Thread: Segmentation Fault

Hey guys. I'm working on making a round robin process scheduling algorithm and I'm working on it bit by bit. However, I seem to have hit a snag. The following code gives me a segmentation fault after the outputs have displayed. I'm sure I'm missing something completely obvious, but I don't see it.

The data file is located here:
Data file
The pogram takes the time quanta and context switch time as arguments, eg. "rr 5 5" is 5 ms burst and 5 ns sontext switch.

Here is my code:

Code:

#include <iostream>
#include <fstream>
using namespace std;

/* Process Data Structure */
struct process {
    int arrive;             /* Process Arrival Time */
    int burst;              /* CPU Burst Time */
    float working;            /* Working time*/
    float waiting;            /* Waiting time*/
    struct process *next;
};

struct process *init_process (int pid, int burst);
float rr (struct process *proc);

int  main(int argc, char* argv[]) {

	struct process *plist,*ptmp;

	int init = 0;
	int quanta = (int) argv[1];
	int context_switch = (int) argv[2];
	float cswitch = context_switch/100;

	ifstream input;
	input.open("rrdata.txt");
	if (input.fail()) {
       cerr << "unable to open file rrdata.txt for reading" << endl;
       exit(1);
	}
	ofstream output;
	output.open("output.txt");
	if (output.fail()) {
       cerr << "unable to open file output.txt" << endl;
       exit(1);

	}

	

	//Initialize  the list of processes
	int arrive,burst;
	input>>arrive>>burst;
	output<<arrive<<' '<<burst<<endl;
	plist = init_process(arrive,burst);
  init++;
  input>>arrive>>burst;
  plist -> next = init_process(arrive,burst);ptmp = plist->next;
  init++;
  output<<arrive<<' '<<burst<<endl;
  for (int i = 0; i < 97; i++) {
    input>>arrive>>burst;
    ptmp -> next = init_process(arrive,burst);ptmp = plist->next;
    init++;
    output<<arrive<<' '<<burst<<endl;
    }
  input>>arrive>>burst;
  ptmp -> next = init_process(arrive,burst);          
	init++;
  output<<arrive<<' '<<burst<<endl;
  cout<<init<<endl;
	cout<<argv[1]<<' '<<argv[2]<<endl;
  float avgburst = rr(plist);
	return 0;
}

struct process *init_process (int arrive, int burst) {
	struct process *proc;
	proc = (struct process*) malloc(sizeof(struct process));
    if (proc == NULL) {
        printf("Fatal error: memory allocation failure.\nTerminating.\n");
        exit(1);
     };
    proc->arrive = arrive;
    proc->burst = burst;
    proc->working = 0;
    proc->waiting = 0;
    proc->next = NULL;
    return(proc);
};

float rr (struct process *proc){
  float burst = 0;
  for(int i=0; i<5; i++){
  burst = burst + proc->burst;
  proc = proc -> next;
  }
  return(burst/100);
 
};

>int quanta = (int) argv[1];
>int context_switch = (int) argv[2];
>float cswitch = context_switch/100;
None of these will work like you want them to. Simply casting a string to int doesn't perform a conversion. To actually convert a string to int (without using atoi because it sucks), you need to use strtol:

Code:

#include <cstdlib>

int quanta = static_cast<int>(std::strtol(argv[1], NULL, 0));
int context_switch = static_cast<int>(std::strtol(argv[2], NULL, 0));

Of course, it's also a very good idea to verify your assumption that argc is at least 3. The reason cswitch won't work is because you're using integer division, which truncates and precision in the value. So if context_switch is less than 100, the result will be 0.

>The following code gives me a segmentation fault after the outputs have displayed.
That's a hint. What do you do after the results are displayed?

Code:

float avgburst = rr(plist);
return 0;

Okay, you can either assume that something is going wrong in rr, or you've managed to corrupt memory so that the cleanup code after returning from main crashes. Let's look at rr first though. There's not much there, so it's a safe bet that the problem would be here:

Code:

for(int i=0; i<5; i++){

And the symptom of the problem (segmentation fault) would be here:

Code:

burst = burst + proc->burst;

Most likely, your list isn't 5 nodes long. You're walking off the end into a null pointer and then trying to dereference that pointer. If this short list is the correct behavior then you need to account for it by changing your loop:

Code:

while (proc != NULL) {

If you really were expecting a 5 node list for correct operation, then your logic is wrong somewhere.

just from a quick look i'm guessing the seg fault is in rr()....you should check to make sure there is a proc->next before setting proc to proc->next.

a few other points:
1. use "new" instead of malloc
2. no need for "struct" any more. this works fine

Code:

struct xyz{ int i; int j; };
xyz mystructure;

3. unless you just want to play with linked lists, i suggest using a vector
4. close your files. being an automatic object the destructors
should do it for you, but i think it's a good idea to make sure.

Originally Posted by Shinobi-wan

The condition is triggeresd all the time, so I need to figure that out. Any suggestions?

uh, yea, my suggestion is to read prelude's post again

>Also, my arguments for my program are always going to be two arguments greater then 5.
Can you prove that? What if you give the program to someone else to use? That's a really bad excuse for shoddy coding, but who am I to tell you otherwise.

>Any suggestions?
Well, it doesn't seg fault anymore...

Originally Posted by Prelude

>Any suggestions?
Well, it doesn't seg fault anymore...

lol...you're such a....