).
COMBINING SIGNAL AND DATA EVENTS
pselect must be used if(3,n) you are waiting for a signal(2,7) as well as data
from a file(1,n) descriptor. Programs that receive signals as events nor-
mally use the signal(2,7) handler only to raise(3,n) a global flag. The global
flag will indicate that the event must be processed in(1,8) the main loop of
the program. A signal(2,7) will cause the select(2,7,2 select_tut) (or pselect) call to return
with errno set(7,n,1 builtins) to EINTR. This behavior is essential so that signals can
be processed in(1,8) the main loop of the program, otherwise select(2,7,2 select_tut) would
block indefinitely. Now, somewhere in(1,8) the main loop will be a condi-
tional to check the global flag. So we must ask: what if(3,n) a signal(2,7)
arrives after the conditional, but before the select(2,7,2 select_tut) call? The answer
is that select(2,7,2 select_tut) would block indefinitely, even though an event is actu-
ally pending. This race condition is solved by the pselect call. This
call can be used to mask out signals that are not to be received except
within the pselect call. For instance, let us say that the event in(1,8)
question was the exit(3,n,1 builtins) of a child process. Before the start of the main
loop, we would block SIGCHLD using sigprocmask. Our pselect call would
enable SIGCHLD by using the virgin signal(2,7) mask. Our program would look(1,8,3 Search::Dict)
like:
int child_events = 0;
void child_sig_handler (int x) {
child_events++;
signal(2,7) (SIGCHLD, child_sig_handler);
}
int main (int argc, char **argv) {
sigset_t sigmask, orig_sigmask;
sigemptyset (&sigmask);
sigaddset (&sigmask, SIGCHLD);
sigprocmask (SIG_BLOCK, &sigmask,
&orig_sigmask);
signal(2,7) (SIGCHLD, child_sig_handler);
for (;;) { /* main loop */
for (; child_events > 0; child_events--) {
/* do event work here */
}
r = pselect (nfds, &rd, &wr, &er, 0, &orig_sigmask);
/* main body of program */
}
}
Note that the above pselect call can be replaced with:
sigprocmask (SIG_BLOCK, &orig_sigmask, 0);
r = select(2,7,2 select_tut) (nfds, &rd, &wr, &er, 0);
sigprocmask (SIG_BLOCK, &sigmask, 0);
but then there is still the possibility that a signal(2,7) could arrive
after the first sigprocmask and before the select(2,7,2 select_tut). If you do do this,
it is prudent to at least put a finite timeout(1,3x,3x cbreak) so that the process does
not block. At present glibc probably works this way. The Linux kernel
does not have a native pselect system call as yet so this is all proba-
bly much of a moot point.
PRACTICAL
So what is the point of select(2,7,2 select_tut)? Can't I just read(2,n,1 builtins) and write(1,2) to my
descriptors whenever I want? The point of select(2,7,2 select_tut) is that it watches
multiple descriptors at the same time(1,2,n) and properly puts(3,n) the process to
sleep(1,3) if(3,n) there is no activity. It does this while enabling you to han-
dle multiple simultaneous pipes and sockets. Unix programmers often
find themselves in(1,8) a position where they have to handle IO from more
than one file(1,n) descriptor where the data flow may be intermittent. If
you were to merely create a sequence of read(2,n,1 builtins) and write(1,2) calls, you would
find that one of your calls may block waiting for data from/to a file(1,n)
descriptor, while another file(1,n) descriptor is unused though available
for data. select(2,7,2 select_tut) efficiently copes with this situation.
A classic example of select(2,7,2 select_tut) comes from the select(2,7,2 select_tut) man(1,5,7) page:
#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int
main(void) {
fd_set rfds;
struct timeval tv;
int retval;
/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(0, &rfds);
/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;
retval = select(2,7,2 select_tut)(1, &rfds, NULL, NULL, &tv);
/* Don't rely on the value of tv now! */
if(3,n) (retval == -1)
perror(1,3)("select(2,7,2 select_tut)()");
else if(3,n) (retval)
printf(1,3,1 builtins)("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf(1,3,1 builtins)("No data within five seconds.\n");
exit(3,n,1 builtins)(0);
}
PORT FORWARDING EXAMPLE
Here is an example that better demonstrates the true utility of select(2,7,2 select_tut).
The listing below a TCP forwarding program that forwards from one TCP
port to another.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <string.h>
#include <signal.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>
static int forward_port;
#undef max
#define max(x,y) ((x) > (y) ? (x) : (y))
static int listen_socket (int listen_port) {
struct sockaddr_in a;
int s;
int yes;
if(3,n) ((s = socket(2,7,n) (AF_INET, SOCK_STREAM, 0)) < 0) {
perror(1,3) ("socket(2,7,n)");
return -1;
}
yes = 1;
if(3,n) (setsockopt
(s, SOL_SOCKET, SO_REUSEADDR,
(char *) &yes, sizeof (yes)) < 0) {
perror(1,3) ("setsockopt");
close(2,7,n) (s);
return -1;
}
memset (&a, 0, sizeof (a));
a.sin_port = htons (listen_port);
a.sin_family = AF_INET;
if(3,n) (bind(2,n,