readlink - read the contents of a symbolic link
#include <unistd.h>
ssize_t readlink(const char *restrict path, char *restrict buf,
size_t bufsize);
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 return the count of bytes placed in the buffer. Otherwise, it shall return a value of -1, leave the buffer unchanged, and set errno to indicate the error.
The readlink() function 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 the path argument exceeds {PATH_MAX} or a pathname component 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 is not a directory.
The readlink() function may fail if:
- [EACCES]
- Read permission is denied for the directory.
- [ELOOP]
- More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
- [ENAMETOOLONG]
- As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname string exceeded {PATH_MAX}.
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]; ssizet_t len; ... if ((len = readlink("/modules/pass1", buf, sizeof(buf)-1)) != -1) buf[len] = '\0';
Conforming applications should not assume that the returned contents of the symbolic link are null-terminated.
Since IEEE Std 1003.1-2001 does not require any association of file times with symbolic links, there is no requirement that file times be updated by readlink(). 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.
None.
lstat(), stat(), symlink(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>
First released in Issue 4, Version 2.
Moved from X/OPEN UNIX extension to BASE.
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.