pipes in unix

[Guy Harris]

>In UNIX V, file descriptor table can have up to 25 entries.

Actually, that depends on what *flavor* of System V you have.  It's a
configurable parameter in S5R3, or at least in the 3B2 reference port of
S5R3.1.  I think it's also configurable in S5R4, and, in addition, S5R4
has separate "soft" and "hard" limits for the size of the file
descriptor table (any resemblance between that notion of "soft" and
"hard" limits, and the notion of "soft" and "hard" resource limits in
BSD, is purely intentional).

The default value of the parameter in S5R3.1 appears to be 20, not 25. 
It appears that you can crank it up to 100 or so.

S5R4 will, hopefully, choose SunOS 4.1-ish default values, i.e. 64 for
the soft limit and 256 for the hard limit.

>In BSD UNIX, file descriptor table can have up to 99 entries.

In 4.3BSD, 4.3-tahoe, and 4.3-reno, as distributed by Berkeley, it's 64,
not 99.  It can be cranked up, if you have source and don't mind
recompiling the kernel as well as any programs that depend on the layout
of the U area.

>(UNIX Version 7 has a dup2() system call which takes care both close() and
>dup() in one system call.)

It's also in POSIX, and S5R3 has an implementation of it atop
"fcntl(F_DUPFD)".