chmod(3p) — Linux manual page


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

PROLOG         top

       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         top

       chmod, fchmodat — change mode of a file

SYNOPSIS         top

       #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         top

       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>:

             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

RETURN VALUE         top

       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

ERRORS         top

       These functions shall fail if:

       EACCES Search permission is denied on a component of the path

       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

       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.

EXAMPLES         top

   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"

   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.

RATIONALE         top

       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.



SEE ALSO         top

       access(3p), chown(3p), exec(1p), fstatat(3p), fstatvfs(3p),
       mkdir(3p), mkfifo(3p), mknod(3p), open(3p)

       The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),
       sys_stat.h(0p), sys_types.h(0p)

COPYRIGHT         top

       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)

Pages that refer to this page: sys_stat.h(0p)chmod(1p)access(3p)chown(3p)exec(3p)fchmod(3p)fchmodat(3p)fstatat(3p)fstatvfs(3p)lockf(3p)mkdir(3p)mkfifo(3p)mknod(3p)open(3p)posix_spawn(3p)write(3p)