passing tables as arguments to functions.

kobi_hikri · #1 25-Jul-2005, 06:57

Again, the assmbler project ...

I have these tables defined :

CPP / C++ / C Code:

//The data table.
int *data_table;
//The instruction table.
struct inode *instruction_table;

with these definitions atop :

CPP / C++ / C Code:

typedef enum {a,e,r} instruction_type;
//This structure is defined to hold an instruction.
struct inode
{
	int					the_instruction:16;
	instruction_type	the_instruction_type;
};

Now, in my main function, I add data and instructions into the tables.
the data and instructions are added o.k. (I can print the data from the main function).

Now, I pass the tables to another function to create output files.

This function is defined as :

CPP / C++ / C Code:

int create_object_file(char *file_name,int *data_table,struct inode *instruction_table,int DC,int IC)
{
	int result = PASSED;
	FILE *objfp;
	//Create object file.
	objfp = fopen(add_suffix(file_name,".ob"),"w");
	if (objfp == NULL)
	{
		fprintf(stderr,"Couldn't create object file : %s.\n",add_suffix(file_name,".ob"));
		result = FAILED;
	}
	else
	{
		int index;
		//File created succesfully.
		//Print header info.
		//Print instruction table.
		for (index = 0;index < IC;index++)
			fprintf(objfp,"%.6o\t%.6o\t%c\n",index,instruction_table[index].the_instruction,instruction_table[index].the_instruction_type);
		//Print data table.
		for (index = 0;index < DC;index++)
			fprintf(objfp,"%.6o\t%.6o\n",index + IC,data_table[index]);
	}
	return(result);

}

and I call to it from main like this :

CPP / C++ / C Code:

//Create object file.
create_object_file(filename,data_table,instruction_table,DC,IC);

Now, after debugging I can tell that the data and instruction tables are not passed o.k. since these calls :

CPP / C++ / C Code:

for (index = 0;index < IC;index++)
   fprintf(objfp,"%.6o\t%.6o\t%c\n",index,instruction_table[index].the_instruction,instruction_table[index].the_instruction_type);
//Print data table.
for (index = 0;index < DC;index++)
   fprintf(objfp,"%.6o\t%.6o\n",index + IC,data_table[index]);

generate access violation ...

What am I doing wrong ? How should I pass the tables correctly ?

Best regards,
Kobi.

Dave Sinkula · #2 25-Jul-2005, 08:04

CPP / C++ / C Code:

struct inode *instruction_table;

Where do you allocate memory into which you might put stuff?

kobi_hikri · #3 25-Jul-2005, 10:22

Quote:

Originally Posted by Dave Sinkula

CPP / C++ / C Code:

struct inode *instruction_table;

Where do you allocate memory into which you might put stuff?

The memory allocation is done within the function :

CPP / C++ / C Code:

struct inode *add_instruction(struct inode *instruction_table,int instruction
							  ,instruction_type the_instruction_type,int *instruction_counter)
{
	//Increment instruction counter.
	(*instruction_counter)++;
	//(Re)allocate the memory for the instruction table.
	instruction_table = (struct inode*)realloc(instruction_table,
						(*instruction_counter) * sizeof(struct inode));
	if (instruction_table == NULL)
	{
		//Memory allocation failed.
		fprintf(stderr,"Error : Couldn't (re)allocate memory for instruction table.\n");
		return (NULL);
	}
	else
	{
		//Memory (re)allocation success.
		instruction_table[(*instruction_counter) - 1].the_instruction = instruction;
		instruction_table[(*instruction_counter) - 1].the_instruction_type = the_instruction_type;
	}
	return (instruction_table);
}

And :

CPP / C++ / C Code:

int *add_data(int *data_table,int data,int *data_counter)
{
	//Increment data counter.
	(*data_counter)++;
	//(Re)allocate the memory for the data table.
	data_table = (int*)realloc(data_table,(*data_counter) * sizeof(int));
	if (data_table == NULL)
	{
		//Memory allocation failed.
		fprintf(stderr,"Error : Couldn't (re)allocate memory for data table.\n");
		return (NULL);
	}
	else
		//Memory (re)allocation success.
		data_table[(*data_counter) - 1] = data;
	return (data_table);
}

and the call to these functions (an example from the code):

CPP / C++ / C Code:

data_table = add_data(data_table,iterator->token[index],DC);

kobi_hikri · #4 25-Jul-2005, 11:33

Why does my_table doesn't hold the changes ?

CPP / C++ / C Code:

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

void add_numbers(int *the_table,int *DC);

void main()
{
	int *my_table = NULL;
	int DC = 0;
	add_numbers(my_table,&DC);
	if (my_table == NULL)
		printf("NULL\n");
}

void add_numbers(int *the_table,int *DC)
{
	the_table = add_data(the_table,1,DC);
}

The function add_data if defined :

CPP / C++ / C Code:

int *add_data(int *data_table,int data,int *data_counter)
{
	//Increment data counter.
	(*data_counter)++;
	//(Re)allocate the memory for the data table.
	data_table = (int*)realloc(data_table,(*data_counter) * sizeof(int));
	if (data_table == NULL)
	{
		//Memory allocation failed.
		fprintf(stderr,"Error : Couldn't (re)allocate memory for data table.\n");
		return (NULL);
	}
	else
		//Memory (re)allocation success.
		data_table[(*data_counter) - 1] = data;
	return (data_table);
}

nkhambal · #5 25-Jul-2005, 12:25

Quote:

Originally Posted by kobi_hikri

Why does my_table doesn't hold the changes ?

