CODEWITHSUNDEEP
c
Peter Hwang
Peter Hwang

Reputation: 991

Assign a value to double pointer

I allocated memory for the r, which is a double pointer to struct. However, When I was trying to use this variable, I got an error which is saying:

Error message

read-command.c:461:7: error: lvalue required as left operand of assignment
   r+i = postfixToTree(postfix_string, &csPtr->nodeArray[i]);
       ^
make: *** [read-command.o] Error 1

Usage

r+i = postfixToTree(postfix_string, &csPtr->nodeArray[i]);

Declaration

Declaration

   int b;
    struct command** r;
    r = (struct command**) malloc(50 * sizeof(struct command*));
    for( b=0; b<50; b++){
      *(r+b)=(struct command*) malloc(50*sizeof(struct command));
    }

How do I assign a value to r?

Upvotes: 1

Views: 7902

Answers (3)

Mark Tolonen
Mark Tolonen

Reputation: 177640

Is this what you want? Make sure to include stdlib.h, the malloc casts are not needed, and *(r+b) is equivalent to r[b].

#include <stdlib.h>

struct command {
    int x; // just a definition to compile
};

int main()
{
    int b;
    struct command** r;
    r = malloc(50 * sizeof(struct command*));
    for( b=0; b<50; b++) {
        r[b] = malloc(50*sizeof(struct command));
    }
}

Upvotes: 1

ajay_t
ajay_t

Reputation: 2385

I think you are doing wrong with the pointers.

Do like this,

 int b;
    struct command** r;
    r = (struct command**) malloc(50 * sizeof(struct command*));
    for( b=0; b<50; b++){
      *(r) + b=(struct command*) malloc(50*sizeof(struct command));
    }

Upvotes: 0

Gangadhar
Gangadhar

Reputation: 10516

General syntax of C assignment.

    Lvalue=Rvalue;

Above Rvalue can be result of an expression or function call or simply another variable of same type or constant , Lvalue is a variable that can able to store Rvalue.

r+i = postfixToTree(postfix_string, &csPtr->nodeArray[i]);  

^^^

Trying to add i value to r , in the place of Lvalue.

In C there should not be any evaluation part in The Left hand side.

Add first and then assign

  r=r+i; //r+=i; 
  r = postfixToTree(postfix_string, &csPtr->nodeArray[i]);

you can write above statements As below

  r[i] = postfixToTree(postfix_string, &csPtr->nodeArray[i]);

And for every time when you make a call to function postfixToTree() 

for( b=0; b<50; b++)
  *(r+b)=(struct command*) malloc(50*sizeof(struct command));

Here you are creating 50 copies of struct command on each iteration. Doing this leaks lot of memory.

Also free() the memory which is allocated with the malloc() inside function.

Upvotes: 2

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