[clang][gcc][c][osx] inline asm to syscall on OSX amd64

hello-asm.c

/* clang hello-asm.c -o hello-asm */
int main() {
  long write = 0x2000004; /* syscall "write" on MacOS X */
  long stdout = 1;
  char * str = "Hello World\n";
  unsigned long len = 12;
  unsigned long ret = 0;
  /* ret = write(stdout, str, len); */
  __asm__("movq %1, %%rax;\n"
          "movq %2, %%rdi;\n"
          "movq %3, %%rsi;\n"
          "movq %4, %%rdx;\n"
          "syscall;\n"
          "movq %%rax, %0;\n"
          : "=g"(ret)
          : "g"(write), "g"(stdout), "g"(str), "g"(len)
          : ); /* assign from generic registers to syscall registers */
  return ret; /* will be 12 ("echo $?") */

hello-asm2.c

/* clang hello-asm2.c -o hello-asm2 */
int main() {
  long write = 0x2000004; /* syscall "write" on MacOS X */
  long stdout = 1;
  char * str = "Hello World\n";
  unsigned long len = 12;
  unsigned long ret = 0;
  /* ret = write(stdout, str, len); */
  __asm__("syscall"
          : "=a"(ret)
          : "a"(write), "D"(stdout), "S"(str), "d"(len)
          : ); /* directly assigned by register attribute */
  return ret; /* will be 12 ("echo $?") */
}