How to debug programs in Fortran
In Fortran, debuggers are used to debug programs. These debuggers check the code line by line to search for errors in the programs.
They do so by examining variable values and data object values throughout execution.
Debuggers typically support the following functionalities while debugging program execution:
- Set up breakpoints
- Step through the source code
- Set up watch points
This shot discusses two frequently used debuggers for Fortran.
GDB debugger
The GNU debugger comes with the operating system Linux. For Windows OS, it comes with a graphical user interface. It is called XXGDB in Windows.
The GNU debugger works for many programming languages. GDB is commonly used for debugging C and C++. It can also be used to debug Fortran, D, Go, and Ada.
Some of the commands in GDB are listed below.
Command | Purpose |
break | to set up a breakpoint |
run | to start execution |
cont | to continue execution |
next | to execute only the next line of code while preventing stepping into any function call |
step | to execute the next line of code while stepping into function in case of a function call |
Debugging Fortran programs with GDB
First, the g77 compile command should be modified to include the -g option.
To compile programs with debugging symbols included, run the following:
g77 -o myprogram -g myprogram.f
To set up a breakpoint:
break main__
Run the following to set up a breakpoint at a particular line number in a particular file:
break myprogram.f:mylinenumber
Here, myprogram.f and mylinenumber are the source file and line number where the breakpoints are set.
To clear breakpoints:
clear myprogram.f:mylinenumber
To step through the next line of code and skipping function calls:
next
To step through the next line of code and also step through function calls:
step
To examine variable values:
print myvariable
Here, myvariable is the variable to be examined.
To resume normal program execution:
continue
To exit GDB:
quit
DBX debugger
Another commonly used debugger is DBX. The DBX debugger is used for the operating system Linux.
Some of the commands in DBX are listed below.
Command | Purpose |
stop[var] | to set up a breakpoint when value of var changes |
stop in[proc] | to stop execution while entering a proc |
stop at[line] | to set up a breakpoint at the specified line |
run | begin execution of the program |
cont | to continue execution |
next | to execute only the next line of code while preventing stepping into any function call |
step | to execute the next line of code while stepping into function in case of a function call |
print <exp> | print the value of the expression |
where | print currently active procedures |
trace <line#> | trace execution of the linee |
trace <proc> | trace calls to the procedure |
trace <var> | trace changes to the variable |
trace <exp> at <line#> | print <exp> when <line> is reached |
status | print trace/stop's in effect |
delete <number> | remove trace or stop of given number |
screen | switch dbx to another virtual terminal |
call <proc> | call a procedure in program |
whatis <name> | print the declaration of the name |
list <line>, <line> | list source lines |
registers | display register set |