CPP / C++ / C Code:

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

void add_numbers(int *the_table,int *DC);

void main()
{
	int *my_table = NULL;
	int DC = 0;
	add_numbers(my_table,&DC);
	if (my_table == NULL)
		printf("NULL\n");
}

void add_numbers(int *the_table,int *DC)
{
	the_table = add_data(the_table,1,DC);
}

The function add_data if defined :

CPP / C++ / C Code:

int *add_data(int *data_table,int data,int *data_counter)
{
	//Increment data counter.
	(*data_counter)++;
	//(Re)allocate the memory for the data table.
	data_table = (int*)realloc(data_table,(*data_counter) * sizeof(int));
	if (data_table == NULL)
	{
		//Memory allocation failed.
		fprintf(stderr,"Error : Couldn't (re)allocate memory for data table.\n");
		return (NULL);
	}
	else
		//Memory (re)allocation success.
		data_table[(*data_counter) - 1] = data;
	return (data_table);
}

You should pass a pointer to the pointer variable my_table in function add_numbers and not the actual pointer.

CPP / C++ / C Code:

int *my_table = NULL;
int DC = 0;
add_numbers(&my_table,&DC);

CPP / C++ / C Code:

void add_numbers(int **the_table,int *DC)
{
	(*the_table) = add_data((*the_table),1,DC);
}

CPP / C++ / C Code:

int *add_data(int *data_table,int data,int *data_counter)
{
	//Increment data counter.
	(*data_counter)++;
	//(Re)allocate the memory for the data table.
	data_table = (int*)realloc(data_table,(*data_counter) * sizeof(int));
	if (data_table == NULL)
	{
		//Memory allocation failed.
		fprintf(stderr,"Error : Couldn't (re)allocate memory for data table.\n");
		return (NULL);
	}
	else
		//Memory (re)allocation success.
		data_table[(*data_counter) - 1] = data;
	return (data_table);
}

Dave Sinkula · #6 25-Jul-2005, 12:36

First, with regard to your use of realloc, is this item to note.

It might be easier if you could post a minimal subset of your code, and any necessary input file, that demonstrates the problem. If I were to go about cobbling together something, it might not be what you actually have.

kobi_hikri · #7 27-Jul-2005, 03:28

Quote:

Originally Posted by Dave Sinkula

First, with regard to your use of realloc, is this item to note.

It might be easier if you could post a minimal subset of your code, and any necessary input file, that demonstrates the problem. If I were to go about cobbling together something, it might not be what you actually have.

Thanks.
I'll be brief next time.

kobi_hikri · #8 27-Jul-2005, 03:31

Quote:

Originally Posted by nkhambal

You should pass a pointer to the pointer variable my_table in function add_numbers and not the actual pointer.

I thought that was the answer but I still don't understand why... And I never do things I don't understand

Can you please explain why passing a plain pointer to a variable isn't "good enough" ?

Dave Sinkula · #9 27-Jul-2005, 07:29

Quote:

Originally Posted by kobi_hikri

I thought that was the answer but I still don't understand why... And I never do things I don't understand

Can you please explain why passing a plain pointer to a variable isn't "good enough" ?

In C, parameters are passed by value. So you give the function a copy of the pointer. You modify the copy. The original remains unchanged. If you want to actually modify the value, you can simulate pass by reference using a pointer and modify what it points to.

[edit]Very much like this

CPP / C++ / C Code:

#include <stdio.h>

void foo(int x)
{
   x += 10;
   printf("x = %d\n", x);
}

void bar(int *x)
{
   *x += 10;
   printf("x = %d\n", *x);
}

int main(void)
{
   int i = 50;
   printf("i = %d\n", i);
   foo(i);
   printf("i = %d\n", i);
   bar(&i);
   printf("i = %d\n", i);
   return 0;
}

/* my output
i = 50
x = 60
i = 50
x = 60
i = 60
*/

Remember that pointers are variables too, and behave much the same.
[edit2]

CPP / C++ / C Code:

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

void foo(void *x)
{
   printf("x = %p\n", x);
   x = malloc(10);
   printf("x = %p\n", x);
}

void bar(int **x)
{
   printf("*x = %p\n", *x);
   *x = malloc(10);
   printf("*x = %p\n", *x);
}

int main(void)
{
   void *p = NULL;
   printf("p = %p\n", p);
   foo(p);
   printf("p = %p\n", p);
   bar(&p);
   printf("p = %p\n", p);
   return 0;
}

/* my output
p = 00000000
x = 00000000
x = 007A323C
p = 00000000
*x = 00000000
*x = 007A324C
p = 007A324C
*/

[edit3]The above is an example only -- its foo leaks memory, the bar doesn't, but the memory should be freed.

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passing tables as arguments to functions.

The question in a different form.

I Understand.

Thanks for the example.