copy_file_range(2) | System Calls Manual | copy_file_range(2) |
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:
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.
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); }
2023-02-05 | Linux man-pages 6.03 |