next command in gdb is not working correctly

Question

I am simply using gdb to step through a code line by line to learn how it is working and what it is doing. It worked fine the first time I did this, but now the next command is not working right. Sometimes it goes forward, sometimes it goes backwards. It doesn't make sense. Every time I do this it seems to be the same pattern. Below is an example. Anyone know what is wrong?

Note: This is an old Mac with old program versions (That I do not have the power to update)

Please be explicit. I am fairly new to linux and programming in general.

Thanks!

Reading symbols for shared libraries ... done
(gdb) b main
Breakpoint 1 at 0x2730: file ../../../../gcc-3.4.6/libf2c/libF77/pow_zi.c, line 14.
(gdb) r
Starting program: /Users/kevin/project/ConstU/main 
nReading symbols for shared libraries +. done
Error in re-setting breakpoint 1:
Function "main" not defined.
Re-enabling shared library breakpoints: 1

Breakpoint 1, 0x00002730 in main () at main.c:34
34      {
(gdb) n
main () at main.c:42
42      for (i=0;i<DpDIM;i++) {
(gdb) n
34      {
(gdb) n
35      runstart=clock();
(gdb) n
39      Init=ReadInit(&CaseDim);/*reads in initial valies from initfile*/
(gdb) n
35      runstart=clock();
(gdb) n
39      Init=ReadInit(&CaseDim);/*reads in initial valies from initfile*/
(gdb) 

Answer 1

Anyone know what is wrong?

The symptoms you've described are very common when debugging programs built with optimization: the compiler re-arranges code in such a way that instructions from different lines become intermixed.

Make sure you compile without any -O flags, or add -O0 at the end of your compilation lines.

here is part of the make file: CC = gcc CFLAGS = -g -O2

That would do it: remove -O2 and your debugging would go much easier.