Signal safety
- C course
- Advanced
- Lesson 17/83
- 5 min
Learn signal safety and reentrant functions.
Signal safety
A signal handler can interrupt the program at any point. So inside a handler you should only call reentrant functions.
Functions like malloc, printf, fopen are not reentrant; they hold internal static state. In a handler, write and _exit are safe. For a flag variable you use volatile sig_atomic_t; its read and write are indivisible.
volatile sig_atomic_t
volatile tells the compiler to read the variable from memory on every access without optimization. sig_atomic_t is a type guaranteed to have indivisible read and write.
The handler sets a flag; the main loop checks the flag and reacts. This is the core pattern of safe signal communication. For example, declare volatile sig_atomic_t flag = 0; and set flag = 1; in the handler. The main code waits with while (!flag).
Practice Safe flag
Declare volatile sig_atomic_t flag = 0;. Write void handler(int sig) with flag = 1;. Register with signal(SIGUSR1, handler);. Call raise(SIGUSR1); and print with printf("flag=%d\n", flag);.
Starter code
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
// Define flag variable
// Write handler
// Register with signal, raise
// Print flag
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
volatile sig_atomic_t flag = 0;
void handler(int sig) {
flag = 1;
}
int main(void) {
signal(SIGUSR1, handler);
raise(SIGUSR1);
printf("flag=%d\n", flag);
return 0;
}Expected output
flag=1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match safety concepts
| Term | Meaning |
|---|---|
volatile |
Prevents optimization |
sig_atomic_t |
Indivisible type |
write |
Safe function |
malloc |
Not safe |
2 Which function is safe in a signal handler?
- write Correct answer
- printf
- malloc
- fopen
Explanation: write is a reentrant system call.
3 Flag type
volatile ____ flag = 0;volatile sig_atomic_t flag = 0;Explanation: sig_atomic_t is used for indivisible flags in handlers.