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Note more clearly that fd_set args can be NULL; various other minor edits and ffixes

This commit is contained in:
Michael Kerrisk 2006-04-20 21:21:42 +00:00
parent 1c382af469
commit 22f348ca27
1 changed files with 31 additions and 13 deletions
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44
man2/select.2
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@ -105,7 +105,8 @@ argument, and behaves as
called with NULL
.IR sigmask .
.PP
Three independent sets of descriptors are watched. Those listed in
Three independent sets of file descriptors are watched.
Those listed in
.I readfds
will be watched to see if characters become
available for reading (more precisely, to see if a read will not
@ -116,7 +117,10 @@ will be watched to see if a write will not block, and
those in
.I exceptfds
will be watched for exceptions. On exit, the sets are modified in place
to indicate which descriptors actually changed status.
to indicate which file descriptors actually changed status.
Each of the three file descriptor sets may be specified as NULL
if no file descriptors are to be watched for the corresponding class
of events.
.PP
Four macros are provided to manipulate the sets.
.BR FD_ZERO ()
@ -124,14 +128,15 @@ clears a set.
.BR FD_SET ()
and
.BR FD_CLR ()
respectively add and remove a given descriptor from a set.
respectively add and remove a given file descriptor from a set.
.BR FD_ISSET ()
tests to see if a descriptor is part of the set; this is useful after
tests to see if a file descriptor is part of the set;
this is useful after
.BR select ()
returns.
.PP
.I n
is the highest-numbered descriptor in any of the three sets, plus 1.
is the highest-numbered file descriptor in any of the three sets, plus 1.
.PP
.I timeout
is an upper bound on the amount of time elapsed before
@ -226,7 +231,7 @@ Some code calls
.BR select ()
with all three sets empty,
.I n
zero, and a non-null
zero, and a non-NULL
.I timeout
as a fairly portable way to sleep with subsecond precision.
.PP
@ -256,8 +261,8 @@ On success,
.BR select ()
and
.BR pselect ()
return the number of descriptors contained in the three returned
descriptor sets (that is, the total number of one bits in
return the number of file descriptors contained in the three returned
descriptor sets (that is, the total number of bits that are set in
.IR readfds ,
.IR writefds ,
.IR exceptfds )
@ -331,7 +336,14 @@ sets the timeout variable before exit, but the BSD variant does not.
is defined in IEEE Std 1003.1g-2000 (POSIX.1g), and in
POSIX 1003.1-2001.
.SH NOTES
An fd_set is a fixed size buffer. Executing FD_CLR or FD_SET with a value of
An
.I fd_set
is a fixed size buffer.
Executing
.BR FD_CLR ()
or
.BR FD_SET ()
with a value of
.I fd
that is negative or is equal to or larger than FD_SETSIZE will result
in undefined behavior. Moreover, POSIX requires
@ -339,8 +351,10 @@ in undefined behavior. Moreover, POSIX requires
to be a valid file descriptor.
Concerning the types involved, the classical situation is that
the two fields of a struct timeval are longs (as shown above),
and the struct is defined in
the two fields of a
.I timeval
structure are longs (as shown above),
and the structure is defined in
.IR <sys/time.h> .
The POSIX 1003.1-2001 situation is
@ -353,9 +367,13 @@ struct timeval {
.fi
.RE
where the struct is defined in
where the structure is defined in
.I <sys/select.h>
and the data types time_t and suseconds_t are defined in
and the data types i
.I time_t
and
.I suseconds_t
are defined in
.IR <sys/types.h> .
.LP
Concerning prototypes, the classical situation is that one should