PROS Troubleshooting and Common Problems

Why do I get a "Could not open port" error when flashing?

You might have seen an error like this:

Uploading bin/output.bin to v5 device on COM11 as hello to slot 1
ERROR - pros.cli.upload:upload - could not open port 'COM11': PermissionError(13, 'Access is denied.', None, 5)
File "c:\users\jonathan\appdata\local\programs\python\python36-32\lib\site-packages\serial\serialwin32.py", line 62, in open
  raise SerialException("could not open port {!r}: {!r}".format(self.portstr, ctypes.WinError()))
serial.serialutil.SerialException: could not open port 'COM11': PermissionError(13, 'Access is denied.', None, 5)

That means that some other resource is using the port. If you also have the V5 Utility open, that would cause this error to occur. Close the V5 Utility if you want to flash.

Compile-Time Issues

"main.h" file not found: : This error occurs when the compilation database compile_commands.json is not up to date. The file contains the exact compiler calls of the project, and should be automatically populated when the code is compiled. If you see this error just after creating a project, it could be because the project was created using the CLI without the after creating the project. Regardless of when this issue after creating the project. Regardless of when this issue appears, running pros make all in the CLI or running PROS -> Build -> All in the editor, the whole project is compiled again, fixing this issue. You may need to close and reopen any files in order for the linter to catch up. It is also possible that a compilation error was not caught by the linter, so running a full build will help discover these hidden errors.

undefined reference to ... or implicit declaration of function ...: : A function name is spelled incorrectly, or the function was incorrectly declared in a header file. Custom headers must be included in main.h or in the file in which they are used.
format ... expects argument of type ..., but argument has type ...: : The value provided to a function like printf or lcd_print does not match the expected type inferred from the format string. Some instances of this warning can be safely ignored, but crashes can occur if types double or long long are mixed with other types.
assignment makes pointer from integer without a cast: : Typically caused when a C pointer has the wrong number of asterisks to dereference it, or when assigning a constant to pointer (instead of *pointer).

Run-Time Issues

Some tasks are running, others are not: : A task is not waiting using delay or task_delay_until. Due to the fact that PROS utilizes a priority based non-preemptive scheduler, tasks of higher or equal priority to the blocking task will still run while lower priority tasks will not. This scenario is also known as starvation. See Tasks/Multithreading for more information.
VEX LCD updates very slowly or is "frozen": : A task is not waiting using delay or From the kernel's perspective, updating the LCD is usually less important than how well the robot is running, so PROS prioritizes user tasks over the LCD.
The LCD is only updated if all other tasks are waiting.
Neither autonomous nor driver control code starts: : The initialize() function may still be running. Some tasks such as analog_calibrate take time.
If the `initialize()` function implements some type of loop or
autonomous selection routine, verify that it actually has a
means of ending.
Code restarts unexpectedly: : A run-time error has caused the program to crash. Debugging may reveal the cause of the error. Examine any newly added code for possible logical errors. Some common error messages include:
Segmentation Fault: : Indicates that an invalid C pointer has been used. Check for confusion between pointers and regular variables and that an invalid pointer has not been passed to a PROS API function.
Stack Overflow: : Often indicates infinite recursion, or that the stack size for a custom task is too small. Calling many layers of functions and declaring large local variables can require large amounts of space on the stack. If this error occurs in a default task like autonomous(), consider changing code to reduce the stack requirements, or creating a new task with a larger stack size using task_create to handle large jobs. Large arrays declared inside functions can usually be declared globally to alleviate pressure on stack space.
System Task Failure: : Too many tasks were running for the system to start a new one. Disable or merge unnecessary tasks to eliminate this error.
Cortex blinking red light after upload: : Turn the Cortex microcontroller off and on again. This usually resolves the problem, and it is generally good practice to re-initialize the robot to simulate conditions at most competitions. If the error persists, see the "**Code restarts unexpectedly**" section above.
[printf](printf_) doesn't work:
[printf](printf_) doesn't work: : printf prints information over a serial connection Debugging, not to the VEX LCD. To print to the LCD, use lcd_print instead.