strange effect: if ... else in C influence a previous statement

Applications Programming
1

I write a short code in c

it may work well, but when I add an if ... else ... structure, a computation before the structure can not give a correct result

compilation and run can do well, but the result is wrong. Very strange

(1)
y2 = expression; //y2 = 0 always 0 error
.................

add : if(a->is_root) c = m2 - y2;
else f = m2 - y2;

(2)

            y2 = expression;  //y2 = 0    always 0         error
              .................

add : if(a->is_root) c = m2 - y2;
if(!a->is_root) f = m2 - y2;

(3)
y2 = expression; // y2 = normal value; normal
.................

add : //if(a->is_root) c = m2 - y2;
if(!a->is_root) f = m2 - y2;

(4)

            y2 = expression;          //   y2 = normal value;    normal
              .................

add : if(a->is_root) c = m2 - y2;
//if(!a->is_root) f = m2 - y2;

(5)
y2 = expression; // y2 = normal value; normal

add : //if(a->is_root) c = m2 - y2;
//if(!a->is_root) f = m2 - y2;

Trouble: can not use if... else ... normally

Question: (1) if... else ... can influence a previous statement, why?
(2) How to solve the problem?

2

It would be helpful if you could provide an compilable example of your problem.

3

This happens in a large function, which has a relatively complicated algorithm.

Simply speaking, its structure is as such:
___________________________________________________________
some basic calculation
------------------------------

                   y = expression;
                 
                  \-------------------------------

____________________________________________________________
some operations
--------------------------------

                 \--------------------------------

                 if \{
                               block 1;
                 \}

                 else\{
                 	if \{     block 2;              \}
                    else \{   block 3;              \}
	     \}

The structure of blocks 2 and 3 will change the way in which y is calculated. The result is wrong. I check it through gdb. The five results above are obtained at the same environment except for the changes in the two blocks

In many executions, y is always equal to 0. Otherwise, it will be far larger than a right value.

But when either or each of the two blocks is removed, y will obtain a normal value.

I can not understand

4

I'm afraid your description of the problem give no clue.
Without a compilable example, nobody is going to guess what is wrong with your code.

5

The only possible situation here is that block2 or block3 are overflowing a variable on your function's stack and your y var is being changed.

A function's stack is composed (growing from high memory to low memory) by variables (in reverse order), eip, esp, function name and esp.

Here is a simple example for your possible situation:

void of()
{
    char a[10];
    char b[10];
    strcpy (a, "aaaaaaaaaa");
    printf ("%s\n", a);
    strcpy(b, "bbbbbbbbbboverflow");
    printf ("%s\n", a);
}

void
main ()
{
    of();
}

As you will be able to see, variable b overflows into variable a.

6

The mistaken value is assigned to y when execute 'y = expression;'

in gdb, 'p y' will give the mistaken value after runing the above statement.

but ' p expression' gives the right value.

block2 and block3 are like this:

if(a->is_root) c = m2 - y2;
else f = m2 - y2;

As said before:
only one statement: if(a->is_root) c = m2 - y2; // y2 normal
only one statement: if(!a->is_root) f = m2 - y2; // y2 normal

but:

if(a->is_root) c = m2 - y2;
else f = m2 - y2;

or

if(a->is_root) c = m2 - y2;
if(!a->is_root) f = m2 - y2;

both make y2 abnormal. 'y2 = expression' is far away from if__ else __ structure

Thanks for your teaching

7

Hum! What kind of var is y2? What is the expression assigned to it? There are really several things that can cause that so is a bit hard to help you without the full compilable code.

8

double, the expression is one such as +-/*
It is sure that the lf_lese_ structure is compiled before y2 is actually assigned.
The compilation will make effect on y2? but before y2 is assigned, 'p y2' can not give a value.
May you list the common possilbe causes of the faults? I consulted about it but many people do not know how to interprete it.
Thanks

9

I ran into several situations similar to this question.

Is it wise to avoid inplementing complicated algorithm in a function?

What skills help overcome such troubles?