05. C: Addressing and address arithmetic

(NB: Sugon IP is changed)

Previous lecture debt: calling a linker.

• Strace — tool for system call tracing

  • -s size: parameter size limit

  • -f: follow subsequent process

  • -e syscall: limit to this syscall/group of syscalls

So try a little cc o.c with strace -f -eexecve -s1024 cc o.c

OMFG!

• cc1 — C → Asm translator (aka cc -S)

  • cc -S o.c -o o.s

• as — Asm → object compiler

  • -mips32r2 — exact architecture (ISA+ABI)

  • as -mips32r2 o.s -o o.o

• ld — linker

  • a lot of supplemental files + libc:

    ld \
             -dynamic-linker /lib/ld.so.1 \
             /usr/lib/crt1.o \
             /usr/lib/crti.o \
             /usr/lib/gcc/mipsel-alt-linux/8/crtbegin.o \
             /usr/lib/gcc/mipsel-alt-linux/8/crtend.o \
             /usr/lib/crtn.o \
             -lc \
             o.o -o prog

Addressing

• #include <stdio.h>
  
  int N;
  char *Str = "The string";
  char Carr[]= "Other string";
  int Array[] = {1,3,5,7,9,12,12,34,45,4556,3,23,23};
  
  int main(int argc, char *argv[]) {
          int L;
          int * A;
          N = L = 1;
          A = &L;
          *A = 7;
          *Array = 100;
          Array[2] = 5;
          *(Array+3) = 6;
          4[Array] = 10500;
          printf("%p %p %p %p\n", &N, &L, Str, Array);
          return 0;
  }

(use cc -O0 -static -mno-explicit-relocs -mno-shared -mno-abicalls -mno-gpopt -g -Wa,-adhlns file.c or compile script)

• Pointer type in C is just an address

  • Array name in C is just a lable!

  • Pointer type variable holds an address

• Pointer is aligned on data size:

  • #include <stdio.h>
    struct STRU {
      int a, b;
      double f;
    };
    
    struct STRU Array[10];
    void main(void) {
      int i=3, j=4;
      printf("%d, %lf\n", Array[i].a, Array[j].f);
     
    }

  TODO

Structs

Alignment

Arrays on stack

Function address

H/W