📗 -> 01/09/26: System Calls

🎤 Vocab

❗ Unit and Larger Context

✒️ -> Scratch Notes

C program example

exec_detector.c

Carefully opens a file, and checks if first bytes indicate that the files are executable

0x74 | 0x45 | 0x4c | 0x46 - Means file is ELF form (Executable and Linkable Format, common linux)
# | ! - Means its a script
Else, not executable

Lesson:
This file uses headers - #include <fcntl.h>
Headers are linked at compile time and dynamically loaded at runtime

Sometimes, needs to request privileged operations from OS

Clib Perms

memset - Regular func, only needs to have access to array buf
open - Always privileged function

Verify file exists/is permitted for user/sucessfully reads
Sensitive operations handled by OS through syscalls
printf - Sometimes privileged function
printf points to stdout which is generally internally buffered
Flushing requires OS to write characters (syscall function write)

printf_write.c

printf("Hello ");
sleep(2);
printf("world!\n");

write(STDOUT_FILENO, "Hello ", 6);
sleep(2);
write(STDOUT_FILENO, "world!\n ", 7);

/*
$ /printf_write
<wait 2 sec>
Hello world!
Hello <wait 2 sec>world!
*\

First one is buffered, not printed immediately.
Second one prints immediately.

Sys calls

Def:

Specific CPU instruction
Immediate transfer of control to kernel code
Serves as a secure API between user apps and OS kernel
Handles:
Process management
Files and dirs
Memory
…

Process Management

Process - a program in execution
Has:

A process ID (PID)
Its own memory space
Representation in the OS through a Process Control Block (PCB)
- A data structure
- PID, state, CPU register copies for context switching, open files, etc

Main syscalls

Process creation and execution
	`fork()`: Create a new (clone) process
	`exec()`: Change executed program within running process
Process termination
	`exit()`: End running process
	`wait()/waitpid()`: Wait for a child process and collect exit code
Process identification
	`getpid()`: Get process PID
	`getppid()`: Get parent process PID

fork()

Duplicates process into a clone process
- Child gets an almost identical copy of parent
  - Open files, CLI args, mem, stack, etc
- Child resumes at fork
Fork returns a value, used to distinguish parent and child
- PID of the child to the parent ( if PID > 0 printf("Parent") )
- zero to the child ( if PID == 0 printf("Child") )
- -1 if error ( if PID < 0 printf("Oops") )
Output might not be guaranteed, OS scheduling is involved

exec()

Process starts executing another program

exit()

Terminate current process
Can return an exit val

wait() / waitpid()

Waits for child processes to finish

system()

??

Difference between wait and waitpid

Fork only has one child, so what is difference between specific and any child? nested forks? different commands?
when multiple children

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