copy_file_range - Copy a range of data from one file to another
Standard C library (
libc,
-lc)
#define _GNU_SOURCE
#include <unistd.h>
ssize_t copy_file_range(int fd_in, off64_t *_Nullable off_in,
int fd_out, off64_t *_Nullable 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.
- EBADF
- One or more file descriptors are not valid.
- EBADF
-
fd_in is not open for reading; or fd_out is
not open for writing.
- EBADF
- The O_APPEND flag is set for the open file
description (see open(2)) referred to by the file descriptor
fd_out.
- EFBIG
- An attempt was made to write at a position past the maximum
file offset the kernel supports.
- EFBIG
- 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.
- EFBIG
- 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.
- EINVAL
- The flags argument is not 0.
- EINVAL
-
fd_in and fd_out refer to the same file and
the source and target ranges overlap.
- EINVAL
- Either fd_in or fd_out is not a regular
file.
- EIO
- A low-level I/O error occurred while copying.
- EISDIR
- Either fd_in or fd_out refers to a
directory.
- ENOMEM
- Out of memory.
- ENOSPC
- There is not enough space on the target filesystem to
complete the copy.
-
EOPNOTSUPP (since Linux 5.19)
- The filesystem does not support this operation.
- EOVERFLOW
- The requested source or destination range is too large to
represent in the specified data types.
- EPERM
-
fd_out refers to an immutable file.
- ETXTBSY
- Either fd_in or fd_out refers to an active
swap file.
-
EXDEV (before Linux 5.3)
- The files referred to by fd_in and fd_out are
not on the same filesystem.
-
EXDEV (since Linux 5.19)
- The files referred to by fd_in and fd_out are
not on the same filesystem, and the source and target filesystems are not
of the same type, or do not support cross-filesystem copy.
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 Linux 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.
Since Linux 5.19, cross-filesystem copies can be achieved when both filesystems
are of the same type, and that filesystem implements support for it. See BUGS
for behavior prior to Linux 5.19.
Applications should target the behaviour and requirements of Linux 5.19, that
was also backported to earlier stable kernels.
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).
In Linux 5.3 to Linux 5.18, cross-filesystem copies were implemented by the
kernel, if the operation was not supported by individual filesystems. However,
on some virtual filesystems, the call failed to copy, while still reporting
success.
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int fd_in, fd_out;
off64_t len, ret;
struct stat stat;
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);
}
lseek(2),
sendfile(2),
splice(2)