copy_file_range - Copy a range of data from one file to another
#define _GNU_SOURCE
#include <unistd.h>
ssize_t copy_file_range(int fd_in, loff_t *off_in,
int fd_out, loff_t *off_out,
size_t len, unsigned int flags);
The copy_file_range() system call performs an
in-kernel copy between two file descriptors without the additional cost
of transferring data from the kernel to user space and then back into
the kernel. It copies up to len
bytes of data from the source
file descriptor fd_in
to the target file descriptor
fd_out
, overwriting any data that exists within the requested
range of the target file.
The following semantics apply for off_in
, and similar
statements apply to off_out
:
If off_in
is NULL, then bytes are read from
fd_in
starting from the file offset, and the file offset is
adjusted by the number of bytes copied.
If off_in
is not NULL, then off_in
must point
to a buffer that specifies the starting offset where bytes from
fd_in
will be read. The file offset of fd_in
is not
changed, but off_in
is adjusted appropriately.
fd_in
and fd_out
can refer to the same file. If
they refer to the same file, then the source and target ranges are not
allowed to overlap.
The flags
argument is provided to allow for future
extensions and currently must be set to 0.
Upon successful completion, copy_file_range() will
return the number of bytes copied between files. This could be less than
the length originally requested. If the file offset of fd_in
is
at or past the end of file, no bytes are copied, and
copy_file_range() returns zero.
On error, copy_file_range() returns -1 and
errno
is set to indicate the error.
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <unistd.h>
/* On versions of glibc before 2.27, we must invoke copy_file_range()
using syscall(2) */
static loff_t
copy_file_range(int fd_in, loff_t *off_in, int fd_out,
loff_t *off_out, size_t len, unsigned int flags)
{
return syscall(__NR_copy_file_range, fd_in, off_in, fd_out,
off_out, len, flags);
}
int
main(int argc, char **argv)
{
int fd_in, fd_out;
struct stat stat;
loff_t len, ret;
if (argc != 3) {
fprintf(stderr, "Usage: %s <source> <destination>\n", argv[0]);
exit(EXIT_FAILURE);
}
fd_in = open(argv[1], O_RDONLY);
if (fd_in == -1) {
perror("open (argv[1])");
exit(EXIT_FAILURE);
}
if (fstat(fd_in, &stat) == -1) {
perror("fstat");
exit(EXIT_FAILURE);
}
len = stat.st_size;
fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
if (fd_out == -1) {
perror("open (argv[2])");
exit(EXIT_FAILURE);
}
do {
ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
if (ret == -1) {
perror("copy_file_range");
exit(EXIT_FAILURE);
}
len -= ret;
} while (len > 0 && ret > 0);
close(fd_in);
close(fd_out);
exit(EXIT_SUCCESS);
}
One or more file descriptors are not valid.
fd_in
is not open for reading; or fd_out
is not
open for writing.
The O_APPEND flag is set for the open file
description (see open(2)) referred to by the file
descriptor fd_out
.
An attempt was made to write at a position past the maximum file offset the kernel supports.
An attempt was made to write a range that exceeds the allowed maximum file size. The maximum file size differs between filesystem implementations and can be different from the maximum allowed file offset.
An attempt was made to write beyond the process's file size resource limit. This may also result in the process receiving a SIGXFSZ signal.
The flags
argument is not 0.
fd_in
and fd_out
refer to the same file and the
source and target ranges overlap.
Either fd_in
or fd_out
is not a regular file.
A low-level I/O error occurred while copying.
Either fd_in
or fd_out
refers to a directory.
Out of memory.
There is not enough space on the target filesystem to complete the copy.
The requested source or destination range is too large to represent in the specified data types.
fd_out
refers to an immutable file.
Either fd_in
or fd_out
refers to an active swap
file.
The files referred to by fd_in
and fd_out
are not
on the same mounted filesystem (pre Linux 5.3).
The copy_file_range() system call first appeared in Linux 4.5, but glibc 2.27 provides a user-space emulation when it is not available.
A major rework of the kernel implementation occurred in 5.3. Areas of the API that weren't clearly defined were clarified and the API bounds are much more strictly checked than on earlier kernels. Applications should target the behaviour and requirements of 5.3 kernels.
First support for cross-filesystem copies was introduced in Linux 5.3. Older kernels will return -EXDEV when cross-filesystem copies are attempted.
The copy_file_range() system call is a nonstandard Linux and GNU extension.
If fd_in
is a sparse file, then
copy_file_range() may expand any holes existing in the
requested range. Users may benefit from calling
copy_file_range() in a loop, and using the
lseek(2) SEEK_DATA and
SEEK_HOLE operations to find the locations of data
segments.
copy_file_range() gives filesystems an opportunity to implement "copy acceleration" techniques, such as the use of reflinks (i.e., two or more inodes that share pointers to the same copy-on-write disk blocks) or server-side-copy (in the case of NFS).
This page is part of release 5.10 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/.