_onexit, _onexit_m

Registers a routine to be called at exit time.

_onexit_t _onexit(
   _onexit_t function
_onexit_t_m _onexit_m(
   _onexit_t_m function


  • function
    Pointer to a function to be called at exit.

Return Value

_onexit returns a pointer to the function if successful or NULL if there is no space to store the function pointer.


The _onexit function is passed the address of a function (function) to be called when the program terminates normally. Successive calls to _onexit create a register of functions that are executed in LIFO (last-in-first-out) order. The functions passed to _onexit cannot take parameters.

In the case when _onexit is called from within a DLL, routines registered with _onexit run on a DLL's unloading after DllMain is called with DLL_PROCESS_DETACH.

_onexit is a Microsoft extension. For ANSI portability, use atexit. The _onexit_m version of the function is for mixed mode use.



Required header



For more compatibility information, see Compatibility in the Introduction.


// crt_onexit.c

#include <stdlib.h>
#include <stdio.h>

/* Prototypes */
int fn1(void), fn2(void), fn3(void), fn4 (void);

int main( void )
   _onexit( fn1 );
   _onexit( fn2 );
   _onexit( fn3 );
   _onexit( fn4 );
   printf( "This is executed first.\n" );

int fn1()
   printf( "next.\n" );
   return 0;

int fn2()
   printf( "executed " );
   return 0;

int fn3()
   printf( "is " );
   return 0;

int fn4()
   printf( "This " );
   return 0;


This is executed first.
This is executed next.

.NET Framework Equivalent


See Also


Process and Environment Control


exit, _exit