CLONEFILE(2) System Calls Manual CLONEFILE(2)

clonefilecreate copy on write clones of files

#include <sys/attr.h>
#include <sys/clonefile.h>


int
clonefile(const char * src, const char * dst, int flags);

clonefileat(int src_dirfd, const char * src, int dst_dirfd, const char * dst, int flags);

fclonefileat(int srcfd, int dst_dirfd, const char * dst, int flags);

The () function causes the named file src to be cloned to the named file dst. The cloned file dst shares its data blocks with the src file but has its own copy of attributes and extended attributes which are identical to those of the named file src with the exceptions listed below

  1. ownership information is set as it would be if dst was created by openat(2) or mkdirat(2) or symlinkat(2) if the current user does not have privileges to change ownership. If the optional flag CLONE_NOOWNERCOPY is passed, the ownership information is the same as if the the current user does not have privileges to change ownership

  2. setuid and setgid bits are turned off in the mode bits for regular files.

Subsequent writes to either the original or cloned file are private to the file being modified (copy-on-write). The named file dst must not exist for the call to be successful. Since the clonefile() system call might not allocate new storage for data blocks, it is possible for a subsequent overwrite of an existing data block to return ENOSPC. If src names a directory, the directory hierarchy is cloned as if each item was cloned individually. However, the use of copyfile(3) is more appropriate for copying large directory hierarchies instead of clonefile(2)

The () function is equivalent to clonefile() except in the case where either src or dst specifies a relative path. If src is a relative path, the file to be cloned is located relative to the directory associated with the file descriptor src_dirfd instead of the current working directory. If dst is a relative path, the same happens only relative to the directory associated with dst_dirfd. If clonefileat() is passed the special value AT_FDCWD in either the src_dirfd or dst_dirfd parameters, the current working directory is used in the determination of the file for the respective path parameter.

The () function is similar to clonefileat() except that the source is identified by file descriptor srcfd rather than a path (as in clonefile() or clonefileat())

The flags parameter specifies the options that can be passed. Options are specified in the flags argument by or'ing the following values:

CLONE_NOFOLLOW
Don't follow the src file if it is a symbolic link (applicable only if the source is not a directory). The symbolic link is itself cloned if src names a symbolic link.
CLONE_NOOWNERCOPY
Don't copy ownership information from the source when run called with superuser privileges. The symbolic link is itself cloned if src names a symbolic link.

The The (), clonefileat() and fclonefileat() functions are expected to be atomic i.e. the system call will result all new objects being created successfully or no new objects will be created. POSIX conforming applications cannot use clonefile().

Upon successful completion, clonefile() returns 0. Otherwise, a value of -1 is returned and errno is set to indicate the error.

Not all volumes support clonefile(). A volume can be tested for clonefile() support by using getattrlist(2) to get the volume capabilities attribute ATTR_VOL_CAPABILITIES, and then testing the VOL_CAP_INT_CLONE flag.

The clonefile() function will fail if:

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Read permissions are denied on the source or write permissions are on the destination parent.
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The underlying filesystem does not support this call.
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The named file dst exists.
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src and dst are not on the same filesystem.
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The value of the flags parameter is invalid.
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There is no free space remaining on the file system containing the file.
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An I/O error occurred while reading from or writing to the file system.
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The calling process does not have appropriate privileges.
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src is the root of the Filesystem.
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A loop exists in symbolic links encountered during in resolution of the src or dst path arguments.
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The requested operation requires writing in a directory on a read-only file system.
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The length of a component of a pathname is longer than {NAME_MAX}.
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A component of path src or the path dst does not name an existing file or path is an empty string.
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A component of path prefix of either src or dst 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.
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A component of either pathname refers to a “dataless” directory that requires materialization and the I/O policy of the current thread or process disallows dataless directory materialization (see getiopolicy_np(3)).
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The src pathname refers to a “dataless” file that must be materialized before being cloned and the I/O policy of the current thread or process disallows file materialization (see getiopolicy_np(3)).

In addition, the clonefileat() or fclonefileat() functions may fail with the following errors

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The src or dst argument does not specify an absolute path and the src_dirfd or dst_dirfd argument is neither AT_FDCWD nor a valid file descriptor open for searching.
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The src or dst argument is not an absolute path and src_dirfd or dst_dirfd is neither AT_FDCWD nor a file descriptor associated with a directory.

copyfile(3) chown(2)

The clonefile(), clonefileat() and fclonefileat() function calls appeared in OS X version 10.12

June 3, 2021 Darwin