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A high-level overview of the Linux kernel's system call interface, which handles communication between its various components and the userspace. In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system [a] on which it is executed.
The exec calls named ending with an e alter the environment for the new process image by passing a list of environment settings through the envp argument. This argument is an array of character pointers; each element (except for the final element) points to a null-terminated string defining an environment variable .
The system call interface of a kernel is the set of all implemented and available system calls in a kernel. In the Linux kernel, various subsystems, such as the Direct Rendering Manager (DRM), define their own system calls, all of which are part of the system call interface.
The operating system code and application code are not executed in separated protection domains (there is only a single application running, at least conceptually, so there is no need to prevent interference between applications) and OS services are accessed via simple library calls (potentially inlining them based on compiler thresholds ...
Modern operating systems also provide system calls that allow a process's thread to create other threads and wait for them to terminate ("join" them) in a similar fashion. An operating system may provide variations of the wait call that allow a process to wait for any of its child processes to exit , or to wait for a single specific child ...
A system call usually takes the form of a "system call vector", in which the desired system call is indicated with an index number. For instance, exit() might be system call number 1, and write() number 4. The system call vector is then used to find the desired kernel function for the request.
For a process to start the execution of a different program, it first forks to create a copy of itself. Then, the copy, called the "child process", calls the exec system call to overlay itself with the other program: it ceases execution of its former program in favor of the other. The fork operation creates a separate address space for the ...
The system call approach has the interesting property that there is not necessarily any OS process. Instead, a process executing in user mode changes to kernel mode when it is executing kernel code, and switches back to user mode when it returns from the OS call. If, on the other hand, the OS is designed as a set of separate processes, it is ...