static variables in multiple processes (Signals) - c

Question

I have 2 processes running test.c. There is a signal handler in test.c which executes an execlp. In test.c, I have a static variable which needs to be only initialized once, and incremented each time before the execlp call. When either process reaches 99, they exit.
Unfortunately, right now, it's not getting incremented, my guess is because there are 2 processes that each have a copy of the static variable. Here is test.c:
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <unistd.h>
static int i = 0;
static int foo(int j)
{
printf("In the foo...\n");
j++;
printf("%d\n", j);
return j;
}
void main(int argc, char *argv[])
{
int pid, pid2, k;
int *h = malloc(sizeof(int));
int g = 0;
h = &g;
static char s[15];
pid = fork();
if (pid > 0)
{
sleep(1);
}
if (pid == 0)
{
k = foo(*h);
sprintf(s, "%d", k);
if (k >= 99)
{
printf("k=99\n");
exit(0);
}
execlp("./a.out", "forktest", s, NULL);
}
pid2 = fork();
if (pid2 == 0)
{
k = foo(*h);
sprintf(s, "%d", k);
if (k >= 99)
{
printf("k=99\n");
exit(0);
}
execlp("./a.out", "forktest", s, NULL);
}
wait(pid2);
wait(pid);
}
Can anyone please explain why there is an infinite loop? Why isn't the static variable get incremented?
Thank you.

Answer 1

Use Interprocess communication concepts (pipe, fifo, shared memory) here, execlp function overwrites memory of current program with new program. So when ever you call execlp gets called your program get refreshed and starts from begining and static int i is always 0.
I recommend to use pipe Refer this.

Answer 2

You need to use memory projection (mmap function) if you want to use the concept of shared memory between process.
In your code, the variable 'h' is the shared variable between the three process.It should defined using mmap function and initialized in the main process and then incremented in the two child process.
The answers to your two questions are related: either of the two child process never exits (exit(0)) because the if(k>=99) is never statisfied. This is due to the non-shared variable h which doesn't get incremented.
I will rather use a while loop and a return type main function.
By the way, you don't need the 'g' varibale, you can initialize directly 'h'. And there is no need of declaring the function foo as static (static functions are only useful when you want them to visible only with the file where they are defined). The buffer 's' can be declared non static (it is only a buffer which contains the value of k)
Here is a modified version of your code, it compiles and works fine.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
int foo(int* j)
{
printf("In the foo...\n");
(*j)++;
printf("%d\n", *j);
return *j;
}
int main(void)
{
int pid, pid2, k;
char s[15];
int * h = (int*)mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (h == MAP_FAILED) {
printf("map failed\n");
return -1;
}
*h = 0;
pid = fork();
if (pid < 0) {
printf("fork failed pid\n");
return -1;
}
if (pid > 0) {
sleep(1);
}
else {
while(1) {
k = foo(h);
sprintf(s, "%d", k);
if (k>=99) {
printf("k>=99\n");
printf("%s\n", s);
exit(0);
}
execlp("./a.out", "forktest", s, NULL);
}
}
pid2 = fork();
if (pid2 < 0) {
printf("fork failed pid2\n");
}
if (pid2 > 0) {
sleep(1);
}
else {
while(1) {
k = foo(h);
sprintf(s, "%d", k);
if (k>=99) {
printf("k>=99\n");
exit(0);
}
execlp("./a.out", "forktest", s, NULL);
}
}
wait(pid);
wait(pid2);
return 0;
}
Here is the output (only the last strings) click on the link:
output