c programming dynamic allocation of struct pointers

Question

The program reads the input '1z' as (str+0)->input_name, '2y' as (str+1)->input_name, and '3x' as (str+2)->input_name. Now what i want to do is to insert a new string to (str+0)->input_name without eliminating the initial elements. So initial (str+0)->input_name would become (str+1)->input_name, (str+1)->input_name becomes (str+2)->input_name, and (str+2)->input_name becomes (str+3)->input_name. How would i do this??

Initialize items

Number of products: 3

Enter name: 1z

Enter name: 2y

Enter name: 3x

Output

1 -> 1z

2 -> 2y

3 -> 3x

=====================

Input: 4a

Output

1 -> 4a

2 -> 2y

3 -> 3x

4 -> Hello

What i want to do is:

Output

1 -> 4a

2 -> 1z

3 -> 2y

4 -> 3x

What should i do?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 1000

struct input{
  char input_name[SIZE];
};

int main(){    
  int i, startNum;
  struct input *str;
  char _startNum[SIZE];
  char _mode[SIZE];

  fputs("****Choose Menu****\n", stderr);    
  fputs("\n**Initialize items**\nNumber of products: ", stderr);
  fgets(_startNum, SIZE, stdin);
  startNum = atoi(_startNum);

  str = (struct input*)malloc(startNum * sizeof(struct input));
  if (str == NULL){
    printf("Memory allocation error\n");
    exit(EXIT_FAILURE);
  }

  for(i = 0; i < startNum; i++){
    printf("Enter name: ");
    fgets((str+i)->input_name, sizeof(struct input), stdin);
  }
  printf("\n");


  fputs("\n**Output**\n", stderr);  
  for(i = 0; i < startNum; i++){
    printf("%d -> %s", i+1, (str+i)->input_name);
  }
  printf("\n");

  startNum++;

  strcpy((str+startNum-1)->input_name, "Hello");
  for(i = startNum - 1; i = 0; i--){
    strcpy((str+i)->input_name,(str+i-1)->input_name);
  }

  fputs("Input: ", stderr); 
  fgets((str+0)->input_name, SIZE, stdin);

  fputs("\n**Output**\n", stderr);  
  for(i = 0; i < startNum; i++){
    printf("%d -> %s", i+1, (str+i)->input_name);
  }
  fputs("\n", stderr);

  return EXIT_SUCCESS;
}

Answer 1

If I have understood correctly then you need the following

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SIZE 1000

struct input{
  char input_name[SIZE];
};

int main( void )
{    
  int i, startNum;
  struct input *str;
  char _startNum[SIZE];
  char _mode[SIZE];

  fputs("****Choose Menu****\n", stderr);    
  fputs("\n**Initialize items**\nNumber of products: ", stderr);
  fgets(_startNum, SIZE, stdin);
  startNum = atoi(_startNum);

  str = (struct input*)malloc(startNum * sizeof(struct input));
  if (str == NULL){
    printf("Memory allocation error\n");
    exit(EXIT_FAILURE);
  }

  for(i = 0; i < startNum; i++){
    printf("Enter name: ");
    fgets((str+i)->input_name, SIZE, stdin);
  }
  printf("\n");


  fputs("\n**Output**\n", stderr);  
  for(i = 0; i < startNum; i++){
    printf("%d -> %s", i+1, (str+i)->input_name);
  }
  printf("\n");

  startNum++;


  struct input tmp =  realloc(str, startNum * sizeof(struct input));
  if (tmp == NULL){
    printf("Memory reallocation error\n");
    exit(EXIT_FAILURE);
  }

  str = tmp;

  strcpy((str+startNum-1)->input_name, "Hello");
  for(i = startNum - 1; i != 0; i--){
    strcpy((str+i)->input_name,(str+i-1)->input_name);
  }

  fputs("Input: ", stderr); 
  fgets((str+0)->input_name, SIZE, stdin);

  fputs("\n**Output**\n", stderr);  
  for(i = 0; i < startNum; i++){
    printf("%d -> %s", i+1, (str+i)->input_name);
  }
  fputs("\n", stderr);

  free( str );

  return EXIT_SUCCESS;
}

As for your code then it has undefined behavior because there was not allocated memory for the appended element.

And there is a typo in this loop

for(i = startNum - 1; i = 0; i--){
                      ^^^^^

where instead of comparison there is used assignment.

Also instead of this call

fgets((str+i)->input_name, sizeof(struct input), stdin);

in general you have to use

fgets((str+i)->input_name, sizeof((str+i)->input_name), stdin);

or

fgets((str+i)->input_name, SIZE, stdin);