05-22-2007
The exact particulars vary depending on OS. I will use HP-UX as an example. The number of possible file decriptors is under the control of
setrlimit(2). (A less powerful interface,
ulimit() is also available.) A process cannot have more fd's than the "soft" limit . Using setrlimit(2), a process may raise or lower its soft limit. But a process cannot raise the soft limit above the hard limit. A process can lower the hard limit. Only a root process can raise the hard limit. Kernel parameters define the initial value of the hard and soft limit. Even root cannot raise the hard limit above the initial value for the hard limit. The kernel paramters:
maxfiles
maxfiles_lim
I have cheated a little bit by picking HP-UX as my sample OS. HP-UX allows dynamic reconfiguration of the kernel. Only root can reconfigure the kernel. But a root process could, in theory, raise maxfiles_lim and then raise its hard limit and then relower maxfiles_lim. Not all versions of Unix give that much power to a root process.
I don't believe that cron fiddles with these limits.
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LEARN ABOUT HPUX
getdtablesize
getdtablesize(2) System Calls Manual getdtablesize(2)
NAME
getdtablesize() - get the size of the per-process file descriptor table
SYNOPSIS
DESCRIPTION
The function returns the maximum number of file descriptors that can currently be stored in a process' file descriptor table. This maximum
number is also known as the soft limit for open files, and can be adjusted up to the hard limit by calling The entries in the descriptor
table are numbered with small integers starting at 0 (zero).
The function returns the total number of file descriptors that a process can have open simultaneously. Each process is limited to a cur-
rent maximum (soft limit) and a fixed upper bound (hard limit) of open file descriptors. This limit is at least 32. The system-defined
limits are configurable. See the descriptions of the and kernel parameters in maxfiles_lim(5) and maxfiles(5), respectively, for informa-
tion about changing the system-defined, per-process limit on open file descriptors.
RETURN VALUES
The function returns the size of the descriptor table (soft limit), and is always successful.
SEE ALSO
close(2), getrlimit(2), open(2), select(2), setrlimit(2), sysconf(2), maxfiles(5), maxfiles_lim(5).
STANDARDS CONFORMANCE
getdtablesize(2)