8.5. exit Functions
As we described in Section 7.3, a process can terminate normally in five ways:Executing a return from the main function. As we saw in Section 7.3, this is equivalent to calling exit.Calling the exit function. This function is defined by ISO C and includes the calling of all exit handlers that have been registered by calling atexit and closing all standard I/O streams. Because ISO C does not deal with file descriptors, multiple processes (parents and children), and job control, the definition of this function is incomplete for a UNIX system.Calling the _exit or _Exit function. ISO C defines _Exit to provide a way for a process to terminate without running exit handlers or signal handlers. Whether or not standard I/O streams are flushed depends on the implementation. On UNIX systems, _Exit and _exit are synonymous and do not flush standard I/O streams. The _exit function is called by exit and handles the UNIX system-specific details; _exit is specified by POSIX.1.In most UNIX system implementations, exit(3) is a function in the standard C library, whereas _exit(2) is a system call.Executing a return from the start routine of the last thread in the process. The return value of the thread is not used as the return value of the process, however. When the last thread returns from its start routine, the process exits with a termination status of 0.Calling the pthread_exit function from the last thread in the process. As with the previous case, the exit status of the process in this situation is always 0, regardless of the argument passed to pthread_exit. We'll say more about pthread_exit in Section 11.5.The three forms of abnormal termination are as follows:Calling abort. This is a special case of the next item, as it generates the SIGABRT signal.When the process receives certain signals. (We describe signals in more detail in Chapter 10). The signal can be generated by the process itselffor example, by calling the abort functionby some other process, or by the kernel. Examples Sections 11.5 and 12.7.Regardless of how a process terminates, the same code in the kernel is eventually executed. This kernel code closes all the open descriptors for the process, releases the memory that it was using, and the like.For any of the preceding cases, we want the terminating process to be able to notify its parent how it terminated. For the three exit functions (exit, _exit, and _Exit), this is done by passing an exit status as the argument to the function. In the case of an abnormal termination, however, the kernel, not the process, generates a termination status to indicate the reason for the abnormal termination. In any case, the parent of the process can obtain the termination status from either the wait or the waitpid function (described in the next section).Note that we differentiate between the exit status, which is the argument to one of the three exit functions or the return value from main, and the termination status. The exit status is converted into a termination status by the kernel when _exit is finally called (recall Figure 7.2). Section 10.7.Section 9.2) or a process whose parent has terminated and has been subsequently inherited by init. |
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