Monday Bonus Exercise 1.7: Compile and Run Some C Code

The goal of this exercise is to show that compiled code works just fine in HTCondor. It is mainly of interest to people who have their own C code to run (or C++, or really any compiled code, although Java would be handled a bit differently).

Preparing a C Executable

When preparing a C program for HTCondor, it is best to compile and link the executable statically, so that it does not depend on external libraries and their particular versions. Why is this important? When your compiled C program is sent to another machine for execution, that machine may not have the same libraries that you have on your submit machine (or wherever you compile the program). If the libraries are not available or are the wrong versions, your program may fail or, perhaps worse, silently produce the wrong results.

Here is a simple C program to try using (thanks, Alain Roy):

#include <stdio.h>

int main(int argc, char **argv)
{
    int sleep_time;
    int input;
    int failure;

    if (argc != 3) {
        printf("Usage: simple <sleep-time> <integer>\n");
        failure = 1;
    } else {
        sleep_time = atoi(argv[1]);
        input      = atoi(argv[2]);

        printf("Thinking really hard for %d seconds...\n", sleep_time);
        sleep(sleep_time);
        printf("We calculated: %d\n", input * 2);
        failure = 0;
    }
    return failure;
}

Save that code to a file, for example, simple.c.

Compile the program with static linking:

[email protected] $ gcc -static -o simple simple.c

As always, test that you can run your command from the command line first. First, without arguments to make sure it fails correctly:

[email protected] $ ./simple

and then with valid arguments:

[email protected] $ ./simple 5 21

Running a Compiled C Program

The rest is simple. In fact, it is no different than running any other program. Here is a basic submit file for the C program (call it simple.sub):

executable = simple
arguments = "60 64"

output = c-program.out
error = c-program.err
log = c-program.log

queue

Then submit the job as usual!

In summary, it is easy to work with statically linked compiled code. It is possible to handle dynamically linked compiled code, but it is trickier. We will only mention this topic briefly on Wednesday.