Debugging a Program With dbx

Chapter 16

Debugging Fortran Using dbx

This chapter introduces dbx features you might use with Fortran. Sample requests to dbx are also included to provide you with assistance when debugging Fortran code using dbx.

This chapter includes the following topics:

• Debugging Fortran
• Debugging Segmentation Faults
• Locating Exceptions
• Tracing Calls
• Working With Arrays
• Showing Intrinsic Functions
• Showing Complex Expressions
• Showing Logical Operators
• Viewing Fortran 95 Derived Types
• Pointer to Fortran 95 Derived Type

Debugging Fortran

The following tips and general concepts are provided to help you while debugging Fortran programs.

Current Procedure and File

During a debug session, dbx defines a procedure and a source file as current. Requests to set breakpoints and to print or set variables are interpreted relative to the current function and file. Thus, stop at 5 sets different breakpoints, depending on which file is current.

Uppercase Letters

If your program has uppercase letters in any identifiers, dbx recognizes them. You need not provide case-sensitive or case-insensitive commands, as in some earlier versions.

FORTRAN 77, Fortran 95, and dbx must be in the same case-sensitive or case-insensitive mode:

• Compile and debug in case-insensitive mode without the -U option. The default value of the dbx input_case_sensitive environment variable is then false.

If the source has a variable named LAST, then in dbx, both the print LAST or print last commands work. FORTRAN 77, Fortran 95, and dbx consider LAST and last to be the same, as requested.

• Compile and debug in case-sensitive mode using -U. The default value of the dbx input_case_sensitive environment variable is then true.

If the source has a variable named LAST and one named last, then in dbx, print LAST works, but print last does not work. FORTRAN 77, Fortran 95, and dbx distinguish between LAST and last, as requested.

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Note – File or directory names are always case-sensitive in dbx, even if you have set the dbx input-_case_sensitive environment variable to false.

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Optimized Programs

To debug optimized programs:

• Compile the main program with -g but without -On.
• Compile every other routine of the program with the appropriate -On.
• Start the execution under dbx.
• Use the fix -g any.f command on the routine you want to debug, but without -On.
• Use the cont command with that routine compiled.

Main program for debugging:

a1.f	PARAMETER ( n=2 ) REAL twobytwo(2,2) / 4 *-1 / CALL mkidentity( twobytwo, n ) PRINT *, determinant( twobytwo ) END

Subroutine for debugging:

a2.f	SUBROUTINE mkidentity ( array, m ) REAL array(m,m) DO 90 i = 1, m DO 20 j = 1, m IF ( i .EQ. j ) THEN array(i,j) = 1. ELSE array(i,j) = 0. END IF 20 CONTINUE 90 CONTINUE RETURN END

Function for debugging:

a3.f	REAL FUNCTION determinant ( a ) REAL a(2,2) determinant = a(1,1) * a(2,2) - a(1,2) / a(2,1) RETURN END

Sample dbx Session

The following examples use a sample program called my_program.

1. Compile and link with the -g option.

You can do this in one or two steps.

Compile and link in one step, with -g:

demo% f95 -o my_program -g a1.f a2.f a3.f

Or, compile and link in separate steps:

demo% f95 -c -g a1.f a2.f a3.f demo% f95 -o my_program a1.o a2.o a3.o

2. Start dbx on the executable named my_program.

demo% dbx my_program Reading symbolic information...

3. Set a simple breakpoint by typing stop in subnam, where subnam names a subroutine, function, or block data subprogram.

To stop at the first executable statement in a main program.

(dbx) stop in MAIN (2) stop in MAIN

Although MAIN must be all uppercase, subnam can be uppercase or lowercase.

4. Type the run command, which runs the program in the executable files named when you started dbx.

(dbx) run Running: my_program stopped in MAIN at line 3 in file "a1.f" 3 call mkidentity( twobytwo, n )

When the breakpoint is reached, dbx displays a message showing where it stopped--in this case, at line 3 of the a1.f file.

5. To print a value, type the print command.

Print value of n:

(dbx) print n n = 2

Print the matrix twobytwo; the format might vary:

(dbx) print twobytwo twobytwo = (1,1) -1.0 (2,1) -1.0 (1,2) -1.0 (2,2) -1.0

Print the matrix array:

(dbx) print array dbx: "array" is not defined in the current scope (dbx)

The print fails because array is not defined here--only in mkidentity.

