Comtrol eCos - Manual (Page 570)

Chapter 38. TCP/IP Library Reference
imum number of descriptors supported by the system.
If timeout is a non-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
non-null, pointing to a zero-valued timeval structure. timeout is not
changed by select(), and may be reused on subsequent calls; however, it
is good style to re-initialize it before each invocation of select().
Any of readfds, writefds, and exceptfds may be given as null pointers if
no descriptors are of interest.
RETURN VALUES
select() returns the number of ready descriptors that are contained in
the descriptor sets, or -1 is an error occurred. If the time limit
expires, select() returns 0. If select() returns with an error, includ-
ing one due to an interrupted call, the descriptor sets will be unmodi-
fied.
ERRORS
An error return from select() indicates:
[EFAULT] One or more of readfds, writefds, or exceptfds points
outside the process’s allocated address space.
[EBADF] One of the descriptor sets specified an invalid
descriptor.
[EINTR] A signal was delivered before the time limit expired
and before any of the selected events occurred.
[EINVAL] The specified time limit is invalid. One of its com-
ponents is negative or too large.
SEE ALSO
accept(2), connect(2), gettimeofday(2), poll(2), read(2), recv(2),
send(2), write(2), getdtablesize(3)
BUGS
Although the provision of getdtablesize(3) 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 bit size of fd_set is based on the symbol
FD_SETSIZE (currently 256), but that is somewhat smaller than the current
kernel limit to the number of open files. However, in order to accommo-
date 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 FD_SETSIZE before the inclusion of
< sys/types.h>. The kernel will cope, and the userland libraries provided
with the system are also ready for large numbers of file descriptors.
Alternatively, to be really safe, it is possible to allocate fd_set bit-
arrays dynamically. The idea is to permit a program to work properly
even if it is execve(2)’d with 4000 file descriptors pre-allocated. The
following illustrates the technique which is used by userland libraries:
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