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 64