What this means

MeUmesh · 01-26-2006, 11:10 PM

Hello
Can any one tell about the meaning of this statement &(*IR)->func;

where IR is the structure pointer & func is the member funtion in the structure in C.

Stack · 01-26-2006, 11:38 PM

&(*IR)->func;

1). We dereference IR meaning we get the value which IR is pointing to
2). We then get the address of the value
3). We use the address to select the func element

Hivemind · 01-27-2006, 07:49 AM

If IR is of type pointer-to-some-struct and func is the name of a pointer to a function, that piece of code is not valid C and will not compile. Provide more context next time before asking any questions. If you're having trouble printing the address of a function pointer that's inside a struct, you should carefully inspect the follwing program and its output (it's somewhat difficult to understand what you want to do from just an invalid code snippet):

Code:

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

typedef void (* fptr)(void);

static void
myfunc(void)
{
}

struct foo
{
   char c; /* Just to make sizeof(struct foo) > sizeof(fptr) */
   fptr func;
};

int
main(void)
{
   struct foo *ptr = malloc(sizeof(struct foo));

   ptr->func = myfunc;

   printf("Printing the address of the struct: %#x\n", (unsigned int)ptr);
   printf("Print the address of the pointer to the struct: %#x\n",
          (unsigned int)&ptr);
   printf("Printing the address of the function the function pointer "
          "points to: %#x\n", (unsigned int)ptr->func);
   printf("Printing the address of the stand-alone function myfunc "
          "(will be same as above): %#x\n", (unsigned int)myfunc);
   printf("Printing the address of the function pointer (will be the same "
          "as the address of the struct if sizeof(struct foo) == "
          "sizeof(fptr)): %#x\n", (unsigned int)&(ptr->func));

   free(ptr);

   return EXIT_SUCCESS;
}

Output:

Code:

$ ./what_this_means.exe 
Printing the address of the struct: 0x460160
Print the address of the pointer to the struct: 0x22eec4
Printing the address of the function the function pointer points to: 0x401050
Printing the address of the stand-alone function myfunc (will be same as above): 0x401050
Printing the address of the function pointer (will be the same as the address
of the struct if sizeof(struct foo) == sizeof(fptr)): 0x460164

Makefile

Code:

CC = gcc
CFLAGS = -Wall -W -ansi -pedantic -g -O0 -c
LDFLAGS = -o $(EXEC)
EXEC = what_this_means.exe
OBJECTS = what_this_means.o

all: $(OBJECTS)
	$(CC) $^ $(LDFLAGS)

%.o: %.c
	$(CC) $(CFLAGS) $<

clean:
	rm -f $(OBJECTS) $(EXEC) *~ *.stackdump

MeUmesh · 01-29-2006, 10:43 PM

Can any one tell about the difference b/w Heap & stack in c programs.

xhi · 01-29-2006, 11:05 PM

declaring variables like so goes on the stack
int foo=1;
char x = 'x';
char* str = "hello";
etc....

when you use malloc to allocate memory that goes on the heap..

graemef · 01-30-2006, 07:40 AM

When a program is compiled the compiler will try and determine the amount of memory that is required for each function. The memory requirements that it can determine beforehand will be placed on the stack. Each time the function is called the required amount of memory is allocated and push'ed on the stack. When the program leaves a function the memory requirements are pop'ed from the stack. This makes the variables that were used in the function inaccessible once the function has finished. So parameters that are required will be copied into variables that are local to the calling function.

Memory that is dynamically allocated can't be predetermined before a function call and these variables may be passed from the function that allocated the memory and used in another function. Typically these are held in a different area of memory to help with the memory management of the program as a whole and hence of the computer. The heap is kept at a different location in memory because it is a much harder beast to manage than the stack.

Hope that helps a little.

graeme.

MeUmesh · 01-30-2006, 09:22 AM

Mr.graeme

U mean to say that the variable or location which are to needed untill excution of program are stored in area(heap).

will global variables stored in a heap.

graemef · 01-30-2006, 11:28 AM

The basic rule is, if the compiler can determine the amount of memory required to hold a variable then it will store it on the stack, otherwise it will need to resort to the heap.

For example the compiler will know how much memory to allocate when an integer is declared. However memory can be allocated dynamically using functions such as malloc. The malloc will receive an argument that states just how much memory is required. Now because this could be a variable, or it could be in a loop which is executed a variable number of times the compiler is unable to determine how much memory will be required and so it will ensure that at runtime the memory is allocated from the heap and not the stack.

As I understand it global variables are considered to be "local" to the main function and so they reside on the stack of the main function.

This is the case with any extern variable, it will reside on the stack of the function for where they have been declared. The compiler will then make a link from another function that defines the external variable to the offset with the function where the variable actually resides.

graeme.

MeUmesh · 01-31-2006, 12:44 AM

Mr.graeme.

Thank for your suggestion

Plz can u tell about type-safe Language & type-unsafe Language.
C is called as weak type-safe language.why ?.