listxattr(2) — Linux manual page

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

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

NAME         top

       listxattr, llistxattr, flistxattr - list extended attribute names

SYNOPSIS         top

       #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 to indicate the error.

ERRORS         top

       E2BIG  The size of the list of extended attribute names is larger
              than the maximum size allowed; the list cannot be
              retrieved.  This can happen on filesystems that support an
              unlimited number of extended attributes per file such as
              XFS, for example.  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.

EXAMPLES         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 5.13 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                          2021-03-22                   LISTXATTR(2)

Pages that refer to this page: getxattr(2)removexattr(2)setxattr(2)syscalls(2)symlink(7)xattr(7)