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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 BSD

sigstack

SIGSTACK(2)							System Calls Manual						       SIGSTACK(2)

NAME

       sigstack - set and/or get signal stack context

SYNOPSIS

       #include <signal.h>

       struct sigstack {
       caddr_t	 ss_sp;
       int  ss_onstack;
       };

       sigstack(ss, oss);
       struct sigstack *ss, *oss;

DESCRIPTION

       Sigstack allows users to define an alternate stack on which signals are to be processed.  If ss is non-zero, it specifies a signal stack on
       which to deliver signals and tells the system if the process is currently executing on that stack.  When a signal's  action  indicates  its
       handler	should execute on the signal stack (specified with a sigvec(2) call), the system checks to see if the process is currently execut-
       ing on that stack.  If the process is not currently executing on the signal stack, the system arranges a switch to the signal stack for the
       duration of the signal handler's execution.  If oss is non-zero, the current signal stack state is returned.

NOTES

       Signal stacks are not ``grown'' automatically, as is done for the normal stack.	If the stack overflows unpredictable results may occur.

RETURN VALUE

       Upon successful completion, a value of 0 is returned.  Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS

       Sigstack will fail and the signal stack context will remain unchanged if one of the following occurs.

       [EFAULT]       Either ss or oss points to memory that is not a valid part of the process address space.

SEE ALSO

       sigvec(2), setjmp(3)

4.2 Berkeley Distribution					   June 30, 1985						       SIGSTACK(2)
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