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



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

NAME
       chmod - change mode of a file

SYNOPSIS
       #include <sys/stat.h>

       int chmod(const char *path, mode_t mode);


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 appro-
       priate 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.

       The effect on file descriptors for files open at the time of a call to chmod() is implementation-defined.

       Upon successful completion, chmod() shall mark for update the st_ctime field of the file.

RETURN VALUE
       Upon successful completion, 0 shall be returned; otherwise, -1 shall be returned and errno set to indicate  the
       error. If -1 is returned, no change to the file mode occurs.

ERRORS
       The chmod() function 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 the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.

       ENOTDIR
              A component of the path prefix is not a directory.

       ENOENT A component of path does not name an existing file or path is an empty string.

       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 chmod() function 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
              As a result of encountering a symbolic link in resolution of the path argument, the length of  the  sub-
              stituted pathname strings exceeded {PATH_MAX}.


       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  IEEE Std 1003.1-2001  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  sys-
       tem 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 lock-
       ing) 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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       chown(), mkdir(), mkfifo(), open(), stat(), statvfs(), the Base  Definitions  volume  of  IEEE Std 1003.1-2001,
       <sys/stat.h>, <sys/types.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (C) 2001-2003 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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                            CHMOD(3P)