AMSC 662 / CMSC 662 Fall 2013

Frequently Asked Questions for Homework 3

Some reminders and advice for Matlab programmers programming in C:

VERY IMPORTANT: C is fussy, and its error messages are cryptic, so if you are a novice:

Start with a working program, make a single change, check it, change again, etc.

Save your last working C file so that you can back up to it if necessary.

If gcc fails, it leaves its older output unchanged.

So, suppose you type
gcc −g −Wall −fopenmp −o omp_run omp_hello.c
and then
omp_run

Then suppose you change omp_hello.c, type
gcc −g −Wall −fopenmp −o omp_run omp_hello.c
get an error message and then type
omp_run

It will happily run the old version, the one before your change, and that can be very confusing.

So if you don't pay close attention to error messages, you can keep changing omp_hello.c and not change omp_run at all.

To avoid that confusion, you might want to always remove the file omp_run before calling gcc.

Use brackets { ... } to enclose statements in a "for" loop, a "while" loop, an "if" statement, a function, and statements governed by a "pragma".

Declare all variables and functions, indicating int, float, void, or char.

If you declare
int A[10];
then A is an array of integers with entries indexed as A[0], A[1], ... , A[9]
(square brackets, not round, and indices counting from 0, not 1).

In print statements, the format string uses double quotes, not single, and you probably want to end the string with \n to end the line.

It is ok to break a statement into multiple lines. C does not require anything like Matlab's '...', except for Pragmas.

To make an entire line a comment, begin it with //

To make part of a line a comment, begin the comment with /* and end it with */

(09/26/13) In Matlab, "[A,b] = myfunct(x,y,Z)" might take variables x and y and array Z and compute array A and variable b. A copy of the variables x and y and a copy of all elements of Z are sent to myfunct, and myfunct might change the copy, but after return, the values of x, y, and Z in the calling program are unchanged.

In C, the call to function myfunct must have 5 arguments: x, y, Z, A, and &b. The values x and y are sent to myfunct, and changes made in the copy disappear after return, just as in Matlab. But since Z is an array, the address of its first element of Z is sent to myfunct, so changes made in Z are permanent. Similarly, A can be changed, and the contents of b can be changed because myfunct has a pointer to its address.

Question: 09/29/13 How can I time the parallel code?

Answer: Use the instructions in omp_timing.c

10-05-13 Error discovered: The prototype for the function should say "int nthread" instead of "double* nthread. Sorry for the mistake.

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