what is the Windows equivalent of unix's flock(h, LOCK_SH)? - windows

Question

how do you get a read-lock on Windows? like on Linux you'd do
#include <fcntl.h>
#include <sys/file.h>
int main(int argc, char **argv){
int h=open(argv[0],O_RDONLY);
flock(h,LOCK_SH);
flock(h,LOCK_UN);
close(h);
}
what's the Windows equivalent?

Answer 1

LockFileEx for basic locking. For a very soft lock there are also opportunistic locks.

Answer 2

expanding on #Anders's answer, the answer is LockFileEx, and the type of lock is decided by the dwFlags arguments,
linux=>windows
LOCK_SH => 0
LOCK_EX => LOCKFILE_EXCLUSIVE_LOCK
LOCK_NB => LOCKFILE_FAIL_IMMEDIATELY
the Windows API is quite a bit more complex (and powerful) than the Linux one, allowing you to lock "just parts" of the file, but you're free to effectively lock the whole file by just specifying that you want "the first 18446744073709551615 bytes locked" (meaning you want the first 18.45 exabytes of the file locked...), you're also able to "lock bytes that doesn't exist yet", hence a rough port of the code in the top post would be:
#include <cstdint>
#include <iostream>
#include <windows.h>
int main(int argc, char **argv){
HANDLE handle = CreateFileA(argv[0], GENERIC_READ, FILE_SHARE_READ|FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if (handle == INVALID_HANDLE_VALUE) {
throw std::runtime_error("CreateFile failed with error " + std::to_string(GetLastError()) + " for " + std::string(argv[0]));
}
const DWORD allBitsSet = ~DWORD(0);
DWORD flags = 0; // LOCK_SH = 0, LOCK_EX = LOCKFILE_EXCLUSIVE_LOCK, LOCK_NB = LOCKFILE_FAIL_IMMEDIATELY
_OVERLAPPED overlapped1 = {0};
_OVERLAPPED overlapped2 = {0};
if (!LockFileEx(handle, flags, 0, allBitsSet, allBitsSet, &overlapped1)) {
throw std::runtime_error("LockFileEx failed with error " + std::to_string(GetLastError()));
}
if(!UnlockFileEx(handle, 0, allBitsSet, allBitsSet, &overlapped2)) {
throw std::runtime_error("UnlockFileEx failed with error " + std::to_string(GetLastError()));
}
CloseHandle(handle);
}