llistxattr

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 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
       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
       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
       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.

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

COLOPHON
       This page is part of release 5.10 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                              2020-06-09                       LISTXATTR(2)