Question

When I try to print an array contained in an instance of a struct part of the result is what I’m expecting and other parts seem to be gibberish. What is going on here?

Example output:

$./makevector test

NAME: test 16481592918288327671592918096327670000000000100011809530144490000159291832032767

My code is as follows:

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

static int vec_length = 30;

typedef struct {
  char* name;
  int* vector;
} word_entry;

static word_entry entry_one = {NULL,NULL}; 

void MakeEntry(char* word, word_entry* entry){
 entry->name = word;
 int i;
 int this_vector[vec_length];
 srand(time(NULL));
 for(i=0;i<vec_length;i++){
   this_vector[i] = rand()%2;
 } 
entry->vector = this_vector;
}

int main(int argc, char* argv[]){
  int i;
  MakeEntry(argv[1], &entry_one);
  printf("NAME: %s\n", entry_one.name); 
  for (i=0;i<vec_length;i++){
    printf("%d",entry_one.vector[i]);
  }
  printf("\n");
}

Answer 1

this_vector is a local array to MakeEntry(). When that function ends, that array goes out of scope. So entry->vector is pointing to something invalid and you get undefined behavior.