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   readlink.3p    ( 3 )

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)