The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


chmod - change mode of a file


#include <sys/stat.h>

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


The chmod() function shall change S_ISUID, S_ISGID, [XSI] [Option Start]  S_ISVTX, [Option End] 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, [XSI] [Option Start]  S_ISVTX, [Option End] 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.


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.


The chmod() function shall fail if:

Search permission is denied on a component of the path prefix.
A loop exists in symbolic links encountered during resolution of the path argument.
The length of the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.
A component of the path prefix is not a directory.
A component of path does not name an existing file or path is an empty string.
The effective user ID does not match the owner of the file and the process does not have appropriate privileges.
The named file resides on a read-only file system.

The chmod() function may fail if:

A signal was caught during execution of the function.
The value of the mode argument is invalid.
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname strings exceeded {PATH_MAX}.

The following sections are informative.


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;);


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 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 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.




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


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The following changes were made to align with the IEEE P1003.1a draft standard:

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

End of informative text.

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