Is it true that fork() calls clone() internally?

Question

I read here that clone() system call is used to create a thread in Linux. Now the syntax of clone() is such that a starting routine/function addres

Answer 1

@Dietrich did a great job explaining by looking at the implementation. That's amazing! Anyway, there's another way of discovering that: by looking at the calls strace "sniffs".

We can prepare a very simple program that uses fork(2) and then check our hypothesis (i.e, that there's no fork syscall really happening).

#define WRITE(__fd, __msg) write(__fd, __msg, strlen(__msg))

int main(int argc, char *argv[])
{
  pid_t pid;

  switch (pid = fork()) {
    case -1:
      perror("fork:");
      exit(EXIT_FAILURE);
      break;
    case 0:
      WRITE(STDOUT_FILENO, "Hi, i'm the child");
      exit(EXIT_SUCCESS);
    default:
      WRITE(STDERR_FILENO, "Heey, parent here!");
      exit(EXIT_SUCCESS);
  }

  return EXIT_SUCCESS;
}

Now, compile that code ( clang -Wall -g fork.c -o fork.out ) and then execute it with strace:

strace -Cfo ./fork.strace.log ./fork.out

This will intercept system calls called by our process (with -f we also intercept the child's calls) and then put those calls into ./fork.trace.log; -c option gives us a summary at the end). The result in my machine (Ubuntu 14.04, x86_64 Linux 3.16) is (summarized):

6915  arch_prctl(ARCH_SET_FS, 0x7fa001a93740) = 0
6915  mprotect(0x7fa00188c000, 16384, PROT_READ) = 0
6915  mprotect(0x600000, 4096, PROT_READ) = 0
6915  mprotect(0x7fa001ab9000, 4096, PROT_READ) = 0
6915  munmap(0x7fa001a96000, 133089)    = 0
6915  clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fa001a93a10) = 6916
6915  write(2, "Heey, parent here!", 18) = 18
6916  write(1, "Hi, i'm the child", 17 
6915  exit_group(0)                     = ?
6916  <... write resumed> )             = 17
6916  exit_group(0)                     = ?
6915  +++ exited with 0 +++
6916  +++ exited with 0 +++
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 24.58    0.000029           4         7           mmap
 17.80    0.000021           5         4           mprotect
 14.41    0.000017           9         2           write
 11.02    0.000013          13         1           munmap
 11.02    0.000013           4         3         3 access
 10.17    0.000012           6         2           open
  2.54    0.000003           2         2           fstat
  2.54    0.000003           3         1           brk
  1.69    0.000002           2         1           read
  1.69    0.000002           1         2           close
  0.85    0.000001           1         1           clone
  0.85    0.000001           1         1           execve
  0.85    0.000001           1         1           arch_prctl
------ ----------- ----------- --------- --------- ----------------
100.00    0.000118                    28         3 total

As expected, no fork calls. Just the raw clone syscall with its flags, child stack and etc properly set.