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CHMOD(2)							      CHMOD(2)

       chmod, fchmod - change access permission mode of file

       #include <sys/types.h>
       #include <sys/stat.h>

       int chmod(const char *path, mode_t mode);

       int fchmod(int fildes, mode_t mode);

       The chmod() and fchmod() functions set the access permission portion of
       the mode of the file whose name is given by path or referenced  by  the
       open  file  descriptor  fildes  to  the	bit pattern contained in mode.
       Access permission bits are interpreted as follows:

       S_ISUID	 04000	 Set user ID on execution.
       S_ISGID	 020#0	 Set group ID on execution if # is 7,  5,
			 3,  or	 1.  Enable mandatory file/record
			 locking if # is 6, 4, 2, or 0.
       S_ISVTX	 01000	 Sticky bit.
       S_IRWXU	 00700	 Read, write, execute by owner.
       S_IRUSR	 00400	 Read by owner.
       S_IWUSR	 00200	 Write by owner.
       S_IXUSR	 00100	 Execute  (search  if  a  directory)   by
       S_IRWXG	 00070	 Read, write, execute by group.
       S_IRGRP	 00040	 Read by group.
       S_IWGRP	 00020	 Write by group.
       S_IXGRP	 00010	 Execute by group.
       S_IRWXO	 00007	 Read, write, execute (search) by others.
       S_IROTH	 00004	 Read by others.
       S_IWOTH	 00002	 Write by others.
       S_IXOTH	 00001	 Execute by others.

       Modes are constructed by the bitwise OR operation of the access permis‐
       sion bits.

       The effective user ID of the process must match the owner of  the  file
       or  the	process must have the appropriate privilege to change the mode
       of a file.

       If the process is not a privileged process and the file is not a direc‐
       tory, mode bit 01000 (save text image on execution) is cleared.

       If  neither  the process is privileged nor the file's group is a member
       of the process's	 supplementary group list, and the effective group  ID
       of  the process does not match the group ID of the file, mode bit 02000
       (set group ID on execution) is cleared.

       If a directory is writable and has S_ISVTX (the sticky bit) set,	 files
       within  that directory can be removed or renamed only if one or more of
       the following is true (see unlink(2) and rename(2)):

	   o	  the user owns the file

	   o	  the user owns the directory

	   o	  the file is writable by the user

	   o	  the user is a privileged user

       If a regular file is not executable and has S_ISVTX set,	 the  file  is
       assumed	to  be a swap file. In this case, the system's page cache will
       not be used to hold the file's data. If the S_ISVTX bit is set  on  any
       other file, the results are unspecified.

       If  a  directory	 has  the  set	group ID bit set, a given file created
       within that directory will have	the same group ID  as  the  directory.
       Otherwise,  the newly created file's group ID will be set to the effec‐
       tive group ID of the creating process.

       If the mode bit 02000 (set group ID on execution) is set and  the  mode
       bit   00010  (execute  or  search  by  group)  is  not  set,  mandatory
       file/record locking will exist on a regular  file,  possibly  affecting
       future calls to open(2), creat(2), read(2), and write(2) on this file.

       If  fildes references a shared memory object, fchmod() need only affect
       S_IXGRP, and S_IXOTH file permission bits.

       If  fildes  refers to a socket, fchmod() does not fail but no action is

       If fildes refers to a stream that is attached to an object in the  file
       system  name  space  with  fattach(3C),	the  fchmod() call performs no
       action and returns successfully.

       Upon successful completion, chmod() and fchmod() mark  for  update  the
       st_ctime field of the file.

       Upon  successful	 completion, 0 is returned. Otherwise, −1 is returned,
       the file mode is unchanged, and errno is set to indicate the error.

       The chmod() and fchmod() functions will fail if:

		An I/O error occurred while reading from  or  writing  to  the
		file system.

		The effective user ID does not match the owner of the file and
		the process does not have appropriate privilege.

		The {PRIV_FILE_OWNER} privilege overrides constraints on  own‐
		ership when changing permissions on a file.

