libc/src/unistd/linux/fork.cpp

   1 //===-- Linux implementation of fork --------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "src/unistd/fork.h"
  10
  11 #include "src/__support/OSUtil/syscall.h" // For internal syscall function.
  12 #include "src/__support/common.h"
  13 #include "src/__support/macros/config.h"
  14 #include "src/__support/threads/fork_callbacks.h"
  15 #include "src/__support/threads/identifier.h"
  16 #include "src/__support/threads/thread.h" // For thread self object
  17
  18 #include "src/errno/libc_errno.h"
  19 #include <signal.h>      // For SIGCHLD
  20 #include <sys/syscall.h> // For syscall numbers.
  21
  22 namespace LIBC_NAMESPACE_DECL {
  23
  24 // The implementation of fork here is very minimal. We will add more
  25 // functionality and standard compliance in future.
  26
  27 LLVM_LIBC_FUNCTION(pid_t, fork, (void)) {
  28   invoke_prepare_callbacks();
  29   pid_t parent_tid = internal::gettid();
  30   // Invalidate parent's tid cache before forking. We cannot do this in child
  31   // process because in the post-fork instruction windows, there may be a signal
  32   // handler triggered which may get the wrong tid.
  33   internal::force_set_tid(0);
  34 #ifdef SYS_fork
  35   pid_t ret = syscall_impl<pid_t>(SYS_fork);
  36 #elif defined(SYS_clone)
  37   pid_t ret = syscall_impl<pid_t>(SYS_clone, SIGCHLD, 0);
  38 #else
  39 #error "fork and clone syscalls not available."
  40 #endif
  41
  42   if (ret == 0) {
  43     // Return value is 0 in the child process.
  44     // The child is created with a single thread whose self object will be a
  45     // copy of parent process' thread which called fork. So, we have to fix up
  46     // the child process' self object with the new process' tid.
  47     internal::force_set_tid(syscall_impl<pid_t>(SYS_gettid));
  48     invoke_child_callbacks();
  49     return 0;
  50   }
  51
  52   if (ret < 0) {
  53     // Error case, a child process was not created.
  54     libc_errno = static_cast<int>(-ret);
  55     return -1;
  56   }
  57   // recover parent's tid.
  58   internal::force_set_tid(parent_tid);
  59   invoke_parent_callbacks();
  60   return ret;
  61 }
  62
  63 } // namespace LIBC_NAMESPACE_DECL