Why does srand(time(NULL)) gives me segmentation fault on main?

Question

Need some help here.

I want to understand what's happening in this code.

I'm trying to generate random numbers as tickets to the TCP_t struct created inside ccreate function.

The problem is, everytime I executed this code WITHOUT the srand(time(NULL)) it returned the same sequence of "random" numbers over and over, for example:

TID: 0 | TICKET : 103
TID: 1 | TICKET : 198

So I seeded it with time, to generate really random numbers.

When I put the seed inside the newTicket function, it brings different numbers in every execution, but the same numbers for every thread. Here is an example of output:

Execution 1:

TID: 0 | TICKET : 148
TID: 1 | TICKET : 148

Execution 2:

TID: 0 | TICKET : 96
TID: 1 | TICKET : 96

So, after some research, I found out I shouldn't seed it everytime I call rand but only once, in the beginning of the program. Now, after putting the seed inside the main function, it gives me segmentation fault, and I have NO IDEA why.

This might be a stupid question, but I really want to understand what's happening.

Is the seed screwing anything, somehow? Am I missing something? Should I generate random number in another way?

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

#define MAX_TICKET 255
#define STACK_SIZE 32000

typedef struct s_TCB {
  int threadId;
  int ticket;
  ucontext_t context;
} TCB_t;

void test();
int newTicket();
int newThreadId();
int ccreate (void* (*start)(void*), void *arg);
int threadId = 0;

int main(){
  srand(time(NULL)); //<<<============== HERE = SEGMENTATION FAULT
  ccreate((void*)&test, 0);
  ccreate((void*)&test, 0);
}

int ccreate (void* (*start)(void*), void *arg){
    if(start == NULL) return -1;

    ucontext_t threadContext;
    getcontext(&threadContext);
    makecontext(&threadContext, (void*)start, 0);
    threadContext.uc_stack.ss_sp = malloc(STACK_SIZE);
    threadContext.uc_stack.ss_size = STACK_SIZE;

    TCB_t * newThread = malloc(sizeof(TCB_t));
    if (newThread == NULL) return -1;

    int threadThreadId = newThreadId();
    newThread->threadId = threadThreadId;
    newThread->ticket = newTicket();

    printf("TID: %d | TICKET : %d\n", newThread->threadId, newThread->ticket);

    return threadThreadId;
}

int newThreadId(){
  int newThreadId = threadId;
  threadId++;
  return newThreadId;
}

int newTicket(){
  //srand(time(NULL)); //<<<============== HERE = IT PARTIALLY WORKS
  return (rand() % (MAX_TICKET+1));
}

void test(){
  printf("this is a test function");
}

Thanks to everyone who lends me a hand here.

And sorry if the code is too ugly to read. Tried to simplify it as much as I could.

Answer 1

The problem is not with srand(time(NULL)) , but with makecontext .
You can run your code through a sanatizer to confirm:

gcc-6 -fsanitize=undefined -fsanitize=address -fsanitize=leak -fsanitize-recover=all -fuse-ld=gold -o main main.c
./main 
ASAN:DEADLYSIGNAL
=================================================================
==8841==ERROR: AddressSanitizer: SEGV on unknown address 0x7fc342ade618 (pc 0x7fc340aad235 bp 0x7ffd1b945950 sp 0x7ffd1b9454f8 T0)
    #0 0x7fc340aad234 in makecontext (/lib/x86_64-linux-gnu/libc.so.6+0x47234)
    #1 0x400d2f in ccreate (/home/malko/Desktop/main+0x400d2f)
    #2 0x400c19 in main (/home/malko/Desktop/main+0x400c19)
    #3 0x7fc340a87f44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21f44)
    #4 0x400b28  (/home/malko/Desktop/main+0x400b28)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/lib/x86_64-linux-gnu/libc.so.6+0x47234) in makecontext
==8841==ABORTING

You can solve the problem by setting a stack size before making the context:

char stack[20000];
threadContext.uc_stack.ss_sp = stack;
threadContext.uc_stack.ss_size = sizeof(stack);
makecontext(&threadContext, (void*)start, 0);

---

Unrelated, but make sure you also free that malloc'd memory in your sample code.