This is files/file_perms.c (Listing 15-4, page 296), an example from the book, The Linux Programming Interface.

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

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/* file_perms.c

   Return a string representation of a file permissions mask.
#include <sys/stat.h>
#include <stdio.h>
#include "file_perms.h"                 /* Interface for this implementation */

#define STR_SIZE sizeof("rwxrwxrwx")
char *          /* Return ls(1)-style string for file permissions mask */
filePermStr(mode_t perm, int flags)
    static char str[STR_SIZE];

    /* If FP_SPECIAL was specified, we emulate the trickery of ls(1) in
       returning set-user-ID, set-group-ID, and sticky bit information in
       the user/group/other execute fields. This is made more complex by
       the fact that the case of the character displayed for this bits
       depends on whether the corresponding execute bit is on or off. */

    snprintf(str, STR_SIZE, "%c%c%c%c%c%c%c%c%c",
        (perm & S_IRUSR) ? 'r' : '-', (perm & S_IWUSR) ? 'w' : '-',
        (perm & S_IXUSR) ?
            (((perm & S_ISUID) && (flags & FP_SPECIAL)) ? 's' : 'x') :
            (((perm & S_ISUID) && (flags & FP_SPECIAL)) ? 'S' : '-'),
        (perm & S_IRGRP) ? 'r' : '-', (perm & S_IWGRP) ? 'w' : '-',
        (perm & S_IXGRP) ?
            (((perm & S_ISGID) && (flags & FP_SPECIAL)) ? 's' : 'x') :
            (((perm & S_ISGID) && (flags & FP_SPECIAL)) ? 'S' : '-'),
        (perm & S_IROTH) ? 'r' : '-', (perm & S_IWOTH) ? 'w' : '-',
        (perm & S_IXOTH) ?
            (((perm & S_ISVTX) && (flags & FP_SPECIAL)) ? 't' : 'x') :
            (((perm & S_ISVTX) && (flags & FP_SPECIAL)) ? 'T' : '-'));

    return str;


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