Section 4.24. Summary of File Access Permission Bits

4.24. Summary of File Access Permission Bits

We've covered all the file access permission bits, some of which serve multiple purposes. Figure 4.26 summarizes all these permission bits and their interpretation when applied to a regular file and a directory.

Figure 4.26. Summary of file access permission bits
Constant
Description
Effect on regular file
Effect on directory
S_ISUID
set-user-ID
set effective user ID on execution
(not used)
S_ISGID
set-group-ID
if group-execute set then set effective group ID on execution; otherwise enable mandatory record locking (if supported)
set group ID of new files created in directory to group ID of directory
S_ISVTX
sticky bit
control caching of file contents (if supported)
restrict removal and renaming of files in directory
S_IRUSR
user-read
user permission to read file
user permission to read directory entries
S_IWUSR
user-write
user permission to write file
user permission to remove and create files in directory
S_IXUSR
user-execute
user permission to execute file
user permission to search for given pathname in directory
S_IRGRP
group-read
group permission to read file
group permission to read directory entries
S_IWGRP
group-write
group permission to write file
group permission to remove and create files in directory
S_IXGRP
group-execute
group permission to execute file
group permission to search for given pathname in directory
S_IROTH
other-read
other permission to read file
other permission to read directory entries
S_IWOTH
other-write
other permission to write file
other permission to remove and create files in directory
S_IXOTH
other-execute
other permission to execute file
other permission to search for given pathname in directory

The final nine constants can also be grouped into threes, since

      S_IRWXU = S_IRUSR | S_IWUSR | S_IXUSR
      S_IRWXG = S_IRGRP | S_IWGRP | S_IXGRP
      S_IRWXO = S_IROTH | S_IWOTH | S_IXOTH