这不会导致比赛。fork()创建一个新进程（不是线程），并拥有所有变量的副本。从手册页：
fork（）创建一个新进程。新进程（子进程）是调用进程（父进程）的精确副本，但以下情况除外：

•   The child process has a unique process ID.

       •   The child process has a different parent process ID (i.e., the
           process ID of the parent process).

       •   The child process has its own copy of the parent's descriptors.
           These descriptors reference the same underlying objects, so
           that, for instance, file pointers in file objects are shared
           between the child and the parent, so that an lseek(2) on a
           descriptor in the child process can affect a subsequent read or
           write by the parent.  This descriptor copying is also used by
           the shell to establish standard input and output for newly
           created processes as well as to set up pipes.

       •   The child processes resource utilizations are set to 0; see
           setrlimit(2).

请注意，(*ptr)++;会导致UB，因为ptr指向未初始化的内存。