How to allocate memory for array in struct using realloc?

Question

Why I can't allocate memory for array xa this way? Compiler says that xa is not initialized in main, but I want to initialize array in function fun . In this function, I want to make the memory for the first element and put it in the array, make the memory for second and put it in the array etc. What should I change in this code?

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

typedef struct {
  int *a;

} array;

void fun(int *a,int num)
{
  int i;

  for(i=0;i<num;i++)
  {
    a=(int*)realloc(a,(i+1)*sizeof(int));
    scanf("%d",(a+i));
  }
}
int main(int argc,char *argv[]) 
{
  array x;
  int i,num;

  scanf("%d",&num); //number of elements in array *a (in struct)

  fun(x.a,num);

  for(i=0;i<num;i++)
   printf("%2d",x.a[i]); //prints elements
}

Answer 1

正如你需要传递的地址 num有scanf分配给它，你需要传递的地址xa （并相应地改变匹配参数的类型，以及使用a内部的fun ），以从fun分配给它。

Answer 2

You have two problems. The first is that xa is not set to a value before it is used. You define x at the start of main :

array x;

That creates x but does not put anything in it. Then you pass xa to fun in:

fun(x.a,num);

This says to pass the value of xa to fun . But xa does not have a value yet, so that is wrong. (By the way, make your code readable. Put spaces after commas and semicolons, the same as in normal English text.)

Then, in fun , you use a in:

a=(int*)realloc(a,(i+1)*sizeof(int));

The first time that is executed, a does not have a proper value because it was passed the value from xa , which was never set. realloc is for reallocating existing allocations. You have to pass it an initialized pointer value. (That includes NULL ; you are allowed to pass it NULL to say you want to perform an initial allocation instead of a reallocation.)

The easiest way to fix these problems is to give xa an initial value, which you can do by changing the definition of x to:

array x = { NULL };

The second problem is that fun never passes an updated value back to its caller. Your declaration of fun is:

void fun(int *a,int num)

This says that fun accepts a value that is a pointer to an int. When fun is called with:

fun(x.a,num);

only the value of xa is passed. The actual object xa is not passed, just its value. So fun has no way to return a new value for xa . There are two ways to fix this. One is to change fun to accept a pointer to xa :

void fun(int **a,int num)

and change the main routine to pass the address of xa :

fun(&x.a,num)

and change every use of a inside fun to *a :

*a=(int*)realloc(*a,(i+1)*sizeof(int));
scanf("%d",(*a+i));

The other way is to return the new pointer as the return value of fun . Change the declaration of fun to:

int *fun(int *a,int num)

and change main to update xa when fun returns it:

x.a = fun(x.a,num);

and change fun to return the value (do not change a to *a as above) by inserting this statement at the end:

return a;

Once that is done, your program will be largely working, but there are two more issues you ought to address:

• It is wasteful to realloc repeatedly when you already know the final size you want. You ought to just allocate memory once for the size you want. realloc is for when the size you need changes.
• realloc can fail. You should modify the code to handle the case when it fails and returns NULL .