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man:listxattr

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 sym-
     bolic 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 sec-
     ond 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.

EXAMPLE

     The following program demonstrates the usage of listxattr()  and  getx-
     attr(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  pro-
     gram.   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\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),  setx-
     attr(2), stat(2), symlink(7), xattr(7)

COLOPHON

     This  page  is  part of release 4.16 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 2017-09-15 LISTXATTR(2)

/data/webs/external/dokuwiki/data/pages/man/listxattr.txt · Last modified: 2019/05/17 09:32 by 127.0.0.1

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