Get descriptor table size
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.12:
_BSD_SOURCE || !(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600)
Before glibc 2.12: _BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
getdtablesize() returns the maximum number of files a process can have open, one more than the largest possible value for a file descriptor.
The current limit on the number of open files per process.
On Linux, getdtablesize() can return any of the errors described for getrlimit(2); see NOTES below.
The getdtablesize() function is thread-safe.
SVr4, 4.4BSD (the getdtablesize() function first appeared in 4.2BSD). It is not specified in POSIX.1-2001; portable applications should employ sysconf(_SC_OPEN_MAX) instead of this call.
getdtablesize() is implemented as a libc library function. The glibc version calls getrlimit(2) and returns the current RLIMIT_NOFILE limit, or OPEN_MAX when that fails.
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