6. To advance execution to the next line, type the next command.

Advance execution to the next line:

(dbx) next stopped in MAIN at line 4 in file "a1.f" 4 print *, determinant( twobytwo ) (dbx) print twobytwo twobytwo = (1,1) 1.0 (2,1) 0.0 (1,2) 0.0 (2,2) 1.0 (dbx) quit demo%

The next command executes the current source line and stops at the next line. It counts subprogram calls as single statements.

Compare the next command with the step command. The step command executes the next source line or the next step into a subprogram. If the next executable source statement is a subroutine or function call, then:

• The step command sets a breakpoint at the first source statement of the subprogram.
• The next command sets the breakpoint at the first source statement after the call, but still in the calling program.

7. To quit dbx, type the quit command.

(dbx)quit demo%

Debugging Segmentation Faults

If a program gets a segmentation fault (SIGSEGV), it references a memory address outside of the memory available to it.

The most frequent causes for a segmentation fault are:

• An array index is outside the declared range.
• The name of an array index is misspelled.
• The calling routine has a REAL argument, which the called routine has as INTEGER.
• An array index is miscalculated.
• The calling routine has fewer arguments than required.
• A pointer is used before it has been defined.

Using dbx to Locate Problems

Use dbx to find the source code line where a segmentation fault has occurred.

Use a program to generate a segmentation fault:

demo% cat WhereSEGV.f INTEGER a(5) j = 2000000 DO 9 i = 1,5 a(j) = (i * 10) 9 CONTINUE PRINT *, a END demo%

Use dbx to find the line number of a dbx segmentation fault:

demo% f95 -g -silent WhereSEGV.f demo% a.out Segmentation fault demo% dbx a.out Reading symbolic information for a.out program terminated by signal SEGV (segmentation violation) (dbx) run Running: a.out signal SEGV (no mapping at the fault address)     in MAIN at line 4 in file "WhereSEGV.f" 4 a(j) = (i * 10) (dbx)

Locating Exceptions

If a program gets an exception, there are many possible causes. One approach to locating the problem is to find the line number in the source program where the exception occurred, and then look for clues there.

Compiling with -ftrap=common forces trapping on all common exceptions.

To find where an exception occurred:

demo% cat wh.f call joe(r, s) print *, r/s end subroutine joe(r,s) r = 12. s = 0. return end demo% f95 -g -o wh -ftrap=common wh.f demo% dbx wh Reading symbolic information for wh (dbx) catch FPE (dbx) run Running: wh (process id 17970) signal FPE (floating point divide by zero) in MAIN at line 2 in file "wh.f" 2 print *, r/s (dbx)

Tracing Calls

Sometimes a program stops with a core dump, and you need to know the sequence of calls that led it there. This sequence is called a stack trace.

The where command shows where in the program flow execution stopped and how execution reached this point--a stack trace of the called routines.

ShowTrace.f is a program contrived to get a core dump a few levels deep in the call sequence--to show a stack trace.

Note the reverse order: MAIN called calc calc called calcb. Execution stopped, line 23 calcB called from calc, line 9 calc called from MAIN, line 3

demo% f77 -silent -g ShowTrace.f demo% a.out *** TERMINATING a.out *** Received signal 11 (SIGSEGV) Segmentation Fault (core dumped) quil 174% dbx a.out Reading symbolic information for a.out ... (dbx) run Running: a.out (process id 1089) signal SEGV (no mapping at the fault address) in calcb at line 23 in file "ShowTrace.f" 23 v(j) = (i * 10) (dbx) where -V =>[1] calcb(v = ARRAY , m = 2), line 23 in "ShowTrace.f" [2] calc(a = ARRAY , m = 2, d = 0), line 9 in "ShowTrace.f" [3] MAIN(), line 3 in "ShowTrace.f" (dbx)

Show the sequence of calls, starting at where the execution stopped:

Working With Arrays

dbx recognizes arrays and can print them.

