getdtablesize - Man Page

get file descriptor table size


#include <unistd.h>

int getdtablesize(void);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


    Since glibc 2.20:
        _DEFAULT_SOURCE || ! (_POSIX_C_SOURCE >= 200112L)
    Glibc 2.12 to 2.19:
        _BSD_SOURCE || ! (_POSIX_C_SOURCE >= 200112L)
    Before glibc 2.12:
        _BSD_SOURCE || _XOPEN_SOURCE >= 500


getdtablesize() returns the maximum number of files a process can have open, one more than the largest possible value for a file descriptor.

Return Value

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.


For an explanation of the terms used in this section, see attributes(7).

getdtablesize()Thread safetyMT-Safe

Conforming to

SVr4, 4.4BSD (the getdtablesize() function first appeared in 4.2BSD). It is not specified in POSIX.1; portable applications should employ sysconf(_SC_OPEN_MAX) instead of this call.


The glibc version of getdtablesize() calls getrlimit(2) and returns the current RLIMIT_NOFILE limit, or OPEN_MAX when that fails.

See Also

close(2), dup(2), getrlimit(2), open(2)


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2021-03-22 Linux Programmer's Manual