получить настраиваемые переменные пути (get configurable pathname variables)
Пролог (Prolog)
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.
Имя (Name)
fpathconf, pathconf — get configurable pathname variables
Синопсис (Synopsis)
#include <unistd.h>
long fpathconf(int fildes, int name);
long pathconf(const char *path, int name);
Описание (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 │ 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
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 that would
not cross any mount points 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.
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.
Возвращаемое значение (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 without 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 implementation's <limits.h> or <unistd.h>.
If the variable corresponding to name is dependent on an
unsupported option, the results are unspecified.
Ошибки (Error)
The pathconf() 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 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 A loop exists in symbolic links encountered during
resolution of the path argument.
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
resolution 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
variable name with the specified file.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (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}.
Обоснование (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 implementations 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
variables 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, and the [EOVERFLOW] error
that indicates the value to be returned is larger than
{LONG_MAX}.
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.
It was the intention of IEEE Std 1003.1d‐1999 that the following
variables:
{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 implementation (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 resolution no larger than one second. Even on 32-bit
systems, this can be represented without overflow.
Будущие направления (Future directions)
None.
Смотри также (See also)
chown(3p), confstr(3p), sysconf(3p)
The Base Definitions volume of POSIX.1‐2017, limits.h(0p),
unistd.h(0p)
The Shell and Utilities volume of POSIX.1‐2017, getconf(1p)
