Segmentation fault when using recursive sum operation - c

Question

I'm having an issue using two different recursive sum methods, where at some point both fail and return a segmentation fault error. The first, sum_a only fails on high values, passed 260000, which i do not know why, and the second sum_b always fails. Any help would be appreciated.
Thanks
The execution is done as follows : ./sum a x
Where x is the number of recursions wanted for the SUM(1:x) and a is either a or b
#include <signal.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
unsigned count=0;
void signal_segv_handler(int sig) {
char s[50];
sprintf(s, "Segmentation fault! count=%u\n", count);
write(2, s, strlen(s));
_exit(1);
}
unsigned long long *sum_b(unsigned long long x) {
unsigned long long *s;
count++;
if (x>0)
*s = *sum_b(x - 1) + x;
else
*s = 0;
return s;
}
unsigned long long sum_a(unsigned long long x) {
count++;
if (x>0)
return sum_a(x - 1) + x;
else
return 0;
}
int main(int argc, char** argv) {
unsigned long long x;
unsigned long long *sum_result;
char result[100];
static char stack[SIGSTKSZ];
stack_t ss = {
.ss_size = SIGSTKSZ,
.ss_sp = stack,
};
struct sigaction sa = {
.sa_handler = signal_segv_handler,
.sa_flags = SA_ONSTACK
};
sigaltstack(&ss, 0);
sigfillset(&sa.sa_mask);
sigaction(SIGSEGV, &sa, 0);
if (argc < 3) {
printf("Please specify the sum function to use (a or b) and the target number of integers to sum.\n");
return -1;
}
x = atoi(argv[2]);
if (strcmp(argv[1], "a") == 0)
sprintf(result, "sum_a = %llu for x=%llu, count=%u \n", sum_a(x), x, count);
else if (strcmp(argv[1], "b") == 0) {
sum_result = sum_b(x);
sprintf(result, "sum_b = %llu for x=%llu, count=%u \n", sum_result, x, count);
free(sum_result);
}
else {
printf("error: function must be a or b\n");
return -1;
}
write(1, result, strlen(result));
return 0;
}

Answer 1

unsigned long long *sum_b(unsigned long long x) {
unsigned long long *s; <------
count++;
if (x>0)
*s = *sum_b(x - 1) + x;
else
*s = 0;
return s;
}
You are using an uninitialized pointer without any mem allocation to store the sum results.

Answer 2

Both sum_a and sum_b cause stack overflow when their iterations consume more memory than available in the stack. Seems like the stack size for your program is 8192KB(you may check it in the console with ulimit -s), one sum_a iteration consumes 32 bytes, and sum_b needs 48 bytes(because of more variables than in sum_a - unsigned long long *s;, and probably 16 byte stack aligning I'm not sure), so they owerflow the stack on ~260k and ~170k iteration respectively.
But sum_b has another point of segfault, here *s = *sum_b(x - 1) + x; and here *s = 0;
this is assigning to an uninitialized pointer (wild pointer, which points to a random memory address). It happens if no stack overflow took place of course(in that case assignment operation in this lines is never reached).
Some remarks not relevant to the question:
You allocate alt stack on the main stack, if which is overflown, your handler is in a bad stance, it still may work in some situations, but you should change static char stack[SIGSTKSZ]; to malloc function.
You should avoid using not signal-safe functions in the signal handler, the list of functions you may find in the signal-safety man.
This line is probably not necessary, you may remove it sigfillset(&sa.sa_mask);
Another segfault: free(sum_result); You should always balance a malloc with a call to free.