files/file_perms.c

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 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* 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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