In C programming language the _c_exit and _cexit functions perform cleanup operations and return without terminating the calling process.
They are declared in process.h

Process.h

process.h is a C header file which contains function declarations and macros used in working with threads and processes. Most C compilers that target DOS, Windows 3.1x, Win32, OS/2, Novell NetWare or DOS extenders supply this header and the library functions in their C library...

 header file

Header file

Some programming languages use header files. These files allow programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers...

, and are not defined by the ANSI/ISO C standard

ANSI C

ANSI C refers to the family of successive standards published by the American National Standards Institute for the C programming language. Software developers writing in C are encouraged to conform to the standards, as doing so aids portability between compilers.-History and outlook:The first...

 nor POSIX

POSIX

POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...

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The _c_exit function performs a quick C library termination procedure and returns to the caller without processing registered exit functions (atexit or _onexit) or flushing buffers.

In contrast, the _cexit function performs a complete C library termination procedure by calling registered exit functions in LIFO order, flushing all I/ O buffers, and closing all open streams before returning to the caller.

Both functions restore interrupt vectors altered by the startup code.