NAME¶
copy_file_range - Copy a range of data from one file to another
SYNOPSIS¶
#include <sys/syscall.h>
#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);
DESCRIPTION¶
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 file descriptor
fd_in to 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.
The
flags argument is provided to allow for future extensions and
currently must be to 0.
RETURN VALUE¶
Upon successful completion,
copy_file_range() will return the number of
bytes copied between files. This could be less than the length originally
requested.
On error,
copy_file_range() returns -1 and
errno is set to
indicate the error.
ERRORS¶
- EBADF
- One or more file descriptors are not valid; or fd_in is not open
for reading; or fd_out is not open for writing; or the
O_APPEND flag is set for the open file description referred to by
fd_out.
- EINVAL
- Requested range extends beyond the end of the source file; or the
flags argument is not 0.
- EIO
- A low-level I/O error occurred while copying.
- ENOMEM
- Out of memory.
- ENOSPC
- There is not enough space on the target filesystem to complete the
copy.
- EXDEV
- The files referred to by file_in and file_out are not on the
same mounted filesystem.
VERSIONS¶
The
copy_file_range() system call first appeared in Linux 4.5.
The
copy_file_range() system call is a nonstandard Linux extension.
NOTES¶
If
file_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 i-nodes that share pointers to the same copy-on-write disk blocks)
or server-side-copy (in the case of NFS).
EXAMPLE¶
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <unistd.h>
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);
close(fd_in);
close(fd_out);
exit(EXIT_SUCCESS);
}
SEE ALSO¶
lseek(2),
sendfile(2),
splice(2)
COLOPHON¶
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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/.