Question

I am stuck in a situation. I am working on an embedded device (based on linux OS and ARM processor, 32 bit). It is a touch screen device that has many peripherals like smartcard reader, GPS, GPRS . I am coding in C and my application is crashing after some time when I am redirecting my debug statements as shown below. I have around 300 debug print statements that I am printing using this function like macro. The device can also be connected to the system terminal through a USB cable. When I am printing these debug statements in my system terminal, the application is not crashing anywhere, but when I am not using my computer’s terminal and run the application in the device only then it crashes after some time:

#ifdef DEBUG_TEST
  #define DEBUG_TEST 1
  #else
  #define DEBUG_TEST 0
  #endif

  #define DEBUG_PRINT(fmt, ...) \
             do { if (DEBUG_TEST) fprintf(stderr, fmt, ##__VA_ARGS__); } while (0)

But when I am turning off these debug statements, the application doesn’t crash anywhere. I don’t understand why is it happening.
According to my guess, since the device does not have its own standard terminal, so printing these debug messages is creating a buffer due to which it crashes after some time, while on the other hand when I turn off the debug statements then it works fine. Please suggest why it might be happening?

Answer 1

Perhaps one of the arguments to one of your DEBUG_PRINT calls is causing fprintf’s string-interpolator routine to reference invalid memory? For example, something like this in your code would crash your app inside fprintf():

const char * badPointer = (const char *) 0xDEADBEEF;  // deliberately pointing to invalid memory
DEBUG_PRINT("Crashing now!  %s\n", badPointer);

… either that, or it’s possible there is a bug in your system’s stdio or USB implementation that is causing the crash.

To narrow things down, you could either try commenting out various DEBUG_PRINT statements until the crash goes away (at which point you can reasonably suspect that it was one of the now-commented-out DEBUG_PRINT statements that was causing the crash, and comment some back in until you figure out who the culprit is)… or if you suspect a bug in fprintf(), you could make a torture-test like this:

while(1) DEBUG_PRINT("I think I %s!\n", "can");

… and see if running that gives you the expected output (infinite output of the string) or crashes. If it crashes, then that would suggests a bug outside of your program.