what are parent and child processes all about?
In the Introduction to Keys blog we talked about foreign keys. Foreign keys are keys that reference a primary key in another table. Foreign keys. Each UNIX System implementation has its own set of kernel processes that . Some operating systems combine the operations from step 2, a fork followed by an .. For a parent and child relationship, we often have the following scenario. Child Process Definition - A child process is the creation of a parent process, which In systems such as Unix and Linux, the first process, "init", is created by the.
From the above output, we can observe 7 attribute fields listed for each file. From right to left, the attribute fields are: If the file has not been modified within the last year or six months for Linuxthe year of last modification is displayed.
Note also if the file is a directory, this is the size of the structure needed to manage the directory hierarchy. Understanding and Modifying File Permissions If you look closely at the permission field above, you will notice the permission field for each file consists of 10 characters as described by the diagram below: The first leftmost character indicates the "type" of the file.
Another way to describe this is whether the file has any special attributes associated with it. If it is an ordinary file i. If it is a directory file, it will have the letter d in this position. Or, if it is a link to another file it will have the letter l ell in this first position.
You can see examples of an ordinary file and a directory in the ls -l output above. Other special attributes exist but do not merit discussion here. The next 9 characters are arranged in 3 groups of 3 characters each; that is 3 characters to describe the permissions for the owner of the file, 3 characters to describe the permissions for the group and 3 characters for all other users permissions. The 3 characters indicate whether the particular user has read denoted by rwrite denoted by wor execute denoted by x permissions on that particular file.
Thus in the diagram above, the owner of the file has all available permissions indicated by rwxa user belonging to this group has read and execute indicated by r-x permissions, and everyone else also has read and execute permissions. Observe if a user does not have a particular permission, a dash will appear instead of the corresponding letter.
Changing the permission of an existing file is accomplished using the chmod command, that is to change file protection modes.
Changing the permission for a file or files can only be done by the owner of the file or the root user. The usage of this command using octal mode 1 is chmod ijk file s where ijk represent 3 octal numbers ; where i selects the user permissions, j selects the group permissions and k selects the permissions for all others.
To disable all read, write and execute permissions for any group, you set the respective i, j or k value to 0. To set read permission for any grouping i. The values of 4, 2, and 1 are derived from the first three powers of two, i. The command to select this would then look as follows: As a user, you will not have to remember every combination of 4, 2, and 1; typically you will standard combinations such as,etc. The permissions of read, write and execute take on a slightly different meaning with respect to directory files.
Thus if a file is a directory: This allows a user with write permission to a directory to have the ability to delete files in the directory even if they don't have write permissions for the file see Note: Watch out for this gotcha!
The Unix File System
This is somewhat similar to chmod, but the permissions are set on newly created files only. While similar to chmod, rather than setting which permissions are set, umask sets which permissions are "removed" or unset.
General syntax is as follows: If the -S option is specified and mode omitted, umask reports the umask mode in symbolic mode. If a 3 digit mode value is given without the -Sthe mode value specified is "removed from" e. Bounded capacity- There is a certain pre-determined finite capacity in the queue. Senders must block if the queue is full, until space becomes available in the queue, but may be either blocking or non-blocking otherwise. Unbounded capacity - The queue has a theoretical infinite capacity, so senders are never forced to block.
The second parameter indicates how big the shared memory segment is to be, in bytes.
UNIX for Dummies Questions & Answers
The third parameter is a set of bitwise ORed flags. In this case the segment is being created for reading and writing. The return value of shmget is an integer identifier Any process which wishes to use the shared memory must attach the shared memory to their address space, using shmat: NULL indicates that the system should decide.
The third parameter is a flag for read-only operation. Zero indicates read-write; One indicates readonly. In this example it is being used as a character pointer. Then processes may access the memory using the pointer returned by shmat, for example using sprintf: The name provided will be the name of the memory-mapped file.
In this example the process that created the shared memory will be writing to it: Note that the "borrower" must use the same file name as the "lender" who created it. This information could have been passed using messages. Mach Recall that the Mach kernel is a micro kernel, which performs few services besides delivering messages between other tasks both system tasks and user tasks.
