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Full Discussion: Parameters placement on stack in C
Top Forums Programming Parameters placement on stack in C Post 303035998 by migurus on Wednesday 12th of June 2019 02:21:00 AM
Old 06-12-2019
Parameters placement on stack in C
I am trying to illustrate the reverse order of parameters on the stack when passed to a function in C:


Code:
#include <stdio.h>

void  p(int p1, int p2, double p3)
{
        printf("params:\n"
                        "1) %p offset = %li\n"
                        "2) %p offset = %li\n"
                        "3) %p\n",
                        (void *)&p1,    (void *)&p1 - (void *)&p2,
                        (void *)&p2,    (void *)&p2 - (void *)&p3,
                        (void *)&p3);
}

int main(int argc, char *argv[])
{
  struct A a = {10,11,12, {1,2,3}};

        p(-1, 0, +1);
        return(0);
}

Result is:
Code:
params:
1) 0x7ffe2f20afac offset = 4
2) 0x7ffe2f20afa8 offset = 8
3) 0x7ffe2f20afa0

This is as expected on the 64 bit system (Ubuntu 19.04)


When I pass a structure as a parameter the stack looks puzzling to me:
Code:
#include <stdio.h>
struct A {
        int     n1;
        int     n2;
        int     n3;
        int     arr[3];
};

void    p(int p1, struct A p2, double p3)
{
        printf("params:\n"
                        "1) %p offset = %li\n"
                        "2) %p offset = %li\n"
                        "3) %p\n",
                        (void *)&p1,    (void *)&p1 - (void *)&p2,
                        (void *)&p2,    (void *)&p2 - (void *)&p3,
                        (void *)&p3);
}

int main(int argc, char *argv[])
{
  struct A a = {10,11,12, {1,2,3}};
        p(-1, a, +1);
        return(0);
}

Now result is:
Code:
params:
1) 0x7ffee5f7ddbc offset = -20
2) 0x7ffee5f7ddd0 offset = 32
3) 0x7ffee5f7ddb0

Parameter 3 is close to 1 and parameter 2 is not between 1 and 3? offsets look wrong to me.Please shed some light here, thanks in advance.
The compiler is gcc 7.4.0
 

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LEARN ABOUT XFREE86

offsetof

OFFSETOF(3)						     Linux Programmer's Manual						       OFFSETOF(3)

NAME

       offsetof - offset of a structure member

SYNOPSIS

       #include <stddef.h>

       size_t offsetof(type, member);

DESCRIPTION

       The macro offsetof() returns the offset of the field member from the start of the structure type.

       This  macro  is	useful	because the sizes of the fields that compose a structure can vary across implementations, and compilers may insert
       different numbers of padding bytes between fields.  Consequently, an element's offset is not necessarily given by the sum of the  sizes	of
       the previous elements.

       A compiler error will result if member is not aligned to a byte boundary (i.e., it is a bit field).

RETURN VALUE

       offsetof() returns the offset of the given member within the given type, in units of bytes.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, C89, C99.

EXAMPLE

       On a Linux/i386 system, when compiled using the default gcc(1) options, the program below produces the following output:

	   $ ./a.out
	   offsets: i=0; c=4; d=8 a=16
	   sizeof(struct s)=16

   Program source

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

       int
       main(void)
       {
	   struct s {
	       int i;
	       char c;
	       double d;
	       char a[];
	   };

	   /* Output is compiler dependent */

	   printf("offsets: i=%zd; c=%zd; d=%zd a=%zd
",
		   offsetof(struct s, i), offsetof(struct s, c),
		   offsetof(struct s, d), offsetof(struct s, a));
	   printf("sizeof(struct s)=%zd
", sizeof(struct s));

	   exit(EXIT_SUCCESS);
       }

COLOPHON

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

GNU
								    2017-09-15							       OFFSETOF(3)
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