92110

On Pentium, getcontext saves EFLAGS and setcontext restores it.
When CF is set in EFLAGS, setcontext treats it as an system call error.

How-To-Repeat: % cat t.c
#include <stdlib.h>
#include <stdio.h>
#include <ucontext.h>

ucontext_t c;
int first;

volatile int carry_set;

int main()
{
  first = 1;

  carry_set = ~0;
  carry_set += 1;
  if (getcontext(&c) != 0) {
    perror("getcontext");
    exit(1);
  }

  printf("first:%d\n", first);

  if (first == 0)
    exit(0);

  first = 0;

  if (setcontext(&c) != 0) {
    perror("setcontext");
    exit(1);
  }

  printf("setcontext returns\n");
  exit(1);
}

% gcc -march=pentium4 t.c      
% ./a.out 
first:1
setcontext: Unknown error: 0

If "carry_set += 1" is changed to "carry_set -= 1", it works fine:

% ./a.out 
first:1
first:0