64 lines
1.5 KiB
Markdown
64 lines
1.5 KiB
Markdown
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+++
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date="2022-12-24"
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author="spanskiduh"
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title="fork"
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description="click to read about fork"
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# Fork
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### What happenes when you call `fork()` in your program?
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Parent creates a child and spawns it, it is basically a copy of a parent process
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excluding:
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- `PID`, `PPID`
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- file locks, mutex locks, any other locks...
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including:
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- same code
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- !copy! of same heap and stack
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- opened files
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### Example
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```c
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int pid = fork()
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if (pid > 0){
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// parent process
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} else if (pid == 0) {
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// child process
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} else { // pid < 0
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// we encountered err!
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}
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```
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### What happenes when cild process is exited?
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Exit status is saved in process descriptor, until parent process waits for it.
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Until exit status is not taken then we say that the process is **zombie**:
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- proces is exited, and it does not take any system resources.
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- only its process descriptor is not yet taken
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- and we do not want many zombies in our system
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### How parent knows, that the child process has been terminated?
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Well we can use `singal()` to notify parent when the child process exits. Namely signal `WAITCHLD`.
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Waiting for specific child:
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```c
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waitpid(pid, &status, options)
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```
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Waiting for any child:
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```c
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wait(&status);
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// or
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waitpid(-1, &status, 0):
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```
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### Exit statuses
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- 8bit value
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- 0 means success
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- 1-127 error
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- 128-255 termination becouse of singal (`sig_num = status - 127`)
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## `vfork()` function
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V fork is a modified fork, which does not copy any data. It is meant to be used with comibnation with function `exec()`.
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