Debug Tutorial: How to Set a Breakpoint

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stop [ context ] [ in | at ] { line_num | procedure } [ ,count | if condition ]
stop [ context ] { rw | w } { expression | data_address } [ ,count | if condition ]
stopi [ context ] [ at ] text_address [ ,count | if condition ]
trace [ context ] [ in | at ] { line_num | procedure } [ ,count | if condition ]
tracei [ context ] [ at ] text_address [ ,count | if condition ]
delete [ context ] { all | bkpt_num }
status [ context ] [ >filename ]

Breakpoints force execution of a program to stop at points of interest to allow examination of data, registers, stack, and message queues.

A code breakpoint is placed on an instruction address. Execution is stopped before that instruction is executed. A data breakpoint, which is referred to here as a watchpoint, is placed on a data address. In this case, execution stops after that address has been accessed.

• stop followed by a line number sets a code breakpoint at the start of that source line. Note: Do not set a breakpoint on a loop statement and expect it to be hit while the loop is executed. Specify the first line within the loop instead.

• stop followed by a procedure name sets a code breakpoint after the preamble of that procedure. Procedure parameters are therefore defined when execution stops. An attempt to set a breakpoint at the line number that corresponds to this location will fail.

• stop with a -rw or -w switch sets a watchpoint on the variable or address specified. The size of a watchpoint object is currently always assumed to be 4 bytes. Note: The process command's location field indicates the next instruction to be executed. Thus, watchpoints which fire at the very end of a source line will appear with the line number of the next source line to be executed rather than the source line on which the watchpoint occurred.

• Watchpoints are set using hardware registers which makes them fast, but limits their number to 4.

• ,count or if condition can be specified to delay the reported occurrence of a breakpoint or watchpoint until the condition is met. The count option results in the breakpoint or watchpoint being reported after every count times it is encountered. A condition is a simple expression that evaluates to True (non-zero) or False (0). The breakpoint or watchpoint is only reported when the condition evaluates to True. A condition consists of equivalence operators and logical operators. The syntax used must be the same as that of the program under debug, e.g. > for C and .GT. for Fortran.

• stopi sets a code breakpoint on an instruction address.

• trace and tracei are similar to code breakpoints. When they are encountered a message is printed and execution continues.

• delete removes breakpoints, watchpoints, and tracepoints.

• status lists breakpoints, watchpoints, and tracepoints along with their associated number. If the redirection option is specified (>) a file containing these breakpoints, watchpoints, and tracepoints is created. They are written to the file in the form of debug commands suitable for use as a startup file or source command input file.

Examples:

(0) > stop 10
(0) > stop one
(0) > stop 25,2
(0) > stop -rw myvar if myvar < 0
(0) > stop -w one`i
(0) > status

( 1) stop at line 10:hello.c:one():(0) 
( 2) stop in one():hello.c:one():(0) 
( 3) stop at line 25:hello.c:main():(0) 
( 4) stop if access myvar:::(0) 
( 5) stop if write i:hello.c:one():(0) 
(0) > delete all