Integer division

RealView^® Compilation Tools for μVision Compiler User Guide

Version 3.1

Home > Coding Practices > Trapping and identifying division-by-zero errors > Integer division

4.7.1. Integer division

Integer division‑by‑zero errors can be trapped and identified by re‑implementing the appropriate C library helper functions.

Trapping division-by-zero errors in code

There are two ways you can trap integer division-by-zero errors. Either:

Re-implement the C library helper function __aeabi_idiv0 so that division by zero returns some standard result, for example zero.
Integer division is implemented in code through the C library helper functions __aeabi_idiv and __aeabi_uidiv. Both functions check for division by zero.
When integer division by zero is detected, a branch to __aeabi_idiv0 is made. To trap the division by zero, therefore, you need only to place a breakpoint on __aeabi_idiv0.
See the Run-time ABI for the ARM Architecture for more information on the AEABI functions __aeabi_idiv, __aeabi_uidiv, and __aeabi_idiv0. This can be found at http://www.arm.com/products/DevTools/ABI.html.
Re-implement the C library helper function __rt_raise to deal with the signal.
By default, integer division by zero raises a signal. To intercept divide by zero, therefore, you can re-implement __rt_raise. This function has prototype:
```
void __rt_raise(int signal, int type)
```
When a divide-by-zero error occurs, __aeabi_idiv0 calls __rt_raise(2, 2). Therefore, in your implementation of __rt_raise, you must check (signal == 2) && (type == 2) to determine if division by zero has occurred.
For more information, see:
- Integer and FP helper functions in the Libraries Guide
- Exploiting the C library in the Libraries Guide
- Tailoring error signaling, error handling, and program exit in the Libraries Guide
- __rt_raise() in the Libraries Guide.

Identifying division-by-zero errors in code

On entry into __aeabi_idiv0, the link register LR contains the address of the instruction after the call to the __aeabi_uidiv division routine in your application code. To identify the offending line in your source code, you can simply look up the line of C code in the debugger at the address given by LR.

Examining parameters

To save parameters for post-mortem debugging you must intercept __aeabi_idiv0. Use the $Super$$ and $Sub$$ mechanism to intervene in all calls to __aeabi_idiv0:

$Super$$: Prefix __aeabi_idiv0 with $Super$$ to identify the original unpatched function __aeabi_idiv0. Use this to call the original function directly.
$Sub$$: Prefix __aeabi_idiv0 with $Sub$$ to identify the new function to be called in place of the original version of __aeabi_idiv0. Use this to add processing before or after the original function __aeabi_idiv0.

Example 4.4 illustrates the use of the $Super$$ and $Sub$$ mechanism to intercept __aeabi_div0. See Using $Super$$ and $Sub$$ to override symbol definitions in the Linker Guide for more information on using $Super$$ and $Sub$$.

Example 4.4. Intercepting __aeabi_div0 using $Super$$ and $Sub$$

extern void $Super$$__aeabi_idiv0(void);

/* this function is called instead of the original __aeabi_idiv0() */
void $Sub$$__aeabi_idiv0()
{
    // insert code to process a divide by zero
    ...

    // call the original __aeabi_idiv0 function
    $Super$$__aeabi_idiv0();
}

Copyright © 2007 ARM Limited. All rights reserved.	ARM DUI 0375A
PDF version

Home > Coding Practices > Trapping and identifying division-by-zero errors > Integer division