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 relative to directory file

SYNOPSIS         top

       #include <sys/stat.h>

       int chmod(const char *path, mode_t mode);
       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 file descriptor was opened without O_SEARCH, the
       function shall check whether directory searches are permitted using
       the current permissions of the directory underlying the file
       descriptor. If the file descriptor was opened with 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().

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 occurs.

ERRORS         top

       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

       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 fd was not opened with 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

           #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

           #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.1‐2008 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

       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.1‐2008, 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, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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                 2013                           CHMOD(3P)

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