Struct and Malloc Help

**VinrVanyali** · 11-14-2011

Code:

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

struct nodePayload{
 int number;
};

struct node{
 struct nodePayload *payload;
 struct node *next;
} *headSingle, *tailSingle, *temp;

S_ADD(int z){
  temp = malloc(sizeof(struct node)); //Crashes here

  temp->payload->number = z;
  
  if (headSingle == NULL){
   headSingle = temp;
   tailSingle = temp;
  }
  else{
    headSingle->next = temp;
    tailSingle = temp;
  }
}

main(int argc, char *argv[]){
 headSingle = NULL;
 tailSingle = NULL;
 
 int i;
 
 for (i=0; i<10; i++){
  S_ADD(1);
 }

}

Its crashing at the malloc line. I'm guessing the sizeof(struct node) part is wrong because its not taking into account the the size of nodePayload? How should it be written?

**laserlight** · 11-14-2011

It is probably crashing on the next line because temp->payload does not point to anything.

**VinrVanyali** · 11-14-2011

Originally Posted by laserlight

It is probably crashing on the next line because temp->payload does not point to anything.

What should I have done?

Also it crashes on its third running of S_ADD.

**laserlight** · 11-14-2011

Get payload to point to something, say by using malloc.

**VinrVanyali** · 11-14-2011

Originally Posted by laserlight

Get payload to point to something, say by using malloc.

temp->payload = malloc(sizeof(struct nodePayload));
I'm very new to c. I tried adding this before the first malloc, but I don't know what I'm doing. Can you spell it out for me?

**laserlight** · 11-14-2011

What is your current code? By the way, you forgot about the return type for S_ADD. Also, rename S_ADD as fully capitalised names are conventionally reserved to macros, or at least to constants.

**VinrVanyali** · 11-14-2011

Code:

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

struct nodePayload{
 int number;
};

struct node{
 struct nodePayload *payload;
 struct node *next;
} *headSingle, *tailSingle, *temp;

s_display_inorder(){
  tailSingle->next = NULL;
  temp = headSingle;
  printf("\nSingle list:\n");
  while (temp != NULL){
   printf("Left to right output:\t%d\n", temp->payload->number);
   temp = temp->next;
  }
}

s_add(int z){

  temp->payload = malloc(sizeof(struct nodePayload));
  temp = malloc(sizeof(struct node)); //Crashes here
  
  temp->payload->number = z;
  
  if (headSingle == NULL){
   headSingle = temp;
   tailSingle = temp;
  }
  else{
    headSingle->next = temp;
    tailSingle = temp;
  }
  return;
}

main(int argc, char *argv[]){
 headSingle = NULL;
 tailSingle = NULL;
 
 int i;
 
 for (i=0; i<10; i++){
  s_add(1);
 }
 
  s_display_inorder();

}

I just want it to put the nodes in a linked list.

**laserlight** · 11-14-2011

Ah, you need to assign to temp before you assign to temp->payload.

**VinrVanyali** · 11-14-2011

Originally Posted by laserlight

Ah, you need to assign to temp before you assign to temp->payload.

Ah thank you very much; it's working perfectly now.

**laserlight** · 11-14-2011

Good to hear, but there's still work to be done, e.g., you need to free what you malloc.

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