chmod, fchmod, fchmodat - change permissions of a file
#include <sys/stat.h> int chmod(const char *pathname, mode_t mode); int fchmod(int fd, mode_t mode); #include <fcntl.h> /* Definition of AT_* constants */ #include <sys/stat.h> int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): fchmod(): /* Since glibc 2.16: */ _POSIX_C_SOURCE || /* Glibc versions <= 2.19: */ _BSD_SOURCE || /* Glibc versions <= 2.15: */ _XOPEN_SOURCE >= 500 || /* Glibc 2.12 to 2.15: */ _POSIX_C_SOURCE >= 200809L fchmodat(): Since glibc 2.10: _POSIX_C_SOURCE >= 200809L Before glibc 2.10: _ATFILE_SOURCE
The chmod() and fchmod() system calls change a files mode bits. (The file mode consists of the file permission bits plus the set-user-ID, set-group-ID, and sticky bits.) These system calls differ only in how the file is specified: * chmod() changes the mode of the file specified whose pathname is given in pathname, which is dereferenced if it is a symbolic link. * fchmod() changes the mode of the file referred to by the open file descriptor fd. The new file mode is specified in mode, which is a bit mask created by ORing together zero or more of the following: S_ISUID (04000) set-user-ID (set process effective user ID on execve(2)) S_ISGID (02000) set-group-ID (set process effective group ID on execve(2); mandatory locking, as described in fcntl(2); take a new file's group from parent directory, as described in chown(2) and mkdir(2)) S_ISVTX (01000) sticky bit (restricted deletion flag, as described in unlink(2)) S_IRUSR (00400) read by owner S_IWUSR (00200) write by owner S_IXUSR (00100) execute/search by owner ("search" applies for directories, and means that entries within the directory can be accessed) S_IRGRP (00040) read by group S_IWGRP (00020) write by group S_IXGRP (00010) execute/search by group S_IROTH (00004) read by others S_IWOTH (00002) write by others S_IXOTH (00001) execute/search by others The effective UID of the calling process must match the owner of the file, or the process must be privileged (Linux: it must have the CAP_FOWNER capability). If the calling process is not privileged (Linux: does not have the CAP_FSETID capability), and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, the S_ISGID bit will be turned off, but this will not cause an error to be returned. As a security measure, depending on the filesystem, the set-user-ID and set-group-ID execution bits may be turned off if a file is written. (On Linux, this occurs if the writing process does not have the CAP_FSETID capability.) On some filesystems, only the superuser can set the sticky bit, which may have a special meaning. For the sticky bit, and for set-user-ID and set-group-ID bits on directories, see stat(2). On NFS filesystems, restricting the permissions will immediately influence already open files, because the access control is done on the server, but open files are maintained by the client. Widening the permissions may be delayed for other clients if attribute caching is enabled on them. fchmodat() The fchmodat() system call operates in exactly the same way as chmod(), except for the differences described here. If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by chmod() for a relative pathname). If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like chmod()). If pathname is absolute, then dirfd is ignored. flags can either be 0, or include the following flag: AT_SYMLINK_NOFOLLOW If pathname is a symbolic link, do not dereference it: instead operate on the link itself. This flag is not currently implemented. See openat(2) for an explanation of the need for fchmodat().
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
Depending on the filesystem, errors other than those listed below can be returned. The more general errors for chmod() are listed below: EACCES Search permission is denied on a component of the path prefix. (See also path_resolution(7).) EFAULT pathname points outside your accessible address space. EIO An I/O error occurred. ELOOP Too many symbolic links were encountered in resolving pathname. ENAMETOOLONG pathname is too long. ENOENT The file does not exist. ENOMEM Insufficient kernel memory was available. ENOTDIR A component of the path prefix is not a directory. EPERM The effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the CAP_FOWNER capability). EROFS The named file resides on a read-only filesystem. The general errors for fchmod() are listed below: EBADF The file descriptor fd is not valid. EIO See above. EPERM See above. EROFS See above. The same errors that occur for chmod() can also occur for fchmodat(). The following additional errors can occur for fchmodat(): EBADF dirfd is not a valid file descriptor. EINVAL Invalid flag specified in flags. ENOTDIR pathname is relative and dirfd is a file descriptor referring to a file other than a directory. ENOTSUP flags specified AT_SYMLINK_NOFOLLOW, which is not supported.
fchmodat() was added to Linux in kernel 2.6.16; library support was added to glibc in version 2.4.
chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008. fchmodat(): POSIX.1-2008.
C library/kernel differences The GNU C library fchmodat() wrapper function implements the POSIX- specified interface described in this page. This interface differs from the underlying Linux system call, which does not have a flags argument. Glibc notes On older kernels where fchmodat() is unavailable, the glibc wrapper function falls back to the use of chmod(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.