C exit(), abort() and assert() Functions

The C exit(), abort(), and assert() functions are all used to terminate a C program but each of them works differently from the other. In this article, we will learn about exit, abort, and assert functions and their use in C programming language.

1. exit() in C

The C exit() function is a standard library function used to terminate the calling process. When exit() is called, any open file descriptors belonging to the process are closed and any children of the process are inherited by process 1, init, and the process parent is sent a SIGCHLD signal.

It is defined inside the <stdlib.h> header file.

Syntax of exit() in C

void exit(int status);

Parameters

The exit() function in C only takes a single parameter status which is the exit code that is returned to the caller i.e. either the operating system or parent process. There are two valid values we can use as the status each having a different meaning. They are as follows:

• 0 or EXIT_SUCCESS: 0 or EXIT_SUCCESS means that the program has been successfully executed without encountering any error.
• 1 or EXIT_FAILURE: 1 or EXIT_FAILURE means that the program has encountered an error and could be executed successfully.

Note: We can actually return any non-zero return value in case of failure.

Return Value

• This function doesn't return any value to the current process so its return type is void. Instead, it returns the status value to the parent process which is returned through a method different than the return value.

Example 1:  C Program to Illustrate the exit() Function

Error opening file

How exit() in C works?

When called, the exit() function in C performs the following operations:

1. Flushes unwritten buffered data.
2. Closes all open files.
3. Removes temporary files.
4. Returns an integer exit status to the operating system.

Example 2: Passing an integer value greater than 255 to the exit() function in C

Exit code: 15

Explanation

In the above function, instead of 9999, the status value is 15. It is an effect of 8-bit integer overflow. After 255 (all 8 bits set) comes 0. As the exit() supports only 8-bit integer values, the output is "exit code modulo 256". The output above is actually the modulo of the value 9999 and 256 i.e. 15.

The C standard atexit() function can be used to customize exit() to perform additional actions at program termination.

2. abort() in C

The C abort() function is the standard library function that can be used to exit the C program. But unlike the exit() function, abort() may not close files that are open. It may also not delete temporary files and may not flush the stream buffer. Also, it does not call functions registered with atexit().

Syntax of abort() in C

void abort(void);

Parameters

• The C abort() function does not take any parameter and does not have any return value. This function actually terminates the process by raising a SIGABRT signal, and your program can include a handler to intercept this signal.

Return Value

• The abort() function in C does not return any value.

Example: C Program to Illustrate the abort() Function

Output

timeout: the monitored command dumped core
/bin/bash: line 1: 35 Aborted

Note: If we want to make sure that data is written to files and/or buffers are flushed then we should either use exit() or include a signal handler for SIGABRT.

3. assert() in C

The C assert() function is a macro defined inside the <assert.h> header file. It is a function-like macro that is used for debugging. It takes an expression as a parameter,

• If the expression evaluates to 1 (true), the program continue to execute.
• If the expression evaluates to 0 (false), then the expression, source code filename, and line number are sent to the standard error, and then the abort() function is called.

Syntax of assert() in C

void assert(expression);

Parameters

• expression: It is the condition we want to evaluate.

Return Value

• The assert() function does not return any value.

If the identifier NDEBUG ("no debug") is defined with #define NDEBUG then the macro assert does nothing. Common error output is in the form: Assertion failed: expression, file filename, line line-number.

Example: C Program to Illustrate the assert() Function

Output

test.c:7: open_record: Assertion `record_name != ((void *)0)' failed.
timeout: the monitored command dumped core
/bin/bash: line 1: 34 Aborted