demo% dbx a.out Reading symbolic information... (dbx) list 1,25 1 DIMENSION IARR(4,4) 2 DO 90 I = 1,4 3 DO 20 J = 1,4 4 IARR(I,J) = (I*10) + J 5 20 CONTINUE 6 90 CONTINUE 7 END (dbx) stop at 7 (1) stop at "Arraysdbx.f":7 (dbx) run Running: a.out

stopped in MAIN at line 7 in file "Arraysdbx.f" 7 END (dbx) print IARR iarr = (1,1) 11 (2,1) 21 (3,1) 31 (4,1) 41 (1,2) 12 (2,2) 22 (3,2) 32 (4,2) 42 (1,3) 13 (2,3) 23 (3,3) 33 (4,3) 43 (1,4) 14 (2,4) 24 (3,4) 34 (4,4) 44 (dbx) print IARR(2,3) iarr(2, 3) = 23 - Order of user-specified subscripts ok (dbx) quit

For information on array slicing in Fortran, see Array Slicing Syntax for Fortran.

Fortran 95 Allocatable Arrays

The following example shows how to work with allocated arrays in dbx.

Alloc.f95

demo% f95 -g Alloc.f95 demo% dbx a.out (dbx) list 1,99 1 PROGRAM TestAllocate 2 INTEGER n, status 3 INTEGER, ALLOCATABLE :: buffer(:) 4 PRINT *, 'Size?' 5 READ *, n 6 ALLOCATE( buffer(n), STAT=status ) 7 IF ( status /= 0 ) STOP 'cannot allocate buffer' 8 buffer(n) = n 9 PRINT *, buffer(n) 10 DEALLOCATE( buffer, STAT=status) 11 END

Unknown size at line 6 Known size at line 9 buffer(1000) holds 1000

(dbx) stop at 6 (2) stop at "alloc.f95":6 (dbx) stop at 9 (3) stop at "alloc.f95":9 (dbx) run Running: a.out (process id 10749) Size? 1000 stopped in main at line 6 in file "alloc.f95" 6 ALLOCATE( buffer(n), STAT=status ) (dbx) whatis buffer integer*4 , allocatable::buffer(:) (dbx) next continuing stopped in main at line 7 in file "alloc.f95" 7 IF ( status /= 0 ) STOP 'cannot allocate buffer' (dbx) whatis buffer integer*4 buffer(1:1000) (dbx) cont stopped in main at line 9 in file "alloc.f95" 9 PRINT *, buffer(n) (dbx) print n n = 1000 (dbx) print buffer(n) buffer(n) = 1000

Unknown size at line 6 Known size at line 9 buffer(1000) holds 1000

(dbx) stop at 6 (2) stop at "alloc.f95":6 (dbx) stop at 9 (3) stop at "alloc.f95":9 (dbx) run Running: a.out (process id 10749) Size? 1000 stopped in main at line 6 in file "alloc.f95" 6 ALLOCATE( buffer(n), STAT=status ) (dbx) whatis buffer integer*4 , allocatable::buffer(:) (dbx) next continuing stopped in main at line 7 in file "alloc.f95" 7 IF ( status /= 0 ) STOP 'cannot allocate buffer' (dbx) whatis buffer integer*4 buffer(1:1000) (dbx) cont stopped in main at line 9 in file "alloc.f95" 9 PRINT *, buffer(n) (dbx) print n n = 1000 (dbx) print buffer(n) buffer(n) = 1000

Showing Intrinsic Functions

dbx recognizes Fortran intrinsic functions.

To show an intrinsic function in dbx, type:

demo% cat ShowIntrinsic.f INTEGER i i = -2 END (dbx) stop in MAIN (2) stop in MAIN (dbx) run Running: shi (process id 18019) stopped in MAIN at line 2 in file "shi.f" 2 i = -2 (dbx) whatis abs Generic intrinsic function: "abs" (dbx) print i i = 0 (dbx) step stopped in MAIN at line 3 in file "shi.f" 3 end (dbx) print i i = -2 (dbx) print abs(1) abs(i) = 2 (dbx)

Showing Complex Expressions

dbx also recognizes Fortran complex expressions.

To show a complex expression in dbx, type:

demo% cat ShowComplex.f COMPLEX z z = ( 2.0, 3.0 ) END demo% f95 -g -silent ShowComplex.f demo% dbx a.out (dbx) stop in MAIN (dbx) run Running: a.out (process id 10953) stopped in MAIN at line 2 in file "ShowComplex.f" 2 z = ( 2.0, 3.0 ) (dbx) whatis z complex*8 z (dbx) print z z = (0.0,0.0) (dbx) next stopped in MAIN at line 3 in file "ShowComplex.f" 3 END (dbx) print z z = (2.0,3.0) (dbx) print z+(1.0,1.0) z+(1,1) = (3.0,4.0) (dbx) quit demo%

Showing Logical Operators

dbx can locate Fortran logical operators and print them.

