copy_file_range - Copy a range of data from one file to another
Standard C library (libc, -lc)
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <unistd.h>
ssize_t copy_file_range(int fd_in, off_t *_Nullable off_in,
int fd_out, off_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.
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#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;
off_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);
}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 filesystem does not support this operation.
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 filesystem.
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.
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.
Linux, GNU.
Linux 4.5, but glibc 2.27 provides a user-space emulation when it is not available.
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).
_FILE_OFFSET_BITS should be defined to be 64 in code
that uses non-null off_in or off_out or that takes the
address of copy_file_range, if the code is intended to
be portable to traditional 32-bit x86 and ARM platforms where
off_t's width defaults to 32 bits.
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.