`fpathconf` (3) - glibc

Split from the ledger chapter pathconf/confstr into the per-function layout; see the method page and the glibc ledger for coverage status.

Field	Value
Header	`unistd.h` (`pathconf` lines 633-634, `fpathconf` line 637, `confstr` lines 644-645 under `__USE_POSIX2`, lines 642-646; `_PC_`/`_CS_` constants from `bits/confname.h`, included at line 630)
Standards	POSIX.1-2001 (`pathconf`, `fpathconf`); POSIX.2, POSIX.1-2001 (`confstr`; man gates `_POSIX_C_SOURCE >= 2
Exported	`confstr@@GLIBC_2.2.5`, `fpathconf@@GLIBC_2.2.5`, `pathconf@@GLIBC_2.2.5` (nm marks `pathconf`/`fpathconf` `W`, interposable), fortified `__confstr_chk@@GLIBC_2.4` (host glibc 2.41)
Thread-safety	MT-Safe all (`confstr(3)`, `fpathconf(3)` ATTRIBUTES); manual safety notes for `pathconf`/`fpathconf` carry AS/AC caveats (`conf.texi` lines 1283, 1314)
Ledger ID	GLIBC-confstr, GLIBC-fpathconf, GLIBC-pathconf

Declaration and gates

#include <unistd.h>

long int pathconf (const char *path, int name);
long int fpathconf (int fd, int name);
size_t confstr (int name, char *buf, size_t len);   /* __USE_POSIX2 */

pathconf/fpathconf are declared unconditionally (lines 633-637); confstr only when __USE_POSIX2 is in effect (_POSIX_C_SOURCE >= 2 or _XOPEN_SOURCE, confstr(3) synopsis). The name selectors come from bits/confname.h (multiarch path on Debian): _PC_LINK_MAX at line 26 through _PC_2_SYMLINKS at line 66; _CS_PATH at line 541, _CS_GNU_LIBC_VERSION at line 548, _CS_GNU_LIBPTHREAD_VERSION at line 550.

Semantics

confstr reads a string-valued system parameter into buf; it returns the full length including the terminating NUL, so a return value >= len means truncation, 0 with EINVAL means invalid name (confstr(3), manual String-Valued Parameters). Documented values: _CS_PATH (default utility search path) and the glibc-specific _CS_GNU_LIBC_VERSION / _CS_GNU_LIBPTHREAD_VERSION (confstr(3)). pathconf/fpathconf answer the same question for a file (path) or an open descriptor (fd), with _PC_* selectors such as _PC_LINK_MAX, _PC_NAME_MAX, _PC_PATH_MAX, _PC_PIPE_BUF, _PC_CHOWN_RESTRICTED (fpathconf(3)). A -1 return means either an error (errno set) or an indeterminate limit (errno unchanged); the manual calls clearing errno to 0 before the call the only robust usage (conf.texi lines 1294-1300).

Gotchas and portability

_PC_* answers are per-file/per-filesystem on Linux: glibc implements some via statfs and may read /proc/mounts or /proc/sys/fs/xfs/restrict_chown (manual source comments, conf.texi lines 1284-1287).
Some returned values may be huge and are not suitable for allocating memory (fpathconf(3) NOTES).
The man page FIXMEs note many more _PC_*/_CS_* selectors exist in bits/confname.h than the pages document (confstr(3), fpathconf(3)).
musl 1.2.5 declares all three in unistd.h (lines 131-134) with the same POSIX semantics.

Errors

confstr: EINVAL invalid name.
pathconf: EACCES, ELOOP, ENAMETOOLONG, ENOENT, ENOTDIR, EINVAL (fpathconf(3) ERRORS).
fpathconf: EBADF bad descriptor, EINVAL.

References

glibc manual: conf.texi, nodes Using pathconf, String-Valued Parameters
man3: confstr(3), fpathconf(3) (covers pathconf(3) via .so link)
headers: /usr/include/unistd.h, /usr/include/x86_64-linux-gnu/bits/confname.h
symbols: nm -D --with-symbol-versions libc.so.6

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