makecontext(3)

[kkonaka@mac.com]

on the surface(?) calls to makecontext() appear to have to be 
preceded by getcontext() to completely initialize its arg: ucontext_t
struct (that is, if I comment out getcontext() calls from a
program like the below, it causes SEGV). why is it the case?
kenji

---
#include <ucontext.h>
#include <unistd.h>
#include <err.h>

static ucontext_t u0;
static ucontext_t u1;
static ucontext_t u2;

#define STACK_SIZE (1024 * 8)

#define NLOOP 7

static int i;
static int j;

static void f()
{
  for (i = 0; i < NLOOP; i++) {
    printf("f(): i = %d\n", i);
    if (swapcontext(&u1, &u2) < 0)
      err(1, NULL);
  }
}

static void g()
{
  for (j = 0; j < NLOOP; j++) {
    printf("g(): j = %d\n", j);
    if (swapcontext(&u2, &u0) < 0)
      err(1, NULL);
  }
}

main()
{
  static unsigned char _u1_stack[STACK_SIZE];
  static unsigned char _u2_stack[STACK_SIZE];

  getcontext(&u1);
  u1.uc_stack.ss_sp = _u1_stack;
  u1.uc_stack.ss_size = sizeof _u1_stack;
  u1.uc_stack.ss_flags = 0;
  u1.uc_link = NULL;
  makecontext(&u1, f, 0);

  getcontext(&u2);
  u2.uc_stack.ss_sp = _u2_stack;
  u2.uc_stack.ss_size = sizeof _u2_stack;
  u2.uc_stack.ss_flags = 0;
  u2.uc_link = NULL;
  makecontext(&u2, g, 0);

  while (1) {
    if (swapcontext(&u0, &u1) < 0)
      err(1, NULL);
    printf("main: %d %d\n", i, j);
    if (i == NLOOP && j == NLOOP)
      exit(0);
  }
}
--