Signals are the lightest AmigaOS synchronization primitive. Each task has 32 signal bits (`tc_SigAlloc`). A task blocks on `Wait(mask)` until any of the specified bits are set by another task or interrupt handler calling `Signal()`.
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## Signal Bit Constants
```c
/* exec/tasks.h — NDK39 */
/* Bits 0–15: application-allocated via AllocSignal() */
/* Bits 16–31: reserved by exec */
#define SIGB_ABORT 0 /* bit 0: break signal */
#define SIGB_CHILD 1 /* bit 1: child task signal */
/* Allocate an unused signal bit (-1 = any free bit): */
LONG sigBit = AllocSignal(-1); /* LVO -246 */
if (sigBit <0){/*all16userbitsinuse*/}
ULONG sigMask = (1L <<sigBit);
/* Free when done: */
FreeSignal(sigBit); /* LVO -252 */
```
---
## Waiting for Signals
```c
/* Block until any of the listed signals arrive: */
ULONG received = Wait(sigMask | SIGBREAKF_CTRL_C); /* LVO -318 */
if (received & SIGBREAKF_CTRL_C) {
/* user pressed CTRL-C */
cleanup_and_exit();
}
if (received & sigMask) {
/* our custom event occurred */
}
```
`Wait()` returns only after at least one bit in the mask is set. It is equivalent to sleeping — the task is moved to `TaskWait` and no CPU is consumed.
---
## Sending Signals
```c
/* Signal a task from another task or interrupt handler: */
Signal(target_task, sigMask); /* LVO -324 */
```
`Signal()` is safe from interrupt context.
---
## SetSignal — Read and Clear
```c
/* Read and clear specific signal bits atomically: */
ULONG old = SetSignal(new_bits, change_mask); /* LVO -306 */
/* old = previous state of all 32 signal bits */
/* new value = (old & ~change_mask) | (new_bits & change_mask) */
/* Check CTRL-C without blocking: */
if (SetSignal(0, SIGBREAKF_CTRL_C) & SIGBREAKF_CTRL_C) {
