getdtablesize() is implemented as a libc library function. The glibc version callsgetrlimit(2) and returns the current RLIMIT_NOFILE limit, or OPEN_MAX when that fails. The libc4 and libc5 versions return OPEN_MAX (set to 256 since Linux 0.98.4).
遵循于
SVr4, 4.4BSD (the getdtablesize() function first appeared in 4.2BSD).