Technical Solutions

Solution:

This Solution is specific to the GDB debugger on Linux/UNIX/Mac. For other configurations, see the Debugging section of Tech Note 1605, MEX-files Guide:
http://www.mathworks.com/support/tech-notes/1600/1605.html

These instructions assume familiarity with the GDB debugger. For instructions on how to use your debugger once the MEX-file is loaded, refer to your debugger documentation.

This is a step-by-step version of the debugging overview presented here:
Debugging Fortran Language MEX-Files :: Creating Fortran MEX-Files (External Interfaces)
http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_external/f28378.html

These instructions start at the UNIX prompt, before launching MATLAB. This process involves several environments, and lists the prompts provided by each: ">>" for MATLAB, "(gdb)" for GDB, and "unix>" for the UNIX command prompt (which may be different on your machine). Ensure that you do not type these prompts; they are included to demonstrate which program receives each command.

1. First, select your compiler:
unix> mex -setup
2. Copy the YPRIMEF.F and YPRIMEFG.F files to a working directory. These files can be found in $MATLABROOT/extern/examples/mex (where $MATLABROOT is the MATLAB root directory on your machine, as returned by typing
matlabroot
at the MATLAB Command Prompt.)

Alternatively, use your own MEX-function.

3. Compile the MEX-file using -g to include debugging symbols:
unix> mex -g yprimef.F yprimefg.F
4. Load MATLAB into the debugger:
unix> matlab -Dgdb
5. Use the debugger to start the MATLAB process:
(gdb) run -nojvm
6. Enable MEX-file debugging:
>> dbmex on
7. Execute your MEX-file:
>> yprimef(1,1:4)
8. Set a breakpoint at the start of the gateway function:
(gdb) break mexfunction_
Many Fortran compilers change the name of the MEXFUNCTION subroutine; g77 changes it to "mexfunction_". For more information on how to find the equivalent name for other Fortran compilers, see here:
Debugging on UNIX :: Creating Fortran MEX-Files (External Interfaces)
http://www.mathworks.com/access/helpdesk/help/techdoc/matlab_external/f21321.html

9. Execute until you reach the breakpoint:
(gdb) continue

This will show the next line to be executed:

Continuing.

Breakpoint 1, mexfunction_ (nlhs=0x7fffffffa628, plhs=0x7fffffffafe0,
    nrhs=0x7fffffffa62c, prhs=0x7fffffffb0a0) at yprimefg.F:65
65            IF (NRHS .NE. 2) THEN
Current language:  auto; currently fortran

10. Step 1 statement:
(gdb) step

11. Set another breakpoint at line 82 in YPRIMEFG.F, and continue to that point:
(gdb) break 82
(gdb) continue

12. Print the value of some variables:
(gdb) print m
(gdb) print n

These are displayed by GDB in order, but without their names:

$1 = 1
$2 = 4

13. Finish executing the MEX-file and return to MATLAB:
(gdb) continue

14. Use DBMEX to change to the debugger:
>> dbmex stop

15. End the MATLAB and GDB session:
(gdb) quit

GDB will display the following:

The program is running. Quit anyway (and kill it)? (y or n)
Answer "y". You are now back at the UNIX prompt.