Child Parent Relationship and Inheritance in C

Question

I am totally new with C.

1. What are the process items that are inherited in a child created using fork();?

2. What are the process items that are different from the process's parent?

Answer 1

This hasn't got much to do with C, rather with fork(), which is a POSIX system call (and I guess it could behave differently on different systems).

I'd suggest you to read the fork manual, which is really clear about this:

fork() creates a new process by duplicating the calling process. The new referred to as the child, is an exact duplicate of the calling process, referred to as the parent, except for the following points:

• The child has its own unique process ID, and this PID does not match the ID of any existing process group (setpgid(2)).

• The child's parent process ID is the same as the parent's process ID.

• The child does not inherit its parent's memory locks (mlock(2), mlockall(2)).

• Process resource utilizations (getrusage(2)) and CPU time counters (times(2)) are reset to zero in the child.

• The child's set of pending signals is initially empty (sigpending(2)).

• The child does not inherit semaphore adjustments from its parent (semop(2)).

• The child does not inherit record locks from its parent (fcntl(2)).

• The child does not inherit timers from its parent (setitimer(2), alarm(2), timer_create(2)).

• The child does not inherit outstanding asynchronous I/O operations from its parent (aio_read(3), aio_write(3)), nor does it inherit any asynchronous I/O contexts from its parent (see io_setup(2)).

The process attributes in the preceding list are all specified in POSIX.1-2001. The parent and child also differ with respect to the following Linux-specific process attributes:

• The child does not inherit directory change notifications (dnotify) from its parent (see the description of F_NOTIFY in fcntl(2)).

• The prctl(2) PR_SET_PDEATHSIG setting is reset so that the child does not receive a signal when its parent terminates.

• Memory mappings that have been marked with the madvise(2) MADV_DONTFORK flag are not inherited across a fork().

• The termination signal of the child is always SIGCHLD (see clone(2)).

Note the following further points:

• The child process is created with a single thread -- the one that called fork(). The entire virtual address space of the parent is replicated in the child, including the states of mutexes, condition variables, and other pthreads objects; the use of pthread_atfork(3) may be helpful for dealing with problems that this can cause.

• The child inherits copies of the parent's set of open file descriptors. Each file descriptor in the child refers to the same open file description (see open(2)) as the corresponding file descriptor in the parent. This means that the two descriptors share open file status flags, current file offset, and signal-driven I/O attributes (see the description of F_SETOWN and F_SETSIG in fcntl(2)).

• The child inherits copies of the parent's set of open message queue descriptors (see mq_overview(7)). Each descriptor in the child refers to the same open message queue description as the corresponding descriptor in the parent. This means that the two descriptors share the same flags (mq_flags).

• The child inherits copies of the parent's set of open directory streams (see opendir(3)). POSIX.1-2001 says that the corresponding directory streams in the parent and child may share the directory stream positioning; on Linux/glibc they do not.

If you're interested about Linux, you should also check the clone system call, that lets you specify with more accuracy what you want.

Answer 2

The fork(2) man page on your system (man fork) should give you better details but generally the child only inherits the parent's list of file descriptors, including open files, sockets, and process handles.

From my system's man page (Mac OS X 10.6.6):

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 cre- ated processes as well as to set up pipes.

Answer 3

The only things of child that are different its parent are its

PPID i.e parent process id and

PID process id.

And when it comes to similarity child process inherits its parent's FILE DESCRIPTOR table thus you would see that always three FILE DESCRIPTORS of child are always occupied which correspond to STDIN,STDOUT and STDERR.

Answer 4

See the official description of fork(2)

Answer 5

There is no object-oriented inheritence in C.

Fork'ing in C is basically the process being stopped while it is running, and an entire copy of it being made in (effectively) a different memory space, then both processes being told to continue. They will both continue from where the parent was paused. The only way you can tell which process you are in is to check the return value of the fork() call.

In such a situation the child doesn't really inherit everything from the parent process, it's more like it gets a complete copy of everything the parent had.