NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       listxattr, llistxattr, flistxattr - list extended attribute names

SYNOPSIS         top

       #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         top

       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 xattr(7).

       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
       namespaces 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
       possibility 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 ('\0')), like this:

              user.name1\0system.name1\0user.name2\0

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

              system.posix_acl_access\0system.posix_acl_default\0

RETURN VALUE         top

       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         top

       E2BIG  The size of the list of attribute names for this file exceeds
              the system-imposed limit.  See BUGS.

       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         top

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

CONFORMING TO         top

       These system calls are Linux-specific.

BUGS         top

       As noted in xattr(7), the VFS imposes a limit of 64 kB on the size of
       the extended attribute name list returned by listxattr(7).  If the
       total size of attribute names attached to a file exceeds this limit,
       it is no longer possible to retrieve the list of attribute names.

EXAMPLE         top

       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 (perhaps 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\n", 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.\n", 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("\n");

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

           free(buf);
           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-10-08                     LISTXATTR(2)