This is acl/acl_view.c (Listing 17-1, page 335), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* acl_view.c

   Display the access control list (ACL) on a file.

   Usage: acl_view [-d] file

   If the '-d' option is specified, then the default ACL is displayed (and
   'file' must be a directory), otherwise the access ACL is displayed.

   This program is Linux-specific. ACLs are supported since Linux 2.6.
   To build this program, you must have the ACL library (libacl) installed
   on your system.
#include <acl/libacl.h>
#include <sys/acl.h>
#include "ugid_functions.h"
#include "tlpi_hdr.h"
static void
usageError(char *progName)
    fprintf(stderr, "Usage: %s [-d] filename\n", progName);
main(int argc, char *argv[])
    acl_t acl;
    acl_type_t type;
    acl_entry_t entry;
    acl_tag_t tag;
    uid_t *uidp;
    gid_t *gidp;
    acl_permset_t permset;
    char *name;
    int entryId, permVal, opt;

    type = ACL_TYPE_ACCESS;
    while ((opt = getopt(argc, argv, "d")) != -1) {
        switch (opt) {
        case 'd': type = ACL_TYPE_DEFAULT;      break;
        case '?': usageError(argv[0]);

    if (optind + 1 != argc)

    acl = acl_get_file(argv[optind], type);
    if (acl == NULL)

    /* Walk through each entry in this ACL */

    for (entryId = ACL_FIRST_ENTRY; ; entryId = ACL_NEXT_ENTRY) {

        if (acl_get_entry(acl, entryId, &entry) != 1)
            break;                      /* Exit on error or no more entries */

        /* Retrieve and display tag type */

        if (acl_get_tag_type(entry, &tag) == -1)

        printf("%-12s", (tag == ACL_USER_OBJ) ?  "user_obj" :
                        (tag == ACL_USER) ?      "user" :
                        (tag == ACL_GROUP_OBJ) ? "group_obj" :
                        (tag == ACL_GROUP) ?     "group" :
                        (tag == ACL_MASK) ?      "mask" :
                        (tag == ACL_OTHER) ?     "other" : "???");

        /* Retrieve and display optional tag qualifier */

        if (tag == ACL_USER) {
            uidp = acl_get_qualifier(entry);
            if (uidp == NULL)

            name = userNameFromId(*uidp);
            if (name == NULL)
                printf("%-8d ", *uidp);
                printf("%-8s ", name);

            if (acl_free(uidp) == -1)

        } else if (tag == ACL_GROUP) {
            gidp = acl_get_qualifier(entry);
            if (gidp == NULL)

            name = groupNameFromId(*gidp);
            if (name == NULL)
                printf("%-8d ", *gidp);
                printf("%-8s ", name);

            if (acl_free(gidp) == -1)

        } else {
            printf("         ");

        /* Retrieve and display permissions */

        if (acl_get_permset(entry, &permset) == -1)

        permVal = acl_get_perm(permset, ACL_READ);
        if (permVal == -1)
            errExit("acl_get_perm - ACL_READ");
        printf("%c", (permVal == 1) ? 'r' : '-');
        permVal = acl_get_perm(permset, ACL_WRITE);
        if (permVal == -1)
            errExit("acl_get_perm - ACL_WRITE");
        printf("%c", (permVal == 1) ? 'w' : '-');
        permVal = acl_get_perm(permset, ACL_EXECUTE);
        if (permVal == -1)
            errExit("acl_get_perm - ACL_EXECUTE");
        printf("%c", (permVal == 1) ? 'x' : '-');


    if (acl_free(acl) == -1)



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