Advanced Programming in the UNIX® Environment

ptg10805159

Section 7.3 Process Termination 201

These exit handlers first appeared in the ANSI C Standard in 1989. Systems that predate ANSI

C, such as SVR3 and 4.3BSD, did not provide these exit handlers.

ISO C requires that systems support at least 32 exit handlers, but implementations often

support more(see Figure2.15). Thesysconffunction can be used to determine the

maximum number of exit handlers supported by a given platform, as illustrated in Figure2.14.

With ISO C and POSIX.1,exitfirst calls the exit handlers and then closes (via

fclose)all open streams. POSIX.1 extends the ISO C standard by specifying that any

exit handlers installed will be cleared if the program calls any of theexecfamily of

functions. Figure7.2 summarizes how a C program is started and the various ways it

can terminate.

user

functions

main

function

Cstart-up

routine

exit

function

exit handler

...

exit handler

standardI/O

cleanup

return

return

call

call

exec

_exit

or

_Exit

_exit

or

_Exit

_exit

or

_Exit

(does not return)exit

exit

(does not return)

exit

(does not return)

call

return

call

return

call

return

kernel

user process

Figure 7.2 How a C program is started and how it terminates

The only way a program can be executed by the kernel is if one of theexecfunctions is

called. The only way a process can voluntarily terminate is if_exitor_Exitis called,

either explicitly or implicitly (by callingexit). A process can also be involuntarily

terminated by a signal (not shown in Figure7.2).