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Our server (Solaris) accept a socket when got a request. Why can we only create

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Question: Our server (Solaris) accept a socket when got a request. Why can we only create 256 sockets? Is it possible to increase the limit?

Answer: In Solaris and in Unix generally a socket is actually a file descriptor.
In 2.3 in earlier this requires poking the kernel. In Solaris 2.4+, this can be accomplished by adding the following lines to

  • * set hard limit on file descriptors set rlim_fd_max = 4096
  • * set soft limit on file descriptors set rlim_fd_cur = 1024

Raising the soft limit past 256 may confuse certain applications, especially BCP applications. Raising the limit past 1024 may confuse applications that use select(). Select() cannot use more than 1024 file descriptors at this time prior to Solaris 7.
In Solaris 2.6, the RPC code was rewritten to use poll(), which does work with many more fds than select(). Prior to 2.6, all RPC servers will likely crash and burn if you increase the fd soft limit past 1024.

Solaris 7 allows upto 65536 fds passed to select; this is the default for 64 bit applications but it requires recompiling with a larger value for FD_SETSIZE for 32 bit apps.

Programs using stdio or even library calls that use stdio may break when they have more than 256 files open as that is the stdio limit. Programs using many filedescriptors should try and reserve a number of low numbered file descriptors for use by stdio.

The stdio limit is removed for 64 bit applications in Solaris 7;
if you really need more than 256 FILE * in and you can't use Solaris 7+ or need to run in 32 bits, you can use SFIO from AT&T.

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