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

       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.

       chmod, fchmodat — change mode of a file

       #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);

       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

       S_ISUID, S_ISGID, S_ISVTX, and the file permission bits are described in

       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>:

             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().

       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.

       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.

              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.

              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.

              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.

              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.

              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.

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

           #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"

   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 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.


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

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

       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 .

       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 .

IEEE/The Open Group                   2017                             CHMOD(3P)