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examines the I/O descriptor sets whose addresses are passed in and to
see if some of their descriptors are ready for reading, are ready for writing,
or have an exceptional condition pending, respectively. The first descriptors
are checked in each set; i.e., the descriptors from 0 through in the descriptor
sets are examined. On return, replaces the given descriptor sets with subsets
consisting of those descriptors that are ready for the requested operation.
returns the total number of ready descriptors in all the sets. The descriptor
sets are stored as bit fields in arrays of integers. The following macros
are provided for manipulating such descriptor sets: initializes a descriptor
set to the null set. includes a particular descriptor in removes from
is non-zero if is a member of zero otherwise. The behavior of these macros
is undefined if a descriptor value is less than zero or greater than or
equal to which is normally at least equal to the maximum number of descriptors
supported by the system. If is a non-nil pointer, it specifies a maximum
interval to wait for the selection to complete. If is a nil pointer,
the select blocks indefinitely. To affect a poll, the argument should
be non-nil, pointing to a zero-valued timeval structure. Any of and may
be given as nil pointers if no descriptors are of interest.
returns the number of ready descriptors that are contained in the descriptor
sets, or -1 if an error occurred. If the time limit expires, returns 0. If
returns with an error, including one due to an interrupted call, the descriptor
sets will be unmodified.
An error return from indicates: One of
the descriptor sets specified an invalid descriptor. A signal was delivered
before the time limit expired and before any of the selected events occurred.
The specified time limit is invalid. One of its components is negative
or too large.
Although the provision of was intended
to allow user programs to be written independent of the kernel limit on
the number of open files, the dimension of a sufficiently large bit field
for select remains a problem. The default size (currently 256) is somewhat
larger than the current kernel limit to the number of open files. However,
in order to accommodate programs which might potentially use a larger number
of open files with select, it is possible to increase this size within
a program by providing a larger definition of before the inclusion of
should probably return the time remaining from the original timeout,
if any, by modifying the time value in place. This may be implemented in
future versions of the system. Thus, it is unwise to assume that the timeout
value will be unmodified by the call.
The function call appeared
in
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