Thread: Malloc a struct pointer inside an array of structs

How do I malloc this?

Code:

#ifndef HASHTABLE
#define HASHTABLE


#define WORD_LENGTH 45
#define TABLE_LENGTH 26


#include <stdlib.h>


typedef struct node 
{
    char word[WORD_LENGTH + 1];
    struct node *next;
    
} node;


typedef struct table_node 
{
    char letter;
    struct node *next;
    
} table_node;


void create_hash_t( );

#endif

Code:

#include "hashTable.h"


table_node hashtable[TABLE_LENGTH];




void create_hash_t( )
{
    for (int i = 0; i < TABLE_LENGTH; i++)
    {
        if ((hashtable->next[i] = malloc(sizeof(table_node)) == NULL)) //RIGHT HERE
        { }
      
    }
}

Thanks.

You need to be more clear about what your problem is. Compiler error? Please copy-paste the entire error message with line number. Runtime error? Please post input to reproduce, expected and actual outputs, and any bizarre behavior.

1. hashtable is an array. hashtable->next is not.
2. What kind of pointer is next? What does it point to? How much memory should you allocate? Hint sizeof(table_node) is wrong.
3. I usually find it easier and more readable to separate the malloc line from the NULL check.

Note, a common idiom for using malloc is

Code:

Foo *bar;  // bar is of type pointer to Foo
bar = malloc(sizeof(*bar));
if (bar == NULL)

If bar is what you're allocating to, then using *bar in the sizeof ensures you always allocate the right amount of memory. Even if you change the type of bar, the malloc will still be correct.

Here is the error:

Code:

!arortell@gentoo ~/Development/Projects/C_Projects/Data_Structures/HashTable $ make
clang -ggdb3 -O0 -Qunused-arguments -std=gnu99 -Wall -Werror   -c -o hashTable.o hashTable.c
hashTable.c:23:27: error: assigning to 'struct node' from incompatible type 'int'
                if ((hashtable->next[i] = malloc(sizeof(table_node)) == NULL))
                                        ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
<builtin>: recipe for target 'hashTable.o' failed
make: *** [hashTable.o] Error 1

Next is a pointer to the next node. I have two types of nodes one to hold the word and a pointer to the next node. The table nodes hold the letter and a pointer to the nodes. I wanted to have an array of table nodes each holding a letter and a pointer to the nodes that hold the corresponding words 'A' points to words that start with 'A' and so on. I need to load a dictionary file into memory for a spell checker.

Ok I fixed that here is what I have now.

Code:

void create_hash_t( )
{
	hashtable = malloc(TABLE_LENGTH * sizeof(table_node));
	
	for (int i =0; i < TABLE_LENGTH; i++)
	{
		if ((hashtable->next[i] = malloc(sizeof(*node)) == NULL))
		{
			
		}
	}
}

Code:

!arortell@gentoo ~/Development/Projects/C_Projects/Data_Structures/HashTable $ make
clang -ggdb3 -O0 -Qunused-arguments -std=gnu99 -Wall -Werror   -c -o hashTable.o hashTable.c
hashTable.c:23:44: error: unexpected type name 'node': expected expression
                if ((hashtable->next[i] = malloc(sizeof(*node)) == NULL))
                                                         ^
1 error generated.
<builtin>: recipe for target 'hashTable.o' failed
make: *** [hashTable.o] Error 1

You're treating next[i] as if it were an array. But it is only a pointer at this point. However, hashtable is an array at this point so you want to dereference it as such

Code:

hashtable[i].next

Anduril, check me on this one, it's been a while since I did anything in C

I told you, hashtable->next is not an array. That means doing things like [ ] for array indexing is wrong, so hashtable->next[i] is wrong. Even if you had already allocated hashtable->next as an array, it wouldn't be an array of pointers, so you couldn't assign the result of malloc to it.

hashtable is the array. That is what needs the [index].

Notice that my example of malloc put the variable name in the sizeof with a * in front of it, not a type name.

How do you plat on returning the malloc'ed hashtable back to the calling function?

@camel-man: looks about right

I tried this.

Code:

table_node hashtable[TABLE_LENGTH];




void create_hash_t( )
{
	//hashtable = malloc(TABLE_LENGTH * sizeof(table_node));
	
	for (int i =0; i < TABLE_LENGTH; i++)
	{
		if ((hashtable[i].next = malloc(sizeof(*node)) == NULL))
		{
			
		}
	}
}

And her is what I get now

Code:

!arortell@gentoo ~/Development/Projects/C_Projects/Data_Structures/HashTable $ make
clang -ggdb3 -O0 -Qunused-arguments -std=gnu99 -Wall -Werror   -c -o hashTable.o hashTable.c
hashTable.c:15:43: error: unexpected type name 'node': expected expression
                if ((hashtable[i].next = malloc(sizeof(*node)) == NULL))
                                                        ^
1 error generated.
<builtin>: recipe for target 'hashTable.o' failed
make: *** [hashTable.o] Error 1

I tried to use sizeof(hashtable.next) like you said instead of the type but I get this.

Code:

!arortell@gentoo ~/Development/Projects/C_Projects/Data_Structures/HashTable $ make
clang -ggdb3 -O0 -Qunused-arguments -std=gnu99 -Wall -Werror   -c -o hashTable.o hashTable.c
hashTable.c:15:52: error: member reference base type 'table_node [26]' is not a structure or union
                if ((hashtable[i].next = malloc(sizeof(*hashtable.next)) == NULL))
                                                        ~~~~~~~~~^~~~~
1 error generated.
<builtin>: recipe for target 'hashTable.o' failed
make: *** [hashTable.o] Error 1

I got it thanks for your help.