		The  {PRIV_FILE_SETID} privilege overrides constraints on own‐
		ership when adding the setuid or setgid bits to an  executable
		file  or  a  directory.	  When adding the setuid bit to a root
		owned executable, additional restrictions  apply.  See	privi‐

       The chmod() function will fail if:

		       Search  permission is denied on a component of the path
		       prefix of path.	The privilege {FILE_DAC_SEARCH}	 over‐
		       rides file permissions restrictions in that case.

		       The path argument points to an illegal address.

		       A  loop exists in symbolic links encountered during the
		       resolution of the path argument.

		       The length of the path argument	exceeds	 PATH_MAX,  or
		       the  length  of a path component exceeds NAME_MAX while
		       _POSIX_NO_TRUNC is in effect.

		       Either a component of  the  path	 prefix	 or  the  file
		       referred	 to  by path does not exist or is a null path‐

		       The fildes argument points to a remote machine and  the
		       link to that machine is no longer active.

		       A component of the prefix of path is not a directory.

		       The  file  referred  to	by path resides on a read-only
		       file system.

       The fchmod() function will fail if:

		  The fildes argument is not an open file descriptor

		  The path argument points to a remote machine and the link to
		  that machine is no longer active.

		  The  file  referred to by fildes resides on a read-only file

       The chmod() and fchmod() functions may fail if:

		 A signal was caught during execution of the function.

		 The value of the mode argument is invalid.

       The chmod() function may fail if:

		       More than {SYMLOOP_MAX} symbolic links were encountered
		       during the resolution of the path argument.

		       As  a result of encountering a symbolic link in resolu‐
		       tion of thepath argument, the length of the substituted
		       pathname strings exceeds {PATH_MAX}.

       The fchmod() function may fail if:

		 The fildes argument refers to a pipe and the system disallows
		 execution of this function on a pipe.

       Example 1 Set Read Permissions for User, Group, and Others

       The following example sets read permissions for the owner,  group,  and

	 #include <sys/stat.h>
	 const char *path;
	 chmod(path, S_IRUSR|S_IRGRP|S_IROTH);

       Example 2 Set Read, Write, and Execute Permissions for the Owner Only

       The following example sets read, write, and execute permissions for the
       owner, and no permissions for group and others.

	 #include <sys/stat.h>
	 const char *path;
	 chmod(path, S_IRWXU);

       Example 3 Set Different Permissions for Owner, Group, and Other

       The following example sets owner permissions for	 CHANGEFILE  to	 read,
       write,  and  execute,  group permissions to read and execute, and other
       permissions to read.

	 #include <sys/stat.h>
	 #define CHANGEFILE "/etc/myfile"

       Example 4 Set and Checking File Permissions

       The following example sets the file permission bits for	a  file	 named
       /home/cnd/mod1,	then  calls the stat(2) function to verify the permis‐

	 #include <sys/types.h>
	 #include <sys/stat.h>
	 int status;
	 struct stat buffer
	 chmod("home/cnd/mod1", S_IRWXU|S_IRWXG|S_IROTH|S_IWOTH);
	 status = stat("home/cnd/mod1", &buffer;);

       If chmod() or fchmod() is used to change the file group	owner  permis‐
       sions  on a file with non-trivial ACL entries, only the ACL mask is set
       to the new permissions and the  group  owner  permission	 bits  in  the
       file's  mode  field (defined in mknod(2)) are unchanged.	 A non-trivial
       ACL entry is one whose meaning cannot be represented in the file's mode
       field  alone.  The new ACL mask permissions  might change the effective
       permissions for additional users and groups that have  ACL  entries  on
       the file.

       See attributes(5) for descriptions of the following attributes:

       │Interface Stability │ Standard		│
       │MT-Level	    │ Async-Signal-Safe │

       chmod(1),  chown(2),  creat(2),	fcntl(2),  mknod(2), open(2), read(2),
       rename(2), stat(2), write(2), fattach(3C),  mkfifo(3C),	stat.h(3HEAD),
       attributes(5), privileges(5), standards(5)

       Programming Interfaces Guide

				 Sep 12, 2005			      CHMOD(2)

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