𝐄𝐱𝐩𝐥𝐨𝐫𝐢𝐧𝐠 𝐭𝐡𝐞 𝐒𝐲𝐬𝐭𝐞𝐦 𝐜𝐚𝐥𝐥𝐬

To transition from the user space to kernel space, the user/compiler uses the system call. System calls are the entry points into the kernel, enabling necessary actions that are to be performed on kernel on behalf of processes. The system call changes the processor state from user mode to kernel mode, granting the CPU access to protected kernel memory. One example of utilizing system calls is through dynamic memory allocators like malloc() and realloc(), which dynamically allocate memory. It’s important to note that while malloc() and realloc() are library functions, the underlying system calls they rely on are sbrk() and brk().

Let’s delve into one of the system call used in process management:

𝐬𝐲𝐬𝐭𝐞𝐦(): This system call allows a calling program to execute a shell command. It has the following syntax:
𝐢𝐧𝐭 𝐬𝐲𝐬𝐭𝐞𝐦(𝐜𝐨𝐧𝐬𝐭 𝐜𝐡𝐚𝐫 *𝐜𝐨𝐦𝐦𝐚𝐧𝐝);

Internally, system() uses fork() to create a child process, which then executes the specified shell command using execl(). The command is typically executed by calling “/𝐛𝐢𝐧/𝐬𝐡 -𝐜”. The return value of system() is the termination status of the child shell used to execute the command.

Function called by system() -> excel(“/bin/sh”,”sh”,”-c”,command,(char *) NULL);

Inputs to system() can be given at compile time, load time, execution time . The delimiter “;” can be used to give multiple commands. System() is main back drop is its inefficiency. When system() is used, a minimum of 2 processes are created along with it, one for shell and one more for command. It internally uses other process management system calls such as clone(), wait(),rt_sigaction() etc.

𝐜𝐥𝐨𝐧𝐞(): This system call creates a new process, known as a child process, which shares the same memory space as the parent process. It provides a flexible way to create processes with various levels of sharing between the parent and child processes.

𝐰𝐚𝐢𝐭(): This system call suspends the execution of the calling process until one of its child processes terminates. It allows the parent process to wait for the termination of specific child processes or any child process.

𝐫𝐭_𝐬𝐢𝐠𝐚𝐜𝐭𝐢𝐨𝐧(): This system call is used to set up a signal handler for a specified signal. It allows processes to handle various signals asynchronously, enabling them to respond appropriately to events such as interrupts or exceptional conditions.

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An Article by: Yashwanth Naidu Tikkisetty
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