Segmentation fault (core dumped) when using my own struct with pointers to array

Question

Hello can you tell me why is the compiler building sucessfully when there's only one largenumber a, but when I declare another largenumber b and do the exact same things it shows Segmentation fault (core dumped) . I don't understand what's wrong? I can't declare other of type largenumber because it shows segmentation fault

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


struct largenumber{
    int numbers[200];
};
typedef struct largenumber largenumber;

void insert_number_to_largenumber(int* start, int* end, char *char_number){
  int* pointer;
  int i=0;
  printf("array elements\n");
  for(pointer=start; pointer!=end; pointer++){

    switch (char_number[i]) {
      case 48 :
        *pointer = 0;
        break;
      case 49 :
        *pointer = 1;
        break;
      case 50 :
        *pointer = 2;
        break;
      case 51 :
        *pointer = 3;
        break;
      case 52 :
        *pointer = 4;
        break;
      case 53 :
        *pointer = 5;
        break;
      case 54 :
        *pointer = 6;
        break;
      case 55 :
        *pointer = 7;
        break;
      case 56 :
        *pointer = 8;
        break;
      case 57 :
        *pointer = 9;
        break;
       default:
       fprintf(stderr, "array element is not a digit!\n");
       exit(EXIT_FAILURE);
    }

    printf("a.numbers[%d] = %d \n", i, *pointer);
    i++;


  }
}

int main()
{
  char* number1;
  printf("input first number\n");
  scanf("%s", number1);
  largenumber a;
  insert_number_to_largenumber(a.numbers, a.numbers+strlen(number1), number1);


    return 0;
}

The segmentation fault occurs because you declared a pointer number1 which is not initialized, it does not point to valid memory but you try to write the input there. change it to char number1[100] (where 100 should be a meaningful buffer size). — Osiris
– Osiris, Commented Dec 12, 2018 at 19:03
Also note that after inserting you do not know how many digits you wrote in your struct, a possible solution would be to write the least significant digit first and set everything to 0 after the most significant digit. — Osiris
– Osiris, Commented Dec 12, 2018 at 19:09

David Ranieri · Accepted Answer · 2018-12-12 19:14:04Z

5

You are using number1 uninitialized,

Use

char *number1 = malloc(32);

or

char number1[32];

before calling scanf, also use a width specifier in order to avoid buffer overflows:

scanf("%31s", number1);

edited Dec 12, 2018 at 19:14

answered Dec 12, 2018 at 18:57

David Ranieri

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1 Comment

John Bollinger Over a year ago

Or even better, declare number1 as an automatic array (char number1[32];), and avoid messing with dynamic allocation.

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