связать один файл с другим файлом (link one file to another file)
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)
link, linkat — link one file to another file
Синопсис (Synopsis)
#include <unistd.h>
int link(const char *path1, const char *path2);
#include <fcntl.h>
int linkat(int fd1, const char *path1, int fd2,
const char *path2, int flag);
Описание (Description)
The link() function shall create a new link (directory entry) for
the existing file, path1.
The path1 argument points to a pathname naming an existing file.
The path2 argument points to a pathname naming the new directory
entry to be created. The link() function shall atomically create
a new link for the existing file and the link count of the file
shall be incremented by one.
If path1 names a directory, link() shall fail unless the process
has appropriate privileges and the implementation supports using
link() on directories.
If path1 names a symbolic link, it is implementation-defined
whether link() follows the symbolic link, or creates a new link
to the symbolic link itself.
Upon successful completion, link() shall mark for update the last
file status change timestamp of the file. Also, the last data
modification and last file status change timestamps of the
directory that contains the new entry shall be marked for update.
If link() fails, no link shall be created and the link count of
the file shall remain unchanged.
The implementation may require that the calling process has
permission to access the existing file.
The linkat() function shall be equivalent to the link() function
except that symbolic links shall be handled as specified by the
value of flag (see below) and except in the case where either
path1 or path2 or both are relative paths. In this case a
relative path path1 is interpreted relative to the directory
associated with the file descriptor fd1 instead of the current
working directory and similarly for path2 and the file descriptor
fd2. If the access mode of the open file description associated
with the file descriptor is not O_SEARCH, the function shall
check whether directory searches are permitted using the current
permissions of the directory underlying the file descriptor. If
the access mode is O_SEARCH, the function shall not perform the
check.
Values for flag are constructed by a bitwise-inclusive OR of
flags from the following list, defined in <fcntl.h>:
AT_SYMLINK_FOLLOW
If path1 names a symbolic link, a new link for the target
of the symbolic link is created.
If linkat() is passed the special value AT_FDCWD in the fd1 or
fd2 parameter, the current working directory shall be used for
the respective path argument. If both fd1 and fd2 have value
AT_FDCWD, the behavior shall be identical to a call to link(),
except that symbolic links shall be handled as specified by the
value of flag.
If the AT_SYMLINK_FOLLOW flag is clear in the flag argument and
the path1 argument names a symbolic link, a new link is created
for the symbolic link path1 and not its target.
Возвращаемое значение (Return value)
Upon successful completion, these functions shall return 0.
Otherwise, these functions shall return -1 and set errno to
indicate the error.
Ошибки (Error)
These functions shall fail if:
EACCES A component of either path prefix denies search
permission, or the requested link requires writing in a
directory that denies write permission, or the calling
process does not have permission to access the existing
file and this is required by the implementation.
EEXIST The path2 argument resolves to an existing directory entry
or refers to a symbolic link.
ELOOP A loop exists in symbolic links encountered during
resolution of the path1 or path2 argument.
EMLINK The number of links to the file named by path1 would
exceed {LINK_MAX}.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_MAX}.
ENOENT A component of either path prefix does not exist; the file
named by path1 does not exist; or path1 or path2 points to
an empty string.
ENOENT or ENOTDIR
The path1 argument names an existing non-directory file,
and the path2 argument contains at least one non-<slash>
character and ends with one or more trailing <slash>
characters. If path2 without the trailing <slash>
characters would name an existing file, an [ENOENT] error
shall not occur.
ENOSPC The directory to contain the link cannot be extended.
ENOTDIR
A component of either path prefix names an existing file
that is neither a directory nor a symbolic link to a
directory, or the path1 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, or the path1 argument names
an existing non-directory file and the path2 argument
names a nonexistent file, contains at least one
non-<slash> character, and ends with one or more trailing
<slash> characters.
EPERM The file named by path1 is a directory and either the
calling process does not have appropriate privileges or
the implementation prohibits using link() on directories.
EROFS The requested link requires writing in a directory on a
read-only file system.
