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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 .