fork() creates a new process. The new process is a copy of the original process -- unless specified otherwise below. The new process' ID does not match any existing process or group ID. The parent process ID (ppid) of the child process is the process ID (pid) of the parent process (caller of fork()). The opened file descriptors are copied to the child process and refer to the same object. The opened directory streams are copied to the child process. Positioning information may be shared between both processes. The child process gets a copy of the parent message catalog descriptor. tms_utime, tms_stime, tms_cutime, and tms_cstime values are set to 0 in the child process. The time left until an alarm clock signal is reset to zero, and the alarm, if any, is canceled. semadj values are cleared. The file locks are not inherited by the child process. The child process is created with no pending signals. Interval timers are reset in the child process. The opened semaphores are inherited in the child process. The child process does not inherit memory locks set by the parent process with mlock or mlockall. Memory mappings created in the parent are retained in the child process. If the mapping is MAP_PRIVATE, any modification done after the fork() is visible only to the process doing the modification. For the SCHED_RR and SCHED_FIFO scheduling policies, the child process inherits the policy and priority settings of the parent process during the fork() execution. The per-process timers are not inherited. The opened message queue descriptors are copied to the child process and refer to the same object. Asynchronous IO operations are not inherited by the child. The new process has only one thread. The CPU-time clocks of the new process/ new process' thread are initialized to 0. fork() returns 0 to the child and the child PID to the parent process when successful. fork() returns -1, errno is set to EAGAIN, and no child process is created if the system lacks a resource to create the new process or CHILD_MAX process are already running.