Most communication in Mach, including all system calls and inter-process communication is done via messages sent to mailboxes, also known as ports. Whenever a task process is created, it automatically gets two special mailboxes: The kernel communicates with the task using the Kernel mailbox. The kernel sends notification of events to the Notify mailbox. Three system calls are used for message transfer: Only one task at a time can own or receive messages from any given mailbox, but these are transferable.
Messages from the same sender to the same receiver are guaranteed to arrive in FIFO order, but no guarantees are made regarding messages from multiple senders. Messages consist of a fixed-length header followed by variable length data. The header contains the mailbox number address of the receiver and the sender. The data section consists of a list of typed data items, each containing a type, size, and value.
If the receiver's mailbox is full, the sender has four choices: Wait indefinitely until there is room in the mailbox. Wait at most N milliseconds. Do not wait at all. Temporarily cache the message with the kernel, for delivery when the mailbox becomes available.
Only one such message can be pending at any given time from any given sender to any given receiver. Normally only used by certain system tasks, such as the print spooler, which must notify the "client" of the completion of their job, but cannot wait around for the mailbox to become available.
Receive calls must specify the mailbox or mailbox set from which they wish to receive messages. If there are no messages available in a mailbox setthe receiver can either block for N milliseconds, or not block at all. In order to avoid delays caused by copying messages multiple timesMach re-maps the memory space for the message from the sender's address space to the receiver's address space using virtual memory techniques to be covered laterand does not actually move the message anywhere at all.
When the sending and receiving task are both on the same computer. Windows XP Figure 3. Two processes communicating over a network often use a pair of connected sockets as a communication channel. Software that is designed for client-server operation may also use sockets for communication between two processes running on the same computer - For example the UI for a database program may communicate with the back-end database manager using sockets.
If the program were developed this way from the beginning, it makes it very easy to port it from a single-computer system to a networked application. A socket is identified by an IP address concatenated with a port number, e.
For example, telnet servers listen to port 23, ftp servers to port 21, and web servers to port The parent goes back to waiting for the next service request to arrive. When a process wants to execute a different program. This is common for shells. In this case, the child does an exec right after it returns from the fork. Some operating systems combine the operations from step 2, a fork followed by an exec, into a single operation called a spawn.
The UNIX System separates the two, as there are numerous cases where it is useful to fork without doing an exec. The vfork function was intended to create a new process for the purpose of executing a new program step 2 at the end of the previous section. Instead, the child runs in the address space of the parent until it calls either exec or exit.
This optimization is more efficient on some implementations of the UNIX System, but leads to undefined results if the child: When the child calls either of these functions, the parent resumes. This can lead to deadlock if the child depends on further actions of the parent before calling either of these two functions.
The incrementing of the variables done by the child changes the values in the parent. Because the child runs in the address space of the parent. If we call exit instead, the results are indeterminate. Because the process is about to exit, the kernel will close all the file descriptors open in the process.
Closing them in the library simply adds overhead without any benefit. Executing a return from the main function. This is equivalent to calling exit. ISO C does not deal with file descriptors, multiple processes parents and childrenand job control. The definition of this function is incomplete for a UNIX system. 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.
When the last thread returns from its start routine, the process exits with a termination status of 0. The three forms of abnormal termination: When the process receives certain signals. The signal can be generated by: By default, cancellation occurs in a deferred manner: 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 so on. The terminating process is to be able to notify its parent how it terminated by by passing an exit status as the argument to one of the three exit functions.
In the case of an abnormal termination, 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.
If the child terminated normally, the parent can obtain the exit status of the child. The child has a parent process after the call to fork. What happens if the parent terminates before the child? The answer is the init process becomes the parent process of any process whose parent terminates.
This is called "the process has been inherited by init". Whenever a process terminates, the kernel goes through all active processes to see whether the terminating process is the parent of any process that still exists. If so, the parent process ID of the surviving process is changed to be 1 the process ID of init.