Runtime errors

Some mistakes can only be found while the program runs: an array index out of range, a pointer that is null. Compiled with -exx, a program checks for them, and stops with a numbered runtime error that says what went wrong and where, instead of reading memory it does not own.

#cmdline "-exx"

'' -exx checks every array index while the program runs.
dim scores( 0 to 2 ) as int32 = { 70, 85, 90 }
dim as int32 total = 0
for i as int32 = 0 to 3                 '' one too far
    total += scores( i )
next
print total                             '' never reached

Output

Aborting due to runtime error 6 (out of bounds array access) at line 7 of bounds.tiko::(), 
'SCORES' accessed with invalid index = 3, must be between 0 and 2

The loop's last pass asks for scores( 3 ). The check stops the program before the read: runtime error 6, with the line, the array's name, the index and the bounds it should have been within. The program's exit status is the error number, 6, so a script that runs it can tell what happened.

#cmdline "-exx" puts the option in the source, so the file alone reproduces the result. On the command line it is tiko -exx bounds.tiko.

-exx adds the array-bounds and null-pointer checks, and records the procedure each runtime error happens in, so the message can name it.

The null-pointer check is shown in nullptr. Without -exx neither check is made: the read past the end returns whatever is in memory there, and nothing reports it. The message itself goes to standard error, not standard output.

Checks cost speed: a comparison on every array access and a test on every pointer dereference. Use -exx while developing and testing, and decide deliberately whether a release build keeps it.

Activities

1. Remove #cmdline "-exx" and run the program again. What does it print?
2. Change the loop to for i as int32 = -1 to 2. Which index does the message report?