прочитать содержимое символической ссылки (read the contents of a symbolic link)
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)
readlink, readlinkat — read the contents of a symbolic link
Синопсис (Synopsis)
#include <unistd.h>
ssize_t readlink(const char *restrict path, char *restrict buf,
size_t bufsize);
#include <fcntl.h>
ssize_t readlinkat(int fd, const char *restrict path,
char *restrict buf, size_t bufsize);
Описание (Description)
The readlink() function shall place the contents of the symbolic
link referred to by path in the buffer buf which has size
bufsize. If the number of bytes in the symbolic link is less
than bufsize, the contents of the remainder of buf are
unspecified. If the buf argument is not large enough to contain
the link content, the first bufsize bytes shall be placed in buf.
If the value of bufsize is greater than {SSIZE_MAX}, the result
is implementation-defined.
Upon successful completion, readlink() shall mark for update the
last data access timestamp of the symbolic link.
The readlinkat() function shall be equivalent to the readlink()
function except in the case where path specifies a relative path.
In this case the symbolic link whose content is read is relative
to the directory associated with the file descriptor fd instead
of the current working directory. 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.
If readlinkat() is passed the special value AT_FDCWD in the fd
parameter, the current working directory shall be used and the
behavior shall be identical to a call to readlink().
Возвращаемое значение (Return value)
Upon successful completion, these functions shall return the
count of bytes placed in the buffer. Otherwise, these functions
shall return a value of -1, leave the buffer unchanged, and set
errno to indicate the error.
Ошибки (Error)
These functions shall fail if:
EACCES Search permission is denied for a component of the path
prefix of path.
EINVAL The path argument names a file that is not a symbolic
link.
EIO An I/O error occurred while reading from the file system.
ELOOP A loop exists in symbolic links encountered during
resolution of the path argument.
ENAMETOOLONG
The length of a component of a pathname is longer than
{NAME_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 readlinkat() function shall fail if:
EACCES The access mode of the open file description associated
with fd is not O_SEARCH and the permissions of the
directory underlying fd do not permit directory searches.
EBADF The path argument does not specify an absolute path and
the fd argument is neither AT_FDCWD nor a valid file
descriptor open for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd 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 path 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 following sections are informative.
Примеры (Examples)
Reading the Name of a Symbolic Link
The following example shows how to read the name of a symbolic
link named /modules/pass1.
#include <unistd.h>
char buf[1024];
ssize_t len;
...
if ((len = readlink("/modules/pass1", buf, sizeof(buf)-1)) != -1)
buf[len] = '\0';
Использование в приложениях (Application usage)
Conforming applications should not assume that the returned
contents of the symbolic link are null-terminated.
Обоснование (Rationale)
The type associated with bufsiz is a size_t in order to be
consistent with both the ISO C standard and the definition of
read(). The behavior specified for readlink() when bufsiz is
zero represents historical practice. For this case, the standard
developers considered a change whereby readlink() would return
the number of non-null bytes contained in the symbolic link with
the buffer buf remaining unchanged; however, since the stat
structure member st_size value can be used to determine the size
of buffer necessary to contain the contents of the symbolic link
as returned by readlink(), this proposal was rejected, and the
historical practice retained.
The purpose of the readlinkat() function is to read the content
of symbolic links 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
readlink(), resulting in unspecified behavior. By opening a file
descriptor for the target directory and using the readlinkat()
function it can be guaranteed that the symbolic link read is
located relative to the desired directory.
Будущие направления (Future directions)
None.
Смотри также (See also)
fstatat(3p), symlink(3p)
The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),
unistd.h(0p)
