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



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

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.

SEE ALSO
       access(), chown(), exec, fstatat(), fstatvfs(), mkdir(), mkfifo(),
       mknod(), open()

       The Base Definitions volume of POSIX.1-2017, <fcntl.h>, <sys_stat.h>,
       <sys_types.h>

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(3)
