An application can load and execute other applications by using DosExecPgm. The new application, once loaded, is called a child process. The process that starts the new application is called the parent process.
A child process is like any other process. It has its own code, data, and threads. The child process inherits the resources-such as file handles, pipes, and queues-that belong to the parent process at the time the child process is created, although not necessarily with the same access rights. The parent process can place restrictions on the access of the child process to these resources:
Assuming that the parent process gives the child process appropriate access rights, the child process can use the inherited resources without preparing them. For example, if the parent process opens a file for reading and then starts a child process, the child process can read from the file immediately; it does not have to open the file. However, once the child process is created additional resources that the parent process creates are not available to the child process. Similarly, resources the child process creates are not available to the parent process.
The parent process also has control over the meanings of standard input, output, and error for the child process. For example, the parent can write a series of records to a file, open the file as standard input, open a listing file as standard output, and then execute a sort program that takes its input from standard input and writes to standard output.
Note that memory is not included in the list of things that a child process can inherit from its parent process. The child process is created with its own process address space that is separate and distinct from the memory of the parent process. A new linear address space is built for the new process. The only way for a parent process and a child process to access the same memory is to set up a shared memory area.
The executable file of the child process can be started either synchronously or asynchronously to the parent process. If the parent process starts the child process running synchronously, the parent process is suspended and waits until the child process ends before continuing. A child process running asynchronously executes independently of the parent process (that is, both run at the same time). The parent process specifies how the child process is to run by setting a parameter in the call to DosExecPgm.
The OS/2 command processor, CMD.EXE, starts most child processes synchronously. The parent process waits for each child process to end before it prompts the user for the next command. The command processor also enables the user to start asynchronous applications by using the DETACH command. When the user detaches an application, the command processor starts the application asynchronously, in the background, and continues to prompt for input.