Segmentation fault without even running the first line of code [closed] - c

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As you can see in the code I've added printfs everywhere for debugging and have numbered them. I have tried both repl.it and GCC(on gcc it even compiles and everything but when run, it prints the statement that asks for a user input and scans then fails) on ubuntu but both Show segmentation fault without even printing the first printf which is a "1". the code initializes an array in a parent function, sends the array to a child function, then the child prints it. I have to calculate the sum of all the elements in the mentioned array, send it back to the parent, and let the p print it too, but can't even do that if I can't check the functionality of this code
any thoughts?
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <semaphore.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sysexits.h>
#include <assert.h>
#include <time.h>
#include <sys/wait.h>
int main() {
printf("\n1\n");
int size;
sem_t *sem1, *sem2, *sem3;
mode_t perms = S_IRUSR | S_IWUSR;
int shmflags = O_CREAT | O_RDWR | O_NONBLOCK;
int semflags = O_CREAT | O_EXCL;
srand(time(NULL));
sem_t *sem = sem_open("/sem1", semflags, perms, 1);
if (sem == SEM_FAILED) {
printf("semaphore_open error \n");
exit(EXIT_SUCCESS);
}
printf("\n2\n");
pid_t pid = fork();
if (pid > 0) {//parent
printf("enter the size of array\n");
scanf("%d", &size);
int array[size];
for (int i = 0; i < size; i++) {
array[i] = rand() % 1001;
}
char str[512];
int i = 0;
int index = 0;
for (i = 0; i < size; i++) {
index += sprintf(&str[index], "%d,", array[i]);
}
printf("\n3\n");
size_t len = strlen(str);
int fd = shm_open("/shmem", shmflags, perms);
if (fd == -1) {
printf("shm_open error \n");
exit(EXIT_SUCCESS);
}
if (ftruncate(fd, len) == -1) {
printf("ftruncate error \n");
exit(EXIT_SUCCESS);
}
char *addr = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED) {
printf("mmap error \n");
exit(EXIT_SUCCESS);
}
if (close(fd) == -1) {
printf("fd close error \n");
exit(EXIT_SUCCESS);
}
sem_wait(sem);
memcpy(addr, str, strlen(str));
sem_post(sem);
wait(NULL);
} else { //child
struct stat sb;
printf("\n4\n");
int fd = shm_open("/shmem", O_RDONLY, 0);
if (fstat(fd, &sb) == -1) {
printf("fstat error \n");
exit(EXIT_SUCCESS);
}
char *addr = mmap(NULL, sb.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (addr == MAP_FAILED) {
printf("mmap error \n");
exit(EXIT_SUCCESS);
}
if (close(fd) == -1) {
printf("fd close error \n");
exit(EXIT_SUCCESS);
}
printf("\n5\n");
sem_wait(sem);
puts(addr);
sem_post(sem);
if (shm_unlink("/shmem") == -1) {
printf("unlink error \n");
exit(EXIT_SUCCESS);
}
if (sem_unlink("/sem1") == -1) {
printf("unlink error \n");
exit(EXIT_SUCCESS);
}
}
return 0;
}

Answer 1

Here are a few good candidates for undefined behavior:
scanf("%d", &size);
int array[size];
for (int i = 0; i < size; i++) {
array[i] = rand() % 1001;
}
If size is too large, say greater than a few millions, the array allocated with automatic storage will likely cause a stack overflow.
char str[512];
int i = 0;
int index = 0;
for (i = 0; i < size; i++) {
index += sprintf(&str[index], "%d,", array[i]);
}
If size entered by the user is larger than 128, the code converting the array to a string will likely cause a buffer overflow causing undefined behavior.
In the child process:
puts(addr);
addr points to the memory mapped file contents. This file does not contain a null terminator, hence if by chance its size is a multiple of the page size, the memory mapped block will not contain a null terminator and puts(addr) will read beyond the end of the block, causing undefined behavior.
Note that none of the above may cause the process to fail before the first printf call.