EXDEV The link named by path2 and the file named by path1 are on
different file systems and the implementation does not
support links between file systems.
EXDEV path1 refers to a named STREAM.
The linkat() function shall fail if:
EACCES The access mode of the open file description associated
with fd1 or fd2 is not O_SEARCH and the permissions of the
directory underlying fd1 or fd2, respectively, do not
permit directory searches.
EBADF The path1 or path2 argument does not specify an absolute
path and the fd1 or fd2 argument, respectively, is neither
AT_FDCWD nor a valid file descriptor open for reading or
searching.
ENOTDIR
The path1 or path2 argument is not an absolute path and
fd1 or fd2, respectively, is a file descriptor associated
with a non-directory file.
These functions may fail if:
ELOOP More than {SYMLOOP_MAX} symbolic links were encountered
during resolution of the path1 or path2 argument.
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}.
The linkat() function may fail if:
EINVAL The value of the flag argument is not valid.
The following sections are informative.
Примеры (Examples)
Creating a Link to a File
The following example shows how to create a link to a file named
/home/cnd/mod1 by creating a new directory entry named
/modules/pass1.
#include <unistd.h>
char *path1 = "/home/cnd/mod1";
char *path2 = "/modules/pass1";
int status;
...
status = link (path1, path2);
Creating a Link to a File Within a Program
In the following program example, the link() function links the
/etc/passwd file (defined as PASSWDFILE) to a file named
/etc/opasswd (defined as SAVEFILE), which is used to save the
current password file. Then, after removing the current password
file (defined as PASSWDFILE), the new password file is saved as
the current password file using the link() function again.
#include <unistd.h>
#define LOCKFILE "/etc/ptmp"
#define PASSWDFILE "/etc/passwd"
#define SAVEFILE "/etc/opasswd"
...
/* Save current password file */
link (PASSWDFILE, SAVEFILE);
/* Remove current password file. */
unlink (PASSWDFILE);
/* Save new password file as current password file. */
link (LOCKFILE,PASSWDFILE);
Использование в приложениях (Application usage)
Some implementations do allow links between file systems.
If path1 refers to a symbolic link, application developers should
use linkat() with appropriate flags to select whether or not the
symbolic link should be resolved.
Обоснование (Rationale)
Linking to a directory is restricted to the superuser in most
historical implementations because this capability may produce
loops in the file hierarchy or otherwise corrupt the file system.
This volume of POSIX.1‐2017 continues that philosophy by
prohibiting link() and unlink() from doing this. Other functions
could do it if the implementor designed such an extension.
Some historical implementations allow linking of files on
different file systems. Wording was added to explicitly allow
this optional behavior.
The exception for cross-file system links is intended to apply
only to links that are programmatically indistinguishable from
``hard'' links.
The purpose of the linkat() function is to link files in
directories other than the current working directory without
exposure to race conditions. Any part of the path of a file could
be changed in parallel to a call to link(), resulting in
unspecified behavior. By opening a file descriptor for the
directory of both the existing file and the target location and
using the linkat() function it can be guaranteed that the both
filenames are in the desired directories.
The AT_SYMLINK_FOLLOW flag allows for implementing both common
behaviors of the link() function. The POSIX specification
requires that if path1 is a symbolic link, a new link for the
target of the symbolic link is created. Many systems by default
or as an alternative provide a mechanism to avoid the implicit
symbolic link lookup and create a new link for the symbolic link
itself.
Earlier versions of this standard specified only the link()
function, and required it to behave like linkat() with the
AT_SYMLINK_FOLLOW flag. However, historical practice from SVR4
and Linux kernels had link() behaving like linkat() with no
flags, and many systems that attempted to provide a conforming
link() function did so in a way that was rarely used, and when it
was used did not conform to the standard (e.g., by not being
atomic, or by dereferencing the symbolic link incorrectly). Since
applications could not rely on link() following links in
practice, the linkat() function was added taking a flag to
specify the desired behavior for the application.
Будущие направления (Future directions)
None.
Смотри также (See also)
rename(3p), symlink(3p), unlink(3p)
The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),
unistd.h(0p)
