Segmentation fault: unable to fix the problem - c++

Question

I'm new to C++, and I'm trying to write a project that interacts through command line. Right now, whenever I run my main (which is the executable), I always receive a segmentation fault error when the main program finished.
Edit comment:
I'm told by tutor to use as little as C++ features such as vectors or strings ... I'm also very new to C++, so i'm trying to utilize as many basic C functions as I can.
I'm
My main function looks like this:
int main(int argc, char** argv) {
cout << "starting mvote..." << endl;
int run_flag = 1;
char* actionBuffer = (char*)malloc(100 * sizeof(char));
char* action = (char*)malloc(16 * sizeof(char));
char* readPtr;
char exit[4] = { 'e','x','i','t' };
//parse command line argumentand get the filename
char* filename = argv[2];
cout << filename;
FILE* fp;
char line[64];
//from here, I'm opening the file and read it by lines
fp = fopen(filename, "r");
if (fp == NULL) {
cout << "file not exists";
return -1;
}
while (fgets(line, 64, fp) != NULL) {
cout << line << "\n";
}
fclose(fp);
while (run_flag == 1) {
cout << "what do you want?\n " << endl;
cin.getline(actionBuffer, 1024);
if (strcmp(actionBuffer, exit) == 0) {
cout << "bye!";
run_flag = 0;
break;
}
//if not exit, Look for the space in the input
readPtr = strchr(actionBuffer, ' ');
int size = readPtr - actionBuffer;
//extract the operation
strncpy(action, actionBuffer, size);
for (int i = 0; i < size; i++) {
cout << "operation:" << action[i];
}
// depend on the operation specified before the first empty space
run_flag = 0;
}
free(actionBuffer);
free(action);
return 0;
}
Description:
I first try to open up a csv file which lies in the same folder as main, and I read the file line by line. Then, I just implement a simple command where you can type exit and quit the program.
I allocate two memory, actionBuffer and action, which are used to hold command
Problem: a segmentation fault [core dumped] always exists when I type exit and hit enter, and then the process finished.
Research: So I learned that segmentation fault is due to accessing a memory that does not belongs to me. But where in my program am I trying to access such a memory?
Any advice is appreciated! Thank you.

Answer 1

Just to give you an idea, this would be an example of C++ code
#include<iostream>
#include<fstream>
#include<string_view>
#include<string>
#include<sstream>
#include<exception>
int main(int argc, char** argv) {
std::cout << "starting mvote...\n";
//parse command line argumentand get the filename
std::string filename = argv[2]; // NO CHECKS!
std::cout << filename <<'\n';
//from here, I'm opening the file and read it by lines
{
std::ifstream ifs(filename);
if (!ifs) {
throw std::invalid_argument("file not exists");
}
std::string line;
while (std::getline(ifs, line)) {
std::cout << line << '\n';
}
}
bool run_flag = true;
while (run_flag) {
std::cout << "what do you want?\n";
std::string userInput;
std::getline(std::cin, userInput);
if (userInput == "exit") {
std::cout << "bye!\n";
return 0;
}
std::stringstream userInputSs(userInput);
std::string operation;
while(userInputSs >> operation){
std::cout << "operation: " << operation << '\n';
}
}
}