Should we use exit() in C?

Question

There is question about using exit in C++. The answer discusses that it is not good idea mainly because of RAII, e.g., if exit is called somewhere in code, destructors of objects will not be called, hence, if for example a destructor was meant to write data to file, this will not happen, because the destructor was not called.

I was interested how is this situation in C. Are similar issues applicable also in C? I thought since in C we don't use constructors/destructors, situation might be different in C. So is it ok to use exit in C? For example I have seen following functions sometimes used in C:

void die(const char *message)
{
    if(errno) {
        perror(message);
    } else {
        printf("ERROR: %s\n", message);
    }

    exit(1);
}

Answer 1

Rather than abort(), the exit() function in C is considered to be a "graceful" exit.

From C11 (N1570) 7.22.4.4/p2 The exit function (emphasis mine):

The exit function causes normal program termination to occur.

The Standard also says in 7.22.4.4/p4 that:

Next, all open streams with unwritten buffered data are flushed, all open streams are closed, and all files created by the tmpfile function are removed.

It is also worth looking at 7.21.3/p5 Files:

If the main function returns to its original caller, or if the exit function is called, all open files are closed (hence all output streams are flushed) before program termination. Other paths to program termination, such as calling the abort function, need not close all files properly.

However, as mentioned in comments below you can't assume that it will cover every other resource, so you may need to resort to atexit() and define callbacks for their release individually. In fact it is exactly what atexit() is intended to do, as it says in 7.22.4.2/p2 The atexit function:

The atexit function registers the function pointed to by func, to be called without arguments at normal program termination.

Notably, the C standard does not say precisely what should happen to objects of allocated storage duration (i.e. malloc()), thus requiring you be aware of how it is done on particular implementation. For modern, host-oriented OS it is likely that the system will take care of it, but still you might want to handle this by yourself in order to silence memory debuggers such as Valgrind.

Answer 2

Yes, it is ok to use exit in C.

To ensure all buffers and graceful orderly shutdown, it would be recommended to use this function atexit, more information on this here

An example code would be like this:

void cleanup(void){
   /* example of closing file pointer and free up memory */
   if (fp) fclose(fp);
   if (ptr) free(ptr);
}

int main(int argc, char **argv){
   /* ... */
   atexit(cleanup);
   /* ... */
   return 0;
}

Now, whenever exit is called, the function cleanup will get executed, which can house graceful shutdown, clean up of buffers, memory etc.

Answer 3

You don't have constructors and destructors but you could have resources (e.g. files, streams, sockets) and it is important to close them correctly. A buffer could not be written synchronously, so exiting from the program without correctly closing the resource first, could lead to corruption.

Answer 4

Using exit() is OK

Two major aspects of code design that have not yet been mentioned are 'threading' and 'libraries'.

In a single-threaded program, in the code you're writing to implement that program, using exit() is fine. My programs use it routinely when something has gone wrong and the code isn't going to recover.

But…

However, calling exit() is a unilateral action that can't be undone. That's why both 'threading' and 'libraries' require careful thought.

Threaded programs

If a program is multi-threaded, then using exit() is a dramatic action which terminates all the threads. It will probably be inappropriate to exit the entire program. It may be appropriate to exit the thread, reporting an error. If you're cognizant of the design of the program, then maybe that unilateral exit is permissible, but in general, it will not be acceptable.

Library code

And that 'cognizant of the design of the program' clause applies to code in libraries, too. It is very seldom correct for a general purpose library function to call exit(). You'd be justifiably upset if one of the standard C library functions failed to return just because of an error. (Obviously, functions like exit(), _Exit(), quick_exit(), abort() are intended not to return; that's different.) The functions in the C library therefore either "can't fail" or return an error indication somehow. If you're writing code to go into a general purpose library, you need to consider the error handling strategy for your code carefully. It should fit in with the error handling strategies of the programs with which it is intended to be used, or the error handling may be made configurable.

I have a series of library functions (in a package with header "stderr.h", a name which treads on thin ice) that are intended to exit as they're used for error reporting. Those functions exit by design. There are a related series of functions in the same package that report errors and do not exit. The exiting functions are implemented in terms of the non-exiting functions, of course, but that's an internal implementation detail.

I have many other library functions, and a good many of them rely on the "stderr.h" code for error reporting. That's a design decision I made and is one that I'm OK with. But when the errors are reported with the functions that exit, it limits the general usefulness the library code. If the code calls the error reporting functions that do not exit, then the main code paths in the function have to deal with error returns sanely — detect them and relay an error indication to the calling code.

---

The code for my error reporting package is available in my SOQ (Stack Overflow Questions) repository on GitHub as files stderr.c and stderr.h in the src/libsoq sub-directory.

Answer 5

One reason to avoid exit in functions other than main() is the possibility that your code might be taken out of context. Remember, exit is a type of non local control flow. Like uncatchable exceptions.

For example, you might write some storage management functions that exit on a critical disk error. Then someone decides to move them into a library. Exiting from a library is something that will cause the calling program to exit in an inconsitent state which it may not be prepared for.

Or you might run it on an embedded system. There is nowhere to exit to, the whole thing runs in a while(1) loop in main(). It might not even be defined in the standard library.

Answer 6

Depending on what you are doing, exit may be the most logical way out of a program in C. I know it's very useful for checking to make sure chains of callbacks work correctly. Take this example callback I used recently:

unsigned char cbShowDataThenExit( unsigned char *data, unsigned short dataSz,unsigned char status)
{

    printf("cbShowDataThenExit with status %X (dataSz %d)\n", status, dataSz);
    printf("status:%d\n",status);
    printArray(data,dataSz);
    cleanUp();
    exit(0);
}

In the main loop, I set everything up for this system and then wait in a while(1) loop. It is possible to make a global flag to exit the while loop instead, but this is simple and does what it needs to do. If you are dealing with any open buffers like files and devices you should clean them up before close for consistency.