Stack Unwinding (thanks HN!)

.gitignore

*.o
*.obj
*.out
unwind

Makefile

CC		= clang
CFLAGS		= -m64 -fPIC -std=gnu99 -W -Wall
SOURCES		= $(wildcard *.c)
TARGET		= unwind

.PHONY:		all
all:		$(TARGET)
$(TARGET):	$(SOURCES)
.PHONY:		clean
clean:
	$(RM) $(TARGET)

unwind.c

#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>

struct frameptr_t { struct frameptr_t *next; };
struct frameptr_t *get_frameptr(void) {
  struct frameptr_t *ebp;
#if __x86_64__
  asm volatile("movq %%rbp, %0;" : "=r" (ebp));
#else
  asm volatile("movl %%ebp, %0;" : "=r" (ebp));
#endif
  return ebp->next;
}

void unwind(size_t depth, char *addr, size_t size) {
  size_t i;
  struct frameptr_t *ebp;
  ptrdiff_t offset;

  ebp = get_frameptr();
  offset = addr - (char *)ebp;

  for (i = 0; i < depth; i++) ebp = ebp->next;
  for (i = 0; i < size; i++) addr[i] = ((char *)ebp + offset)[i];
}
#define UNWIND(d, v) unwind(d, (char *)&v, sizeof(v))

void test_unwind(int x) {
  x = 42;
  printf("x now:\t\t\t%d\n", x);
  UNWIND(1, x);
  printf("x as it was passed:\t%d\n", x);
}

int main(int argc, const char *argv[]) {
  int x;

  if (argc < 2) {
    fprintf(stderr, "Error: need at least one integer argument.\n");
    exit(1);
  }

  x = atoi(argv[1]);
  test_unwind(x);
  return 0;
}