Method for Clearing GOSUB...RETURN Subroutine Stack in BASIC (44306)

SUMMARY

When a program jumps out of a GOSUB subroutine without executing a RETURN statement, the return address is left on the stack. If the program jumps out multiple times, an "Out of Stack Space" or "Out of Memory" error eventually occurs. You must make your program execute an equal number of GOSUB and RETURN statements to avoid this error.

The program below demonstrates a method for clearing the subroutine stack when a program executes more GOSUB statements than RETURN statements. Generally, a program should get out of a GOSUB subroutine with a RETURN statement, but when that is not feasible, the method below may be used.

This information applies to Microsoft QuickBASIC Versions 1.00, 1.01, 2.00, 2.01, 3.00, 4.00, 4.00b, and 4.50 for MS-DOS; Microsoft GW-BASIC Interpreter Versions 3.20, 3.22, and 3.23 for MS-DOS; Microsoft BASIC Interpreter Version 5.28 for MS-DOS; Microsoft BASIC Compiler Versions 5.35 and 5.36 for MS-DOS; Microsoft BASIC Compiler Versions 6.00 and 6.00b for MS-DOS and MS OS/2; and Microsoft BASIC PDS Version 7.00 for MS-DOS and MS OS/2.

MORE INFORMATION

The following program, STACK.BAS, invokes RETURN in a continuous loop to clear the stack until a "RETURN Without GOSUB" error occurs. This error is trapped, and the program resumes execution. The program STACK.BAS is as follows:

10 A = A + 1
   IF A < 10 THEN GOSUB 10      ' Put 10 GOSUB addresses on the stack.
   ON ERROR GOTO 30
20 RETURN 20                    ' Pop addresses off stack until
30 ON ERROR GOTO 0              ' "RETURN without GOSUB" error.
   END

Note: This method will not work with the SUB...END SUB subprogram procedure. A SUB...END SUB procedure should always be exited with either an EXIT SUB or an END SUB statement.