FPATHCONF(3P) POSIX Programmer's Manual FPATHCONF(3P)
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This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
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 vari-
ables listed in the following table and may support others. The vari-
ables 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 | 4,7 |
|{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 |
|{POSIX2_SYMLINKS} | _PC_2_SYMLINKS | 4 |
|{POSIX_ALLOC_SIZE_MIN} | _PC_ALLOC_SIZE_MIN | 10 |
|{POSIX_REC_INCR_XFER_SIZE} | _PC_REC_INCR_XFER_SIZE | 10 |
|{POSIX_REC_MAX_XFER_SIZE} | _PC_REC_MAX_XFER_SIZE | 10 |
|{POSIX_REC_MIN_XFER_SIZE} | _PC_REC_MIN_XFER_SIZE | 10 |
|{POSIX_REC_XFER_ALIGN} | _PC_REC_XFER_ALIGN | 10 |
|{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 |
|_POSIX_TIMESTAMP_RESOLUTION | _PC_TIMESTAMP_RESOLUTION | 1 |
+----------------------------+--------------------------+--------------+
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 unspeci-
fied whether an implementation supports an association of the vari-
able 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 that would not cross
any mount points when the specified directory is the working direc-
tory.
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.
10. If path or fildes des does not refer to a regular file, it is
unspecified whether an implementation supports an association of
the variable name with the specified file. If an implementation
supports such an association for other than a regular file, the
value returned is unspecified.
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 is described in <limits.h> as a
maximum or minimum value and the variable has no limit for the path or
file descriptor, both pathconf() and fpathconf() shall return -1 with-
out changing errno. Note that indefinite limits do not imply infinite
limits; see <limits.h>.
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 implementa-
tion's <limits.h> or <unistd.h>.
If the variable corresponding to name is dependent on an unsupported
option, the results are unspecified.
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.
EOVERFLOW
The value of name is _PC_TIMESTAMP_RESOLUTION and the resolution
is larger than {LONG_MAX}.
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 vari-
able name with the specified file.
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
resolution of the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_MAX}.
ENAMETOOLONG
The length of a pathname exceeds {PATH_MAX}, or pathname resolu-
tion of a symbolic link produced an intermediate result with a
length that exceeds {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 names an existing file that is
neither a directory nor a symbolic link to a directory, or the
path argument contains at least one non-<slash> character and
ends with one or more trailing <slash> characters and the last
pathname component names an existing file that is neither a
directory nor a symbolic link to a directory.
The fpathconf() function shall fail if:
EINVAL The value of name is not valid.
EOVERFLOW
The value of name is _PC_TIMESTAMP_RESOLUTION and the resolution
is larger than {LONG_MAX}.
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 vari-
able name with the specified file.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
Application developers should check whether an option, such as
_POSIX_ADVISORY_INFO, is supported prior to obtaining and using values
for related variables such as {POSIX_ALLOC_SIZE_MIN}.
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 appropriate privileges 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
current working directory of the process. 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. Note that imple-
mentations are allowed to accept pathnames longer than {PATH_MAX} bytes
long, but are not allowed to return pathnames longer than this unless
the user specifies a larger buffer using a function that provides a
buffer size argument.
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 vari-
ables are potentially more volatile and are not guaranteed to remain
constant throughout the lifetime of the process. 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 fpath-
conf() functions return -1 without changing errno. This can be distin-
guished 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.
It was the intention of IEEE Std 1003.1d-1999 that the following vari-
ables:
{POSIX_ALLOC_SIZE_MIN} {POSIX_REC_INCR_XFER_SIZE}
{POSIX_REC_MAX_XFER_SIZE} {POSIX_REC_MIN_XFER_SIZE}
{POSIX_REC_XFER_ALIGN}
only applied to regular files, but Note 10 also permits implementation
of the advisory semantics on other file types unique to an implementa-
tion (for example, a character special device).
The [EOVERFLOW] error for _PC_TIMESTAMP_RESOLUTION cannot occur on
POSIX-compliant file systems because POSIX requires a timestamp resolu-
tion no larger than one second. Even on 32-bit systems, this can be
represented without overflow.
FUTURE DIRECTIONS
None.
SEE ALSO
chown(), confstr(), sysconf()
The Base Definitions volume of POSIX.1-2008, <limits.h>, <unistd.h>
The Shell and Utilities volume of POSIX.1-2008, getconf
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FPATHCONF(3P)
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