16.5 Tracing with printf

Printing information at key points is a way of tracing or following the execution of the code. With C code, you stick printf's throughout the code that let you know you've reached a particular point in the code or tell you what the value of a variable is. By using this approach, you can zero in on a crucial point in the program and see the value of parameters that may affect the execution of the code. This quick and dirty approach works best when you already have an idea of what might be going wrong. But if you are clueless as to where the problem is, you may need a lot of print statements to zero in on the problem, particularly with large programs. Moreover, it is very easy for the truly useful information to get lost in the deluge of output you create.

On the other hand, there is certainly nothing wrong with printing information that provides the user with some sense of progress and an indication of how the program is working. We did this in our numerical integration program when we printed the process number and the individual areas calculated by each process.

It can be particularly helpful to echo values that are read into the program to ensure that they didn't get garbled in the process. For example, if you've inadvertently coerced a floating point number into an integer, the truncation that occurs will likely cause problems. By printing the value, you may be alerted to the problem.

Including print statements can also be helpful when you are working with complicated data structures since you will be able to format the data in meaningful ways. Examining a large array with a symbolic debugger can be challenging. Since it is straightforward to conditionally print information, print statements can be helpful when the data you are interested in is embedded within a large loop and you want to examine it only under selective conditions.

In developing code, programmers will frequently write large blocks of diagnostic code that they will discard once the code seems to be working. When the code has to be changed at a later date, they will often find themselves rewriting similar code as new problems arise. A better solution is to consider the diagnostic code a key part of the development process and keep it in your program. By using conditional compile directives, the code can be disabled in production versions so that program efficiency isn't compromised, but can be enabled easily should the need arise.

A technique that is often used with printf is deleting extraneous code. The idea is, after making a copy of your program, to start deleting code and retesting to see whether the problem has disappeared. The goal is to produce the smallest piece of code that still exhibits the problem. This can be useful with some types of problems, particularly when you are trying to piece together how some feature of a language works. It can also be helpful when generating a bug report.

With parallel code, the printf approach can be problematic. Earlier in this book, you saw examples of how the output from different processes could be printed in a seemingly arbitrary order. Buffering further complicates matters. If your code is crashing, a process may die before its output is displayed. That output will be lost. Also, output can change timings which can limit its effectiveness if you are dealing with a race problem. Finally, print statements can seriously deteriorate performance.

If you are going to use the printf approach with parallel programs, there are two things you should do. First, if there is any possibility of the source of the output being confused, be sure to label the output with the process number or machine name. Second, follow your calls to printf with a call to fflush so that the output is actually printed at the moment the program generates it. For example,

...

int  processId;       

char processName[MPI_MAX_PROCESSOR_NAME];

...

MPI_Comm_rank(MPI_COMM_WORLD, &processId);

MPI_Get_processor_name(processName, &nameSize);

...

fprintf(stdout, "Process %d on %s at checkpoint 1. \n", processId, 

processName);

fflush(stdout);

...

If you want to control the order of the output, you'll need to have the master process coordinate output.