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Chapter 6. Processes > Process ID and Parent Process ID
6.2. Process ID and Parent Process ID
Each process has a process ID (PID), a positive integer that uniquely identifies the process on the system. Process IDs are used and returned by a variety of system calls. For example, the kill() system call (Section 20.5) allows the caller to send a signal to a process with a specific process ID. The process ID is also useful if we need to build an identifier that is unique to a process. A common example of this is the use of the process ID as part of a process-unique filename.
The getpid() system call returns the process ID of the calling process.
