Advanced Programming in the UNIX® Environment

ptg10805159

Section 7.3 Process Termination 199

Historically,theexitfunction has always performed a clean shutdown of the

standardI/O library: thefclosefunction is called for all open streams. Recall from

Section 5.5 that this causes all buffered output data to be flushed (written to the file).

All three exit functions expect a single integer argument, which we call theexit

status.Most UNIX System shells provide a way to examine the exit status of a process.

If (a) any of these functions is called without an exit status, (b)maindoes areturn

without a return value, or (c) themainfunction is not declared to return an integer,the

exit status of the process is undefined. However, if the return type ofmainis an integer

andmain‘‘falls offthe end’’(an implicit return), the exit status of the process is 0.

This behavior is new with the 1999 version of the ISO C standard. Historically,the exit status

was undefined if the end of themainfunction was reached without an explicitreturn

statement or a call to theexitfunction.

Returning an integer value from themainfunction is equivalent to callingexit

with the same value. Thus

exit(0);

is the same as

return(0);

from themainfunction.

Example

The program in Figure7.1 is the classic ‘‘hello, world’’example.

#include <stdio.h>

main()

{

printf("hello, world\n");

}

Figure 7.1 Classic C program

When we compile and run the program in Figure7.1, we see that the exit code is

random. If we compile the same program on different systems, we arelikely to get

different exit codes, depending on the contents of the stack and register contents at the

time that themainfunction returns:

$gcc hello.c

$./a.out

hello, world

$echo $? print the exit status

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