Array of pointers - need bigger

Question

How can I create an array of pointers that can store more than 1,047,141 pointers? I calculated this number using the following code:

int main(int argc, char const *argv[]) {
  long a = 0;
  while(1==1){
    char * str[a];
    printf("%ld is good.\n", a);
    a++;
    //Loop ends on Segmentation fault
  }

  return 0;
}

I am using the array of pointers to store strings. What are the alternatives?

Edit

The code above is just a way of finding the max size of an array of pointers.

One pointer holds one string, so the max number of strings I can store is 1,047,141. I need a way of storing more than 1,047,141 strings.

Possible duplicate of [When and why to use malloc](http://stackoverflow.com/questions/8800482/when-and-why-to-use-malloc) — a3f, Jul 14 '16 at 14:46

score 4 · Answer 1 · answered Jul 14 '16 at 14:47

4

Allocate the array dynamically via malloc().

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

int main(int argc, char const *argv[]) {
  long a = 0;
  while(1==1){
    char ** str = malloc(sizeof(char*) * a);
    if (str != NULL){
      printf("%ld is good.\n", a);
      free(str);
    } else {
      break;
    }
    a++;
  }

  return 0;
}

answered Jul 14 '16 at 14:47

MikeCAT

61,086
10
41
58

I also need to be able to access previously stored strings. – daka Jul 14 '16 at 14:49
Each pointer in the pointer array is assigned a string, I need to be able to access each and every one later on. – daka Jul 14 '16 at 14:52
@sudoman When is "later"? Where will you assign strings and where will you use them? Write code to do what you want. If you don't know how to do, please ask questions, not just telling what you want to do. – MikeCAT Jul 14 '16 at 14:54
@MikeCAT The code in my question is simply a way to find out the max size of an array of pointers. What I want is a way to store more than the max amount of pointers (which will be used to store strings) in a pointer array. And if thats not possible then another way of storing more than `1,047,141` strings. – daka Jul 14 '16 at 14:59
@sudoman Files on disks are commonly used to store data that is too big to store in the (main) memory. – MikeCAT Jul 14 '16 at 15:02

score 2 · Accepted Answer · answered Jul 14 '16 at 15:02

You have to allocate the arrays on the heap with malloc. This code will allocate an array of pointers long how_many_strings; and for each pointer it will allocate a string long str_length.

char** str = malloc(sizeof(char*)*how_many_strings);
for(int i = 0; i < how_many_strings; i++)
{
    str[i] = malloc(sizeof(char)*str_length);
}

The size is limited to your RAM capacity.

score 0 · Answer 3 · answered Jul 14 '16 at 15:03

0

The OP code has undefined behavior. The array isn't used, so if you use -O2 (gcc), you are just printing a as it increments. Gcc generates:

.L2:
    movq    %rbx, %rdx
    movl    $.LC0, %esi
    movl    $1, %edi
    xorl    %eax, %eax
    addq    $1, %rbx
    call    __printf_chk
    jmp .L2

It won't segfault, but the output will be quite boring.

However, with -O0, gcc generates a much longer loop (that I don't want to paste) that creates larger and larger str buffers on the stack. At some point when running this you will run out of stack space, which can cause a segfault.

answered Jul 14 '16 at 15:03

evaitl

1,285
6
15

Which part of the OP's code provokes "*undefined behavior*", and how, please? – alk Jul 14 '16 at 15:30
Every time through the loop the buffer may grow (with -O0 on gcc). Eventually this causes a stack overflow. – evaitl Jul 14 '16 at 15:32
Ahok, sure, I understood you were referring to something else on top of the SO provoking UB. – alk Jul 14 '16 at 15:47

Array of pointers - need bigger

3 Answers3