NAME
fpathconf, pathconf - get configurable pathname variables
SYNOPSIS
#include <unistd.h>
long fpathconf(int fildes, int name);
long pathconf(const char *path, int name);
DESCRIPTION
The fpathconf() and pathconf() functions shall determine the current
value of a configurable limit or option (variable) that is associated
with a file or directory.
For pathconf(), the path argument points to the pathname of a file or
directory.
For fpathconf(), the fildes argument is an open file descriptor.
The name argument represents the variable to be queried relative to
that file or directory. Implementations shall support all of the
variables listed in the following table and may support others. The
variables in the following table come from <limits.h> or <unistd.h> and
the symbolic constants, defined in <unistd.h>, are the corresponding
values used for name.
Variable Value of name Requirements
{FILESIZEBITS} _PC_FILESIZEBITS 3,4
{LINK_MAX} _PC_LINK_MAX 1
{MAX_CANON} _PC_MAX_CANON 2
{MAX_INPUT} _PC_MAX_INPUT 2
{NAME_MAX} _PC_NAME_MAX 3,4
{PATH_MAX} _PC_PATH_MAX 4,5
{PIPE_BUF} _PC_PIPE_BUF 6
{POSIX_ALLOC_SIZE_MIN} _PC_ALLOC_SIZE_MIN
{POSIX_REC_INCR_XFER_SIZE} _PC_REC_INCR_XFER_SIZE
{POSIX_REC_MAX_XFER_SIZE} _PC_REC_MAX_XFER_SIZE
{POSIX_REC_MIN_XFER_SIZE} _PC_REC_MIN_XFER_SIZE
{POSIX_REC_XFER_ALIGN} _PC_REC_XFER_ALIGN
{SYMLINK_MAX} _PC_SYMLINK_MAX 4,9
_POSIX_CHOWN_RESTRICTED _PC_CHOWN_RESTRICTED 7
_POSIX_NO_TRUNC _PC_NO_TRUNC 3,4
_POSIX_VDISABLE _PC_VDISABLE 2
_POSIX_ASYNC_IO _PC_ASYNC_IO 8
_POSIX_PRIO_IO _PC_PRIO_IO 8
_POSIX_SYNC_IO _PC_SYNC_IO 8
Requirements
1. If path or fildes refers to a directory, the value returned shall
apply to the directory itself.
2. If path or fildes does not refer to a terminal file, it is
unspecified whether an implementation supports an association of
the variable name with the specified file.
3. If path or fildes refers to a directory, the value returned shall
apply to filenames within the directory.
4. If path or fildes does not refer to a directory, it is unspecified
whether an implementation supports an association of the variable
name with the specified file.
5. If path or fildes refers to a directory, the value returned shall
be the maximum length of a relative pathname when the specified
directory is the working directory.
6. If path refers to a FIFO, or fildes refers to a pipe or FIFO, the
value returned shall apply to the referenced object. If path or
fildes refers to a directory, the value returned shall apply to any
FIFO that exists or can be created within the directory. If path or
fildes refers to any other type of file, it is unspecified whether
an implementation supports an association of the variable name with
the specified file.
7. If path or fildes refers to a directory, the value returned shall
apply to any files, other than directories, that exist or can be
created within the directory.
8. If path or fildes refers to a directory, it is unspecified whether
an implementation supports an association of the variable name with
the specified file.
9. If path or fildes refers to a directory, the value returned shall
be the maximum length of the string that a symbolic link in that
directory can contain.
RETURN VALUE
If name is an invalid value, both pathconf() and fpathconf() shall
return -1 and set errno to indicate the error.
If the variable corresponding to name has no limit for the path or file
descriptor, both pathconf() and fpathconf() shall return -1 without
changing errno. If the implementation needs to use path to determine
the value of name and the implementation does not support the
association of name with the file specified by path, or if the process
did not have appropriate privileges to query the file specified by
path, or path does not exist, pathconf() shall return -1 and set errno
to indicate the error.
If the implementation needs to use fildes to determine the value of
name and the implementation does not support the association of name
with the file specified by fildes, or if fildes is an invalid file
descriptor, fpathconf() shall return -1 and set errno to indicate the
error.
Otherwise, pathconf() or fpathconf() shall return the current variable
value for the file or directory without changing errno. The value
returned shall not be more restrictive than the corresponding value
available to the application when it was compiled with the
implementation’s <limits.h> or <unistd.h>.
ERRORS
The pathconf() function shall fail if:
EINVAL The value of name is not valid.
ELOOP A loop exists in symbolic links encountered during resolution of
the path argument.
The pathconf() function may fail if:
EACCES Search permission is denied for a component of the path prefix.
EINVAL The implementation does not support an association of the
variable name with the specified file.
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
The length of the path argument exceeds {PATH_MAX} or a pathname
component is longer than {NAME_MAX}.
ENAMETOOLONG
As a result of encountering a symbolic link in resolution of the
path argument, the length of the substituted pathname string
exceeded {PATH_MAX}.
ENOENT A component of path does not name an existing file or path is an
empty string.
ENOTDIR
A component of the path prefix is not a directory.
The fpathconf() function shall fail if:
EINVAL The value of name is not valid.
The fpathconf() function may fail if:
EBADF The fildes argument is not a valid file descriptor.
EINVAL The implementation does not support an association of the
variable name with the specified file.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
The pathconf() function was proposed immediately after the sysconf()
function when it was realized that some configurable values may differ
across file system, directory, or device boundaries.
For example, {NAME_MAX} frequently changes between System V and BSD-
based file systems; System V uses a maximum of 14, BSD 255. On an
implementation that provides both types of file systems, an application
would be forced to limit all pathname components to 14 bytes, as this
would be the value specified in <limits.h> on such a system.
Therefore, various useful values can be queried on any pathname or file
descriptor, assuming that the appropriate permissions are in place.
The value returned for the variable {PATH_MAX} indicates the longest
relative pathname that could be given if the specified directory is the
process’ current working directory. A process may not always be able to
generate a name that long and use it if a subdirectory in the pathname
crosses into a more restrictive file system.
The value returned for the variable _POSIX_CHOWN_RESTRICTED also
applies to directories that do not have file systems mounted on them.
The value may change when crossing a mount point, so applications that
need to know should check for each directory. (An even easier check is
to try the chown() function and look for an error in case it happens.)
Unlike the values returned by sysconf(), the pathname-oriented
variables are potentially more volatile and are not guaranteed to
remain constant throughout the process’ lifetime. For example, in
between two calls to pathconf(), the file system in question may have
been unmounted and remounted with different characteristics.
Also note that most of the errors are optional. If one of the variables
always has the same value on an implementation, the implementation need
not look at path or fildes to return that value and is, therefore, not
required to detect any of the errors except the meaning of [EINVAL]
that indicates that the value of name is not valid for that variable.
If the value of any of the limits is unspecified (logically infinite),
they will not be defined in <limits.h> and the pathconf() and
fpathconf() functions return -1 without changing errno. This can be
distinguished from the case of giving an unrecognized name argument
because errno is set to [EINVAL] in this case.
Since -1 is a valid return value for the pathconf() and fpathconf()
functions, applications should set errno to zero before calling them
and check errno only if the return value is -1.
For the case of {SYMLINK_MAX}, since both pathconf() and open() follow
symbolic links, there is no way that path or fildes could refer to a
symbolic link.
FUTURE DIRECTIONS
None.
SEE ALSO
confstr() , sysconf() , the Base Definitions volume of
IEEE Std 1003.1-2001, <limits.h>, <unistd.h>, the Shell and Utilities
volume of IEEE Std 1003.1-2001
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .