This page is a, continuously growing, database of commonly asked questions/issues and possible answers/solutions. There is no specific order to the entries. If you have an issue yourself, make sure you search this page (⌘F | Ctrl + F) for some keywords that may be associated with your issue. If you do not find a solution here, you may ask a TA during your Lab session or use the Canvas Discussion board.
General
I am supposed to use function X but how do I know how it works?
One of the great things about using a UNIX-based operating system is that it (very likely) includes manual pages for most (if not all) functions of the C Standard Library.
If, for instance, you want to know how the fopen function works and what arguments you need to pass it, you can simply type
man fopen
in a Terminal window. This will open the manual page (or short man page) for the fopen function. This page should give information about the parameters, functioning, and return value of the function. To close the page simply press 'q'.
Why does my program print a different value at every execution?
You are probably passing uninitialized memory to the printf function. For example, you may be loading a value from an address on the stack before having initialized this value with a prior mov instruction.
Trace the value(s) of the register(s) that printf will use for printing to localize the issue. It may help to use a debugger to follow the register values (and their changes) throughout the program.
Why do I get a (seemingly random) segmentation fault when calling a C library function like printf?
One possible cause of this issue may be the stack alignment. As explained on the page about Calling Subroutines, the stack needs to be 16-byte aligned for any subroutine calls. For some subroutines a misaligned stack may not be an issue, however, functions of the C Standard Library will likely exit with a segmentation fault.
Why does make show an error even though the program runs correctly?
When using the shortcut targets (like make a1) make is checking for the exit code of your program (so the return value of the main routine). You are likely not returning 0 from your main routine and thereby, as non-zero return codes are typically used to indicate unsuccessful termination, make assumes that something went wrong.
macOS
The debugger is ignoring my breakpoints, why?
For reasons that we are not yet certain of, any labels starting with an l (so for example loop_1:) cause the debugger to no longer associate the lines in the source file with the instructions of the assembled program. The workaround for this is to name your labels such that they do not start with an l.
CodeGrade
The test on CodeGrade fails with "No such file or directory"
If CodeGrade cannot find the program to execute, this is likely due to a failure during assembly or linking of your program. To see what error the assembler or linker produced, expand the "Setup" tab at the top of the AutoTest page and examine the "stderr" output of the "Per-student setup script" (at the bottom of the "Setup" tab).
