Re: Gdb and assignment statements

[Jim Blandy]

Ranga Rao Ravuri <address@hidden> writes:
> Hi
> Here is little problem gdb never does
> 
> I have a line in 'c' code like 
> 
>       10      int port;
>       11      .
>       12      .
>       13      .
> >>>>  14      x = htons(x);
>       15      .
>       16      .
>       17
> If I run this part of code on sun sparc with SunOs 5.7 
> the htons is defined like 
> 
> #define htons(x) (x)
> 
> How do I break on line 14 and see what is happening ? It never breaks on
> 14, rather it does on 15. 
> All statements (x) = (x) are ignored by gdb.For each such a case the
> break point was set on next source statement. 
> Is that gcc which optimizes the expressions like 'x=x' by removing from
> the code.

If you give gcc the -g flag, and not the -O flag, it is supposed to
generate code that behaves the way one would expect from the source
code.  If you have compiled your program this way, and GDB doesn't let
you set a breakpoint on line 14, that is a bug.  However, it's most
likely a GCC bug, not a GDB bug.


File: gcc.info,  Node: Optimize Options,  Next: Preprocessor Options,  Prev: 
Debugging Options,  Up: Invoking GCC

Options That Control Optimization
=================================

   These options control various sorts of optimizations:

`-O'
`-O1'
     Optimize.  Optimizing compilation takes somewhat more time, and a
     lot more memory for a large function.

     Without `-O', the compiler's goal is to reduce the cost of
     compilation and to make debugging produce the expected results.
     Statements are independent: if you stop the program with a
     breakpoint between statements, you can then assign a new value to
     any variable or change the program counter to any other statement
     in the function and get exactly the results you would expect from
     the source code.

     Without `-O', the compiler only allocates variables declared
     `register' in registers.  The resulting compiled code is a little
     worse than produced by PCC without `-O'.