As per the man page, I have defined _GNU_SOURCE before the inclusion of <fcntl.h:

#include <stdio.h>

#define _GNU_SOURCE    /* For Linux's fallocate(). */
#define HAVE_FALLOCATE 1

#include <fcntl.h>

int main(void)
{
    printf("%dn", FALLOC_FL_KEEP_SIZE);
}

Yet this code fails to compile with:

error: `FALLOC_FL_KEEP_SIZE` undeclared (first use in this function)

I compiled the code with:

gcc-13 -std=gnu2x c.c

Some stats about the environment:

OS: Linux Mint 21.2 x86_64
Kernel: 5.15.0-112-generic
GLIBC Version: GNU C Library (Ubuntu GLIBC 2.35-0ubuntu3.8) stable release 2.35
Compiler: gcc version 13.1.0 (Ubuntu 13.1.0-8ubuntu1~22.04)

Running grep -r FALLOC_FL_KEEP_SIZE /usr/include returned:

/usr/include/linux/falloc.h:#define FALLOC_FL_KEEP_SIZE 0x01 /* default is extend size */
/usr/include/linux/falloc.h: * with fallocate. Flag FALLOC_FL_KEEP_SIZE should cause the inode