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PROLOG

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

NAME

       chmod, fchmodat — change mode of a file

SYNOPSIS

       #include <sys/stat.h>

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

       #include <fcntl.h>

       int fchmodat(int fd, const char *path, mode_t mode, int flag);

DESCRIPTION

       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 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.

       Upon successful completion, chmod() shall mark for update the last file status change  timestamp  of  the
       file.

       The  fchmodat()  function  shall  be  equivalent  to  the  chmod() function except in the case where path
       specifies a relative path. In this case the file to be changed is determined relative  to  the  directory
       associated  with  the  file descriptor fd instead of the current working directory. If the access mode of
       the open file description associated with the file descriptor is not O_SEARCH, the function  shall  check
       whether  directory  searches  are permitted using the current permissions of the directory underlying the
       file descriptor. If the access mode is O_SEARCH, the function shall not perform the check.

       Values for flag are constructed by a bitwise-inclusive OR of flags from the following  list,  defined  in
       <fcntl.h>:

       AT_SYMLINK_NOFOLLOW
             If path names a symbolic link, then the mode of the symbolic link is changed.

       If  fchmodat()  is  passed  the special value AT_FDCWD in the fd parameter, the current working directory
       shall be used. If also flag is zero, the behavior shall be identical to a call to chmod().

RETURN VALUE

       Upon successful completion, these functions shall return 0.  Otherwise, these functions shall  return  -1
       and set errno to indicate the error. If -1 is returned, no change to the file mode occurs.

ERRORS

These functions 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 a component of a pathname is longer than {NAME_MAX}.

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

ENOTDIR
A component of the path prefix names an existing file that is neither a directory nor a symbolic
link to a directory, or the path argument contains at least one non-<slash> character and ends
with one or more trailing <slash> characters and the last pathname component names an existing
file that is neither a directory nor a symbolic link to a directory.

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 fchmodat() function shall fail if:

EACCES The access mode of the open file description associated with fd is not O_SEARCH and the
permissions of the directory underlying fd do not permit directory searches.

EBADF The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a
valid file descriptor open for reading or searching.

ENOTDIR
The path argument is not an absolute path and fd is a file descriptor associated with a non-
directory file.

These functions 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
The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an
intermediate result with a length that exceeds {PATH_MAX}.

The fchmodat() function may fail if:

EINVAL The value of the flag argument is invalid.

EOPNOTSUPP
The AT_SYMLINK_NOFOLLOW bit is set in the flag argument, path names a symbolic link, and the
system does not support changing the mode of a symbolic link.

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 POSIX.1‐2017 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 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 bit.

       The purpose of the fchmodat() function is to enable changing the mode of files in directories other  than
       the  current working directory without exposure to race conditions.  Any part of the path of a file could
       be changed in parallel to a call to chmod(),  resulting  in  unspecified  behavior.  By  opening  a  file
       descriptor  for  the  target  directory  and  using the fchmodat() function it can be guaranteed that the
       changed file is located relative to the desired directory. Some implementations might allow changing  the
       mode  of symbolic links. This is not supported by the interfaces in the POSIX specification. Systems with
       such support provide an interface named lchmod().  To support such implementations fchmodat() has a  flag
       parameter.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                         CHMOD(3POSIX)