Coding FAQ

Compile-Time Issues

  • 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 lcdPrint() 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 taskDelayUntil(). 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 taskDelayUntil(). 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 gyroInit() or analogCalibrate() 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 taskCreate() 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() doesn’t work:
    printf() prints information over a serial connection (see Debugging), not to the VEX LCD. To print to the LCD, use lcdPrint() instead.