First Steps with C11

Content

First Steps with C11
What we can learn from the Exercise

Some remarks on compiling C code:

Always compile with the -Wall flag to get warnings that often indicate errors.
Compile with -std=c11 because by default many compiler still use the C89 standard by default.

For all examples do the following:

Compile the program, run it and check the exit code. Use the -o flag such that the executable is named like the source file without the .c suffix.
Look at the output the C preprocessor produces (call gcc with the -E flag).
Look at the assembler code produced by the compiler:
- Use the -S flag such that gcc stops after translating the C code into assembler code.
- Also use the -fno-asynchronous-unwind-tables and -fomit-frame-pointer flags (otherwise gcc produces lots of code we do not want to understand at the moment).
For the ULM we used a different assembly language. However, there are many similarities. Try to understand the meaning of the assembler code produced by gcc
Check how the assembler code produced by gcc is different when you compile with optimization flags, e.g.
- Compile with -O1
- Compile with -O2
- Compile with -O3

Example on Arrays

#define N   5

unsigned char x[N] = {1,2,3,4,5};

int main()
{
    register unsigned long long  res = 0;
    for (register unsigned long long i=0; i<N; ++i) {
        res += x[i];
    }
    return res;
}

Example on For loops

session06/for_loop.c

#define N   5

int main()
{
    register unsigned long long  res = 1;
    for (register unsigned long long i=2; i<=N; ++i) {
        res *= i;
    }
    return res;
}

Example on While loops

Explain why the following program will not compile. Then fix it and proceed as before:

session06/while_loop.c

#define N   5

int main()
{
    register unsigned long long  res = 1;
    while (N>1) {
        res *= N;
        --N;
    }
    return res;
}