copy_file_range(2) System Calls Manual copy_file_range(2)
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 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.
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 cur-
rently 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 origi-
nally 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 writ-
ing.
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 re-
source 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 tar-
get 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 repre-
sent 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. Ar-
eas 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 sup-
port 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 plat-
forms 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 filesys-
tems. 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)
Linux man-pages 6.7 2023-10-31 copy_file_range(2)
Generated by dwww version 1.16 on Tue Dec 16 06:17:51 CET 2025.