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copy_file_range(2)
COPY_FILE_RANGE(2) Linux Programmer's Manual COPY_FILE_RANGE(2)
NAME
copy_file_range - Copy a range of data from one file to another
SYNOPSIS
#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);
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 spec‐
ifies 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 appro‐
priately.
The flags argument is provided to allow for future extensions and cur‐
rently 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 origi‐
nally 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 (see open(2)) referred
to by the file descriptor fd_out.
EFBIG An attempt was made to write a file that exceeds the implementa‐
tion-defined maximum file size or the process's file size limit,
or to write at a position past the maximum allowed offset.
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.
EISDIR 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.
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, but
glibc 2.27 provides a user-space emulation when it is not available.
CONFORMING TO
The copy_file_range() system call is a nonstandard Linux and GNU exten‐
sion.
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 inodes 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>
/* 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);
close(fd_in);
close(fd_out);
exit(EXIT_SUCCESS);
}
SEE ALSO
lseek(2), sendfile(2), splice(2)
COLOPHON
This page is part of release 5.02 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/.
Linux 2019-03-06 COPY_FILE_RANGE(2)