01-27-2004
Note that processes do not have a limit of 20 file descriptors
as suggested in your mail - rather the limit is OPEN_MAX.
Here is the formal POSIX definition of a file descriptor:
A per-process unique, non-negative integer used to identify an open file for the purpose of file access. The value of a file descriptor is from zero to OPEN_MAX. A process can have no more than OPEN_MAX file descriptors open simultaneously.
Note also the difference between file DESCRIPTOR and file DESCRIPTION. From SUSv3 ....
(Open) File Description
A record of how a process or group of processes is accessing a file. Each file descriptor refers to exactly one open file description, but an open file description can be referred to by more than one file descriptor. The file offset, file status, and file access modes are attributes of an open file description.
- Finnbarr
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LEARN ABOUT ULTRIX
select
select(2) System Calls Manual select(2)
Name
select - synchronous I/O multiplexing
Syntax
#include <sys/types.h>
#include <sys/time.h>
int select (nfds, readfds, writefds, exceptfds, timeout)
int nfsd;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;
FD_SET (fd, fdset)
FD_CLR (fd, fdset)
FD_ISSET (fd, fdset)
FD_ZERO (fdset)
int fd;
fd_set *fdset;
Description
The system call examines the I/O descriptor sets whose addresses are passed in readfds, writefds, and exceptfds to see if some of their
descriptors are ready for reading, ready for writing, or have an exceptional condition pending. The nfds parameter is the number of bits to
be checked in each bit mask that represent a file descriptor; the descriptors from 0 through nfds-1 in the descriptor sets are examined.
Typically nfds has the value returned by for the maximum number of file descriptors. On return, replaces the given descriptor sets with
subsets consisting of those descriptors that are ready for the requested operation. The total number of ready descriptors in all the sets
is returned.
The descriptor sets are stored as bit fields in arrays of integers. The following macros are provided for manipulating such descriptor
sets: FD_ZERO(fds descriptor set fdset to the null set. FD_SET(fd, fdset) includes a particular descriptor fd in fdset. FD_CLR(fd, fdset)
removes fd from fdset. FD_ISSET(fd, fdset) is nonzero if fd is a member of fdset, zero otherwise. The behavior of these macros is undefined
if a descriptor value is less than zero or greater than or equal to FD_SETSIZE, which is equal to the maximum number of descriptors that
can be supported by the system.
If timeout is not a NULL pointer, it specifies a maximum interval to wait for the selection to complete. If timeout is a NULL pointer, the
select blocks indefinitely. To effect a poll, the timeout argument should be a non-NULL pointer, pointing to a zero-valued timeval struc-
ture.
Any of readfds, writefds, and exceptfds may be given as NULL pointers if no descriptors are of interest.
Selecting true for reading on a socket descriptor upon which a call has been performed indicates that a subsequent call on that descriptor
will not block.
Restrictions
The call may indicate that a descriptor is ready for writing when in fact an attempt to write would block. This can happen if system
resources necessary for a write are exhausted or otherwise unavailable. If an application deems it critical that writes to a file descrip-
tor not block, it should set the descriptor for non-blocking I/O using the F_SETFL request to the call.
Return Values
The call returns a non-negative value on success. A positive value indicates the number of ready descriptors in the descriptor sets. A 0
indicates that the time limit referred to by timeout expired. On failure, returns -1, sets errno to indicate the error, and the descriptor
sets are not changed.
Diagnostics
[EBADF] One of the descriptor sets specified an invalid descriptor.
[EFAULT] One of the pointers given in the call referred to a non-existent portion of the process' address space.
[EINTR] A signal was delivered before any of the selected events occurred, or before the time limit expired.
[EINVAL] A component of the pointed-to time limit is outside the acceptable range; t_sec must be between 0 and 10^8, inclusive. t_usec
must be greater than or equal to 0, and less than 10^6.
See Also
accept(2), connect(2), fcntl(2), gettimeofday(2), listen(2), read(2), recv(2), send(2), write(2), getdtablesize(2)
select(2)