why is copy_file_range() undefined with 4.15 Linux kernel? - linux

Question

The manpage for copy_file_range() at
https://man7.org/linux/man-pages/man2/copy_file_range.2.html
says:
#define _GNU_SOURCE
#include <unistd.h>
ssize_t copy_file_range(
(...)
The copy_file_range() system call first appeared in Linux 4.5
but when I try to compile the user sample code that contains the above, I get
warning: implicit declaration of function ‘copy_file_range’(...)
In function ‘main':
(...)undefined reference to ‘copy_file_range'
and I have Linux 4.15 :
$uname -r
4.15.0-142-generic
NOTE: I don't want to use any user-mode version which may or may not be present in glibc. I want to use the kernel version which should be available according to the manpage and also to all the Googling I could find. I want to know why is it not available - is the documentation completely wrong? Is it missing some crucial requirement for compilation? Am I completely not understanding something basic?
Full data which I think is irrelevant:
Full sample code foobar.c from the documentation:
#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;
struct stat stat;
off64_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 && ret > 0);
close(fd_in);
close(fd_out);
exit(EXIT_SUCCESS);
}
Full errors:
$ gcc foobar.c
foobar.c: In function ‘main’:
foobar.c:40:19: warning: implicit declaration of function ‘copy_file_range’; did you mean ‘sync_file_range’? [-Wimplicit-function-declaration]
40 | ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
| ^~~~~~~~~~~~~~~
| sync_file_range
/tmp/ccfnzueg.o: In function `main':
foobar.c:(.text+0x127): undefined reference to `copy_file_range'
collect2: error: ld returned 1 exit status