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Linux 2.6 - man page for chmod (linux section 3posix)

CHMOD(P)			    POSIX Programmer's Manual				 CHMOD(P)

       chmod - change mode of a file

       #include <sys/stat.h>

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

       The  chmod()  function  shall change S_ISUID, S_ISGID,	 S_ISVTX, and the file permission
       bits of the file named by the pathname pointed to by the path argument to the  correspond-
       ing  bits in the mode argument. The application shall ensure that the effective user ID of
       the process matches the owner of the file or the process  has  appropriate  privileges  in
       order to do this.

       S_ISUID, S_ISGID,    S_ISVTX,  and the file permission bits are described in <sys/stat.h>.

       If  the	calling  process does not have appropriate privileges, and if the group ID of the
       file does not match the effective group ID or one of the supplementary group  IDs  and  if
       the  file  is  a  regular file, bit S_ISGID (set-group-ID on execution) in the file's mode
       shall be cleared upon successful return from chmod().

       Additional implementation-defined restrictions may cause the S_ISUID and S_ISGID  bits  in
       mode to be ignored.

       The  effect  on file descriptors for files open at the time of a call to chmod() is imple-

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

       Upon successful completion, 0 shall be returned; otherwise, -1 shall be returned and errno
       set to indicate the error. If -1 is returned, no change to the file mode occurs.

       The chmod() function shall fail if:

       EACCES Search permission is denied on a component of the path prefix.

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

	      The  length  of  the  path  argument  exceeds {PATH_MAX} or a pathname component is
	      longer than {NAME_MAX}.

	      A component of the path prefix is not a directory.

       ENOENT A component of path does not name an existing file or path is an empty string.

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

       EROFS  The named file resides on a read-only file system.

       The chmod() function may fail if:

       EINTR  A signal was caught during execution of the function.

       EINVAL The value of the mode argument is invalid.

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

	      As a result of encountering a symbolic link in resolution of the path argument, the
	      length of the substituted pathname strings exceeded {PATH_MAX}.

       The following sections are informative.

   Setting Read Permissions for User, Group, and Others
       The following example sets read permissions for the owner, group, and others.

	      #include <sys/stat.h>

	      const char *path;
	      chmod(path, S_IRUSR|S_IRGRP|S_IROTH);

   Setting Read, Write, and Execute Permissions for the Owner Only
       The following example sets read, write, and execute permissions for the owner, and no per-
       missions for group and others.

	      #include <sys/stat.h>

	      const char *path;
	      chmod(path, S_IRWXU);

   Setting 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"

   Setting and Checking File Permissions
       The  following example sets the file permission bits for a file named /home/cnd/mod1, then
       calls the stat() function to verify the permissions.

	      #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;);

       In order to ensure that the S_ISUID and S_ISGID bits are  set,  an  application	requiring
       this should use stat() after a successful chmod() to verify this.

       Any  file  descriptors  currently  open	by  any process on the file could possibly become
       invalid if the mode of the file is changed to a value which  would  deny  access  to  that
       process. One situation where this could occur is on a stateless file system. This behavior
       will not occur in a conforming environment.

       This volume of IEEE Std 1003.1-2001 specifies that the S_ISGID bit is cleared  by  chmod()
       on a regular file under certain conditions. This is specified on the assumption that regu-
       lar files may be executed, and the system should prevent users from making executable set-
       gid() files perform with privileges that the caller does not have. On implementations that
       support execution of other file types, the S_ISGID bit should be cleared  for  those  file
       types under the same circumstances.

       Implementations	that  use  the	S_ISUID bit to indicate some other function (for example,
       mandatory record locking) on non-executable files need not clear this bit on writing. They
       should clear the bit for executable files and any other cases where the bit grants special
       powers to processes that change the file contents.  Similar comments apply to the  S_ISGID


       chown()	,  mkdir() , mkfifo() , open() , stat() , statvfs() , the Base Definitions volume
       of IEEE Std 1003.1-2001, <sys/stat.h>, <sys/types.h>

       Portions of this text are reprinted and	reproduced  in	electronic  form  from	IEEE  Std
       1003.1,	2003  Edition,	Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003  by
       the  Institute  of  Electrical  and  Electronics Engineers, Inc and The Open Group. In the
       event of any discrepancy between this version and the original IEEE  and  The  Open  Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 CHMOD(P)

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