How to manage debug mode of application in runtime? - c

Question

Suppose we have application on Linux, pure C. Application consists of several functions - f1, f2 and f3, which receive a structure. One element of the structure is debug_mode, with values ON or OFF.
How can I switch debug mode in f1, f2 and f3 (for example, some printfs) when the structure has debug_mode == ON?
Should I always use if statements to check debug_mode for every incoming structure or there is some another best practise to not use always if statements?
For example, these could be traces in telecom products, where we could collect some trace data for a specified subscriber.

Answer 1

You can change it during runtime like this :
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#define FALSE 0
#define TRUE 1
volatile sig_atomic_t GLOBAL_DEBUG = FALSE;
void sig_handler(int signo)
{
if (signo == SIGUSR1) {
// printf("received SIGUSR1\n");
GLOBAL_DEBUG = TRUE;
} else if (signo == SIGUSR2) {
GLOBAL_DEBUG = FALSE;
// printf("received SIGUSR2\n");
}
}
int main (int argc, char **argv) {
if (signal(SIGUSR1, sig_handler) == SIG_ERR)
printf("\ncan't catch SIGUSR1\n");
if (signal(SIGUSR2, sig_handler) == SIG_ERR)
printf("\ncan't catch SIGUSR2\n");
while (TRUE) {
if (GLOBAL_DEBUG) printf ("DEBUG == TRUE\n");
printf ("Hello world\n");
sleep(1);
}
}
And the switch GLOBAL_DEBUG during runtime like this :
user#host:~ $ kill -SIGUSR2 <pid of program>
user#host:~ $ kill -SIGUSR1 <pid of program>
BTW, read signal-safety(7) (explaining why printf should be avoided inside signal handlers) and signal(7). sig_atomic_t is specified in the C11 standard (see n1570).

Answer 2

You could use signals (see signal(7) & signal-safety(7)), as explained in mbieren's answer. Of course you cannot call printf (or any non async-signal-safe function) from a signal handler, and the flag should be declared as volatile sig_atomic_t GLOBAL_DEBUG;
You could design your program otherwise. Perhaps your program has already some event loop (e.g. because it is a GUI application). Then you could add some RPCJSON service (or some HTTP one, or your own socket(7) or unix(7) based protocol, probably some text-based one) to your program. You'll find many libraries to help you (for example, libonion enables you to add some HTTP service to your program; so you might change the debug flag via your web server!).
You don't tell enough about your program to get more precise help.