getpid

Getpid is a POSIX-compliant library function that returns the process identifier (PID) of the calling process. It is commonly used in C and other languages that provide POSIX interfaces. The function is declared in unistd.h and returns a value of type pid_t, a type defined in sys/types.h.

The PID is a unique identifier assigned by the kernel to each active process within the system. The value is valid for the lifetime of the process. When a new process is created via fork, the child receives a new, distinct PID, while the parent retains its original PID. After a process terminates, its PID may be reused by the system for new processes.

Threads and PID semantics: All threads within a single process share the same PID, since they belong to the same process. If a program needs to distinguish individual threads, other identifiers such as thread IDs can be used (for example, gettid on Linux or pthread identifiers through the pthread API).

Usage example in C: pid_t pid = getpid(); printf("PID: %d\n", (int)pid);

Portability and related functions: getpid is part of the POSIX standard, and equivalents or wrappers exist in many languages (for example, Python’s os.getpid()). In contrast, Windows uses a different API (for example, GetCurrentProcessId) to obtain the process ID.

See also: getppid (parent process ID), fork (process creation), pid_t, unistd.h, sys/types.h.