listxattr, llistxattr, flistxattr - list extended attribute names

#include <sys/xattr.h>
ssize_t listxattr(const char *path, char *_Nullable list, size_t size);
ssize_t llistxattr(const char *path, char *_Nullable list, size_t size);
ssize_t flistxattr(int fd, char *_Nullable list, size_t size);

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

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.

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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/xattr.h>
int
main(int argc, char *argv[])
{
    char     *buf, *key, *val;
    ssize_t  buflen, keylen, vallen;
    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);
}

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