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<h1 class="main">Atomics</h1>
<p>V has no special support for atomics, yet, nevertheless it's possible to treat variables as atomics
by <a href="v-and-c.html">calling C</a> functions from V. The standard C11 atomic functions like <code>atomic_store()</code>
are usually defined with the help of macros and C compiler magic to provide a kind of
<em>overloaded C functions</em>.
Since V does not support overloading functions by intention there are wrapper functions defined in
C headers named <code>atomic.h</code> that are part of the V compiler infrastructure.</p>
<p>There are dedicated wrappers for all unsigned integer types and for pointers.
(<code>u8</code> is not fully supported on Windows) – the function names include the type name
as suffix. e.g. <code>C.atomic_load_ptr()</code> or <code>C.atomic_fetch_add_u64()</code>.</p>
<p>To use these functions the C header for the used OS has to be included and the functions
that are intended to be used have to be declared. Example:</p>
<div class="language-v">$if windows {
#include "@VEXEROOT/thirdparty/stdatomic/win/atomic.h"
} $else {
#include "@VEXEROOT/thirdparty/stdatomic/nix/atomic.h"
}
// declare functions we want to use - V does not parse the C header
fn C.atomic_store_u32(&u32, u32)
fn C.atomic_load_u32(&u32) u32
fn C.atomic_compare_exchange_weak_u32(&u32, &u32, u32) bool
fn C.atomic_compare_exchange_strong_u32(&u32, &u32, u32) bool
const num_iterations = 10000000
// see section "Global Variables" below
__global (
atom u32 // ordinary variable but used as atomic
)
fn change() int {
mut races_won_by_change := 0
for {
mut cmp := u32(17) // addressable value to compare with and to store the found value
// atomic version of `if atom == 17 { atom = 23 races_won_by_change++ } else { cmp = atom }`
if C.atomic_compare_exchange_strong_u32(&atom, &cmp, 23) {
races_won_by_change++
} else {
if cmp == 31 {
break
}
cmp = 17 // re-assign because overwritten with value of atom
}
}
return races_won_by_change
}
fn main() {
C.atomic_store_u32(&atom, 17)
t := spawn change()
mut races_won_by_main := 0
mut cmp17 := u32(17)
mut cmp23 := u32(23)
for i in 0 .. num_iterations {
// atomic version of `if atom == 17 { atom = 23 races_won_by_main++ }`
if C.atomic_compare_exchange_strong_u32(&atom, &cmp17, 23) {
races_won_by_main++
} else {
cmp17 = 17
}
desir := if i == num_iterations - 1 { u32(31) } else { u32(17) }
// atomic version of `for atom != 23 {} atom = desir`
for !C.atomic_compare_exchange_weak_u32(&atom, &cmp23, desir) {
cmp23 = 23
}
}
races_won_by_change := t.wait()
atom_new := C.atomic_load_u32(&atom)
println('atom: ${atom_new}, #exchanges: ${races_won_by_main + races_won_by_change}')
// prints `atom: 31, #exchanges: 10000000`)
println('races won by\n- `main()`: ${races_won_by_main}\n- `change()`: ${races_won_by_change}')
}
</div>
<p>In this example both <code>main()</code> and the spawned thread <code>change()</code> try to replace a value of <code>17</code>
in the global <code>atom</code> with a value of <code>23</code>. The replacement in the opposite direction is
done exactly 10000000 times. The last replacement will be with <code>31</code> which makes the spawned
thread finish.</p>
<p>It is not predictable how many replacements occur in which thread, but the sum will always
be 10000000. (With the non-atomic commands from the comments the value will be higher or the program
will hang – dependent on the compiler optimization used.)</p>
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