NAME

readlink, readlinkat — read the contents of a symbolic link

SYNOPSIS

#include <unistd.h>

ssize_t readlink(const char *restrict path, char *restrict buf,
       size_t bufsize);

[OH] #include <fcntl.h>

ssize_t readlinkat(int fd, const char *restrict path,
       char *restrict buf, size_t bufsize);


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().

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.

ERRORS

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}.

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';

APPLICATION USAGE

Conforming applications should not assume that the returned contents of the symbolic link are null-terminated.

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.

FUTURE DIRECTIONS

None.

SEE ALSO

fstatat, symlink

XBD <fcntl.h>, <unistd.h>

CHANGE HISTORY

First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Issue 6

The return type is changed to ssize_t, to align with the IEEE P1003.1a draft standard.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• This function is made mandatory.

• In this function it is possible for the return value to exceed the range of the type ssize_t (since size_t has a larger range of positive values than ssize_t). A sentence restricting the size of the size_t object is added to the description to resolve this conflict.

The following changes are made for alignment with the ISO POSIX-1:1996 standard:

• The FUTURE DIRECTIONS section is changed to None.

The following changes were made to align with the IEEE P1003.1a draft standard:

• The [ELOOP] optional error condition is added.

The restrict keyword is added to the readlink() prototype for alignment with the ISO/IEC 9899:1999 standard.

Issue 7

Austin Group Interpretation 1003.1-2001 #143 is applied.

SD5-XSH-ERN-189 is applied, updating the ERRORS section.

The readlinkat() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 2.

The [EACCES] error is removed from the "may fail" error conditions.

Changes are made to allow a directory to be opened for searching.

The [ENOTDIR] error condition is clarified to cover the condition where the last component of a pathname exists but is not a directory or a symbolic link to a directory.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0487 [120], XSH/TC1-2008/0488 [461], XSH/TC1-2008/0489 [143], XSH/TC1-2008/0490 [324], XSH/TC1-2008/0491 [278], XSH/TC1-2008/0492 [278], XSH/TC1-2008/0493 [455], and XSH/TC1-2008/0494 [151,231] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0305 [591], XSH/TC2-2008/0306 [817], XSH/TC2-2008/0307 [817], and XSH/TC2-2008/0308 [591] are applied.