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NAME
copy_file_range - Copy a range of data from one file to another
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
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 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.
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. 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.
ERRORS
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.
VERSIONS
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.
STANDARDS
Linux, GNU.
HISTORY
Linux 4.5, but glibc 2.27 provides a user-space emulation when it is not available.
NOTES
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.
BUGS
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.
EXAMPLES
#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);
}
SEE ALSO
lseek(2), sendfile(2), splice(2)