# chmod

## 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.
    


* * *