When a process completes its execution or is terminated prematurely, the kernel is responsible for releasing all system resources that were allocated to it. This includes closing open files, freeing memory, and other system-level cleanup tasks. The image below illustrates the process of termination in Linux:
**Process Exit in Linux**
In Linux, a process can exit in two main ways: normally or abnormally.
**1. Normal Exit**
A process exits normally when:
- It reaches the end of the `main()` function and returns a value.
- It calls the `exit()` function.
- It calls the `_exit()` function.
**2. Abnormal Exit**
An abnormal exit occurs when:
- The program calls the `abort()` function.
- The process receives a signal that causes it to terminate unexpectedly (e.g., `SIGKILL`, `SIGSEGV`).
Regardless of how the process exits, the kernel performs the same set of operations: it closes file descriptors, frees memory, and cleans up any remaining resources.
**Comparison of Exit Methods**
**Difference Between `exit()` and `return`**
The `exit()` function is a standard library function that terminates the program and returns control to the operating system. On the other hand, `return` is used within a function to return control to the caller. When `main()` returns, it's essentially calling `exit()` implicitly.
**Difference Between `exit()` and `abort()`**
The `exit()` function allows for a clean shutdown, including flushing buffers and running any registered cleanup functions. In contrast, `abort()` causes an immediate termination without performing these steps, which can lead to data loss or inconsistent states.
**`exit()` vs `_exit()`**
Both `exit()` and `_exit()` are used to terminate a process, but they differ in their behavior:
- `exit()` is declared in `stdlib.h`. It performs cleanup tasks such as flushing buffered I/O, closing file descriptors, and executing any registered functions using `atexit()`.
- `_exit()` is declared in `unistd.h`. It immediately transfers control back to the kernel without performing any of the cleanup steps. This means that any buffered data may be lost if not properly flushed.
One key difference is that `exit()` ensures that all output buffers are written to disk before the process terminates, while `_exit()` does not. This is especially important when dealing with standard I/O streams like `stdout`.
For example, consider the following code snippets:
**Example 1: `exit.c`**
```c
#include
#include
int main() {
printf("using exit----");
printf("This is the content in buffer");
exit(0);
}
```
Output:
```
using exit----
This is the content in buffer
```
**Example 2: `_exit.c`**
```c
#include
#include
int main() {
printf("using _exit--");
printf("This is the content in buffer");
_exit(0);
}
```
Output:
```
using _exit--
```
The reason for this difference is that `printf()` uses buffered I/O. The buffer is only flushed when a newline character is encountered or when the program exits cleanly via `exit()`. `_exit()` skips this step, leading to potential data loss.
**Parent and Child Process Termination Order**
The order in which a parent and child process terminate can have significant consequences:
**1. Parent Exits Before Child**
If the parent process terminates first, the child becomes an orphan. The system then reassigns the orphaned process to the `init` process (PID 1), which will eventually wait for it to finish.
**2. Child Exits Before Parent, No `wait()` Called**
If the child process exits before the parent, and the parent doesn’t call `wait()`, the child enters a "zombie" state. In this state, the process is no longer running, but the kernel still keeps some information about it. This prevents the system from fully releasing the process’s resources, potentially reducing the maximum number of processes the system can handle.
**3. Child Exits Before Parent, `wait()` Is Called**
If the parent calls `wait()` after the child has exited, the parent will wait for the child to complete, ensuring that all resources are properly released and no zombie processes are left behind.
Understanding these behaviors is crucial for writing robust and efficient multi-process applications in Linux.
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