Question

I have a function that multiply two matrices A and B then prints the result.
I got two different outputs when running the program in two similar ways.

first:

    FILE *f;
    f = fopen("in.txt","r");
    struct Mat* A = read_mat(f);
    struct Mat* B = read_mat(f);
    print_mat(mat_mul_1(A, B));

the output was the exact multiplication of

A * B

second:

    FILE *f;
    f = fopen("in.txt","r");
    print_mat(mat_mul_1(read_mat(f), read_mat(f)));

the output was the exact multiplication of

B * A

I want to know why the arguments has been reversed ?!

(as the ‘mat_mul_1’ function is a black box)

Answer 1

Did you expect that the first read_mat(f) would be evaluated first?

C offers no such guarantees. The compiler is free to emit code that evaluates the arguments in any order it chooses.

The order of evaluation of the function designator, the actual arguments, and
subexpressions within the actual arguments is unspecified, but there is a sequence point
before the actual call.