To show logical operators in dbx, type:

demo% cat ShowLogical.f LOGICAL a, b, y, z a = .true. b = .false. y = .true. z = .false. END demo% f95 -g ShowLogical.f demo% dbx a.out (dbx) list 1,9 1 LOGICAL a, b, y, z 2 a = .true. 3 b = .false. 4 y = .true. 5 z = .false. 6 END (dbx) stop at 5 (2) stop at "ShowLogical.f":5 (dbx) run Running: a.out (process id 15394) stopped in MAIN at line 5 in file "ShowLogical.f" 5 z = .false. (dbx) whatis y logical*4 y (dbx) print a .or. y a.OR.y = true (dbx) assign z = a .or. y (dbx) print z z = true (dbx) quit demo%

Viewing Fortran 95 Derived Types

You can show structures--Fortran 95 derived types--with dbx.

demo% f95 -g DebStruc.f95 demo% dbx a.out (dbx) list 1,99 1 PROGRAM Struct ! Debug a Structure 2 TYPE product 3 INTEGER id 4 CHARACTER*16 name 5 CHARACTER*8 model 6 REAL cost 7 REAL price 8 END TYPE product 9 10 TYPE(product) :: prod1 11 12 prod1%id = 82 13 prod1%name = "Coffee Cup" 14 prod1%model = "XL" 15 prod1%cost = 24.0 16 prod1%price = 104.0 17 WRITE ( *, * ) prod1%name 18 END (dbx) stop at 17 (2) stop at "Struct.f95":17 (dbx) run Running: a.out (process id 12326) stopped in main at line 17 in file "Struct.f95" 17 WRITE ( *, * ) prod1%name (dbx) whatis prod1 product prod1 (dbx) whatis -t product type product integer*4 id character*16 name character*8 model real*4 cost real*4 price end type product (dbx) n

(dbx) print prod1 prod1 = ( id = 82 name = 'Coffee Cup' model = 'XL' cost = 24.0 price = 104.0 )

Pointer to Fortran 95 Derived Type

You can show structures--Fortran 95 derived types--and pointers with dbx.

DebStruc.f95 Declare a derived type. Declare prod1 and prod2 targets. Declare curr and prior pointers. Make curr point to prod1. Make prior point to prod1. Initialize prior. Set curr to prior. Print name from curr and prior.

demo% f95 -o debstr -g DebStruc.f95 demo% dbx debstr (dbx) stop in main (2) stop in main (dbx) list 1,99 1 PROGRAM DebStruPtr! Debug structures & pointers 2 TYPE product 3 INTEGER id 4 CHARACTER*16 name 5 CHARACTER*8 model 6 REAL cost 7 REAL price 8 END TYPE product 9 10 TYPE(product), TARGET :: prod1, prod2 11 TYPE(product), POINTER :: curr, prior 12 13 curr => prod2 14 prior => prod1 15 prior%id = 82 16 prior%name = "Coffee Cup" 17 prior%model = "XL" 18 prior%cost = 24.0 19 prior%price = 104.0 20 curr = prior 21 WRITE ( *, * ) curr%name, " ", prior%name 22 END PROGRAM DebStruPtr (dbx) stop at 21 (1) stop at "DebStruc.f95":21 (dbx) run Running: debstr

(process id 10972) stopped in main at line 21 in file "DebStruc.f95" 21 WRITE ( *, * ) curr%name, " ", prior%name (dbx) print prod1 prod1 = ( id = 82 name = "Coffee Cup" model = "XL" cost = 24.0 price = 104.0 )

Above, dbx displays all fields of the derived type, including field names.

You can use structures--inquire about an item of an Fortran 95 derived type.

Ask about the variable Ask about the type (-t)

(dbx) whatis prod1 product prod1 (dbx) whatis -t product type product integer*4 id character*16 name character*8 model real cost real price end type product

To print a pointer, type:

dbx displays the contents of a pointer, which is an address. This address can be different with every run.

(dbx) print prior prior = ( id = 82 name = 'Coffee Cup' model = 'XL' cost = 24.0 price = 104.0 )