LISTXATTR(2)						     Linux Programmer's Manual						      LISTXATTR(2)

NAME

       listxattr, llistxattr, flistxattr - list extended attribute names

SYNOPSIS

       #include <sys/types.h>
       #include <sys/xattr.h>

       ssize_t listxattr(const char *path, char *list, size_t size);
       ssize_t llistxattr(const char *path, char *list, size_t size);
       ssize_t flistxattr(int fd, char *list, size_t size);

DESCRIPTION

       Extended  attributes  are  name:value  pairs associated with inodes (files, directories, symbolic links, etc.).	They are extensions to the
       normal attributes which are associated with all inodes in the system (i.e., the stat(2) data).  A complete overview of extended	attributes
       concepts can be found in attr(5).

       listxattr()  retrieves the list of extended attribute names associated with the given path in the filesystem.  The retrieved list is placed
       in list, a caller-allocated buffer whose size (in bytes) is specified in the argument size.  The  list  is  the	set  of  (null-terminated)
       names,  one  after the other.  Names of extended attributes to which the calling process does not have access may be omitted from the list.
       The length of the attribute name list is returned.

       llistxattr() is identical to listxattr(), except in the case of a symbolic link, where the list of names of extended attributes	associated
       with the link itself is retrieved, not the file that it refers to.

       flistxattr() is identical to listxattr(), only the open file referred to by fd (as returned by open(2)) is interrogated in place of path.

       A  single extended attribute name is a null-terminated string.  The name includes a namespace prefix; there may be several, disjoint names-
       paces associated with an individual inode.

       If size is specified as zero, these calls return the current size of the list of extended attribute names (and leave list unchanged).  This
       can be used to determine the size of the buffer that should be supplied in a subsequent call.  (But, bear in mind that there is a possibil-
       ity that the set of extended attributes may change between the two calls, so that it is still necessary to check the return status from the
       second call.)

   Example
       The  list  of  names  is  returned  as an unordered array of null-terminated character strings (attribute names are separated by null bytes
       ('')), like this:

	      user.name1system.name1user.name2

       Filesystems that implement POSIX ACLs using extended attributes might return a list like this:

	      system.posix_acl_accesssystem.posix_acl_default

RETURN VALUE

       On success, a nonnegative number is returned indicating the size of the extended attribute name list.  On failure, -1 is returned and errno
       is set appropriately.

ERRORS

       ENOTSUP
	      Extended attributes are not supported by the filesystem, or are disabled.

       ERANGE The size of the list buffer is too small to hold the result.

       In addition, the errors documented in stat(2) can also occur.

VERSIONS

       These system calls have been available on Linux since kernel 2.4; glibc support is provided since version 2.3.

CONFORMING TO

       These system calls are Linux-specific.

EXAMPLE

       The  following  program	demonstrates  the usage of listxattr() and getxattr(2).  For the file whose pathname is provided as a command-line
       argument, it lists all extended file attributes and their values.

       To keep the code simple, the program assumes that attribute keys and values are constant during the execution of the program.  A production
       program	should	expect	and  handle changes during execution of the program.  For example, the number of bytes required for attribute keys
       might increase between the two calls to listxattr().  An application could handle this possibility using a loop that retries the call (per-
       haps up to a predetermined maximum number of attempts) with a larger buffer each time it fails with the error ERANGE.  Calls to getxattr(2)
       could be handled similarly.

       The following output was recorded by first creating a file, setting some extended file attributes, and then listing the attributes with the
       example program.

   Example output
	   $ touch /tmp/foo
	   $ setfattr -n user.fred -v chocolate /tmp/foo
	   $ setfattr -n user.frieda -v bar /tmp/foo
	   $ setfattr -n user.empty /tmp/foo
	   $ ./listxattr /tmp/foo
	   user.fred: chocolate
	   user.frieda: bar
	   user.empty: <no value>

   Program source (listxattr.c)
       #include <malloc.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/types.h>
       #include <sys/xattr.h>

       int
       main(int argc, char *argv[])
       {
	   ssize_t buflen, keylen, vallen;
	   char *buf, *key, *val;

	   if (argc != 2) {
	       fprintf(stderr, "Usage: %s path
", argv[0]);
	       exit(EXIT_FAILURE);
	   }

	   /*
	    * Determine the length of the buffer needed.
	    */
	   buflen = listxattr(argv[1], NULL, 0);
	   if (buflen == -1) {
	       perror("listxattr");
	       exit(EXIT_FAILURE);
	   }
	   if (buflen == 0) {
	       printf("%s has no attributes.
", argv[1]);
	       exit(EXIT_SUCCESS);
	   }

	   /*
	    * Allocate the buffer.
	    */
	   buf = malloc(buflen);
	   if (buf == NULL) {
	       perror("malloc");
	       exit(EXIT_FAILURE);
	   }

	   /*
	    * Copy the list of attribute keys to the buffer.
	    */
	   buflen = listxattr(argv[1], buf, buflen);
	   if (buflen == -1) {
	       perror("listxattr");
	       exit(EXIT_FAILURE);
	   }

	   /*
	    * Loop over the list of zero terminated strings with the
	    * attribute keys. Use the remaining buffer length to determine
	    * the end of the list.
	    */
	   key = buf;
	   while (buflen > 0) {

	       /*
		* Output attribute key.
		*/
	       printf("%s: ", key);

	       /*
		* Determine length of the value.
		*/
	       vallen = getxattr(argv[1], key, NULL, 0);
	       if (vallen == -1)
		   perror("getxattr");

	       if (vallen > 0) {

		   /*
		    * Allocate value buffer.
		    * One extra byte is needed to append 0x00.
		    */
		   val = malloc(vallen + 1);
		   if (val == NULL) {
		       perror("malloc");
		       exit(EXIT_FAILURE);
		   }

		   /*
		    * Copy value to buffer.
		    */
		   vallen = getxattr(argv[1], key, val, vallen);
		   if (vallen == -1)
		       perror("getxattr");
		   else {
		       /*
			* Output attribute value.
			*/
		       val[vallen] = 0;
		       printf("%s", val);
		   }

		   free(val);
	       } else if (vallen == 0)
		   printf("<no value>");

	       printf("
");

	       /*
		* Forward to next attribute key.
		*/
	       keylen = strlen(key) + 1;
	       buflen -= keylen;
	       key += keylen;
	   }

	   free(buf);
	   exit(EXIT_SUCCESS);
       }

SEE ALSO

       getfattr(1), setfattr(1), getxattr(2), open(2), removexattr(2), setxattr(2), stat(2), attr(5), symlink(7)

Linux								    2015-02-21							      LISTXATTR(2)