Help needed: C Code freezes mid execution

[Shiveen Pandita]

Hi, I'm working on a network queue simulator. Although, I'm not really well versed or experienced in programming in C, I was able to get together the following code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>

#define PACKET_MEAN_SIZE 1250 // Packet Size in bytes
#define QUEUE_SIZE 1000000 // Buffer Size for the queuing system

struct node{
double time;
int size;
int seq;
char *status;
struct node* next;
};

int length(struct node* start){
struct node* current = start;
int count = 0;
while(current!=NULL)
{
count ++;
current = current->next;
}
return count;
}

typedef struct queue {
int value;          /* the value of the current item */
int used;           /* have we used 'value' yet? */
struct queue *next; /* next item */
} queue;

void free_queue(queue *q);
void enqueue(queue *q, int val);
void dequeue(queue **q);
int queue_empty(queue *q);

void free_queue(queue *q)
{
    queue *tmp;
    while(q->next != NULL) {
        tmp = q->next;
        free(q);
        q = tmp;
    }
}

void enqueue(queue *q, int val)
{
    if(q->used == 0) {
        q->value = val;
        q->used = 1;
        return;
    }
    while(q->next != NULL) q = q->next;
    q->next = malloc(sizeof(struct queue));
    q->next->used = 1;
    q->next->value = val;
}

void dequeue(queue **q)
{
    queue *tmp = *q;
   
    if(tmp->next != NULL) {
        *q = tmp->next;
        free(tmp);
    }
    else {
        tmp->used = 0;
	printf("\nQueue is empty");
    }
 }

int queue_empty(queue *q)
{
    return q->used == 0;
}



void main(){

printf("\nIntitializing simulation...\n");
int lambda = 1;
int ton_mean = 9;
int tof_mean = 1;

int ton = 0; 	// Variable Initialization 
int tof = 0;
int pkt = 0;
int n = 0;
int s = 0;
int iart = 0;
int nsim = 5;
int l = 0;
int len = 0;
int snc = 0;
int pkn = 1;

struct timeval timeVal;

queue *val = malloc(sizeof(struct queue));
int sim_time = 0; 	// initialization of the simulation time.

printf("Simulation Started\n");
struct node* head;
struct node* list;

head = malloc(sizeof(struct node));
list = malloc(sizeof(struct node));

do{

if (nsim == 5){
gettimeofday(&timeVal, NULL); // getting the value for seeding the rv.
srand48(timeVal.tv_usec); // seeding the rv.
ton = round(ton_mean*log(drand48()))*-1; // finding the exponentially 						 distributed value of 'on time'

gettimeofday(&timeVal, NULL);
srand48(timeVal.tv_usec);
iart = round(lambda*log(drand48()))*-1;
head->time = sim_time;
head->seq = snc;
head->status = "smooth source turned on";
printf("\ntrigger packet sent at: %d", sim_time);
head->next = list;
snc = snc + 1;
}
else{
list->time = sim_time;
list->seq = snc;
list->status = "source turned on";
printf("\nsource turned on at: %dusec",sim_time);
list->next = malloc(sizeof(struct node));
list = list->next;
snc = snc + 1;
gettimeofday(&timeVal, NULL);
srand48(timeVal.tv_usec);
ton = round(ton_mean*log(drand48()))*-1;
iart = round(lambda*log(drand48()))*-1;
}

for (n = 0; n <= ton; n++){

dequeue(&val);	//Emptying the queue.

if (iart == 0){	//checking if packets arrive or not in this instant
printf("\nnumber of packets arriving are: %d",iart);}

else{
printf("\nNumber of packets arriving in else are: %d",iart);	
for (l = 1; l<=iart; l++){	//Sending the amount of packets
list->time = sim_time;
list->seq = snc;
snc = snc +1;
list->status = "Packet Arrival";
gettimeofday(&timeVal, NULL);
srand48(timeVal.tv_usec);
pkt = round(PACKET_MEAN_SIZE*log(drand48()))*-1;
enqueue(val,pkt);
printf("\npacket number %d arrived at %d usec",pkn,sim_time);
list->next = malloc(sizeof(struct node));
list = list->next;
pkn = pkn + 1;
}
}
gettimeofday(&timeVal, NULL);
srand48(timeVal.tv_usec);
iart = round(lambda*log(drand48()))*-1;
sim_time = sim_time + 1;
}

tof = round(tof_mean*log(drand48()))*-1;
list->time = sim_time;
list->seq = snc;
list->status = "source turned of";
list->next = malloc(sizeof(struct node));
list = list->next;
printf("\nSource is turned off at: %d", sim_time);
snc = snc + 1;

for (s=0; s<=tof;s++){
dequeue(&val);
sim_time = sim_time + 1;}

printf("\nValue of iteration is: %d",nsim);
nsim = nsim - 1;
} while(nsim > 0);

len = length(head);
printf("\nLength of the event list is: %d\n",len);
free_queue(val);
}

The problem I keep on getting is that sometimes the code works just fine (VERY RARELY) while other times, it works fine for few iterations of the on timer (ton) but stops short of completing. This is actually an assignment for a networking lab.

ANY kind of help is appreciated. Just any. Even if you point me in right direction, that'll be heaps good. Thanks!

[Campbell Ritchie]

As usual, you cannot expect people to read or understand unindented code. Please go back and indent the code correctly with spaces not tabs.
I tried running your code, but something went wrong with my use of log; it said it couldn’t find the function, even though I made sure to write #include <math.h>.
Try putting some debugging code in: lots of print statements.
Also divide that main() function; it looks too long to me.