CHMOD(3P)                                           POSIX Programmer's Manual                                          CHMOD(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       chmod - change mode of a file

SYNOPSIS
       #include <sys/stat.h>

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


DESCRIPTION
       The chmod() function shall change S_ISUID, S_ISGID,  S_ISVTX, and the file permission bits of the file named by the path-
       name pointed to by the path argument to the corresponding 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 implementation-defined.

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

RETURN VALUE
       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.

ERRORS
       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.

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

       ENOTDIR
              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.

       ENAMETOOLONG
              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.

EXAMPLES
   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 permissions 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"
              ...
              chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);

   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;);

APPLICATION USAGE
       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.

RATIONALE
       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 regular files may be executed, and the system should  prevent  users
       from making executable setgid() files perform with privileges that the caller does not have. On implementations that sup-
       port 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
       bit.

FUTURE DIRECTIONS
       None.

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

COPYRIGHT
       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, Copy-
       right (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   original   Standard   can   be   obtained   online   at   http://www.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     CHMOD(3P)