How do you design a system call interface that is portable and efficient?

1 System call interface

The system call interface is the set of functions, constants, and data structures that define how user programs can invoke system calls. It is usually provided by a library, such as the standard C library, that wraps the low-level details of the system call invocation. The interface should be consistent across different platforms and operating systems, so that applications can be portable and reusable. However, the interface should also be flexible and extensible, so that new system calls can be added or modified without breaking existing code.

2 System call invocation

The system call invocation is the process of transferring control from user mode to kernel mode, and passing the arguments and return values between the user program and the operating system. Depending on the hardware architecture and the operating system design, there are various methods of system call invocation. For example, a software interrupt can be triggered by the user program, which causes the processor to switch to kernel mode and execute the interrupt handler. Alternatively, a system call instruction can be executed by the user program, which directly transfers control to the kernel mode and jumps to a predefined address. Additionally, a library function call can be used, which executes a small piece of code, called a stub, that performs the system call invocation.

3 System call dispatching

The system call dispatching is the process of finding and executing the system call function that corresponds to the system call number and the arguments. This can be done in various ways, such as table lookup, switch statement, or hash table. In table lookup, the system call number is used as an index to a table of pointers to system call functions, which is stored in memory or in a register. For switch statement, the system call number is used as a case label in a switch statement, which branches to the system call function. The switch statement can be generated by a compiler or a macro, and the system call function is called by a direct or indirect jump. Lastly, in hash table, the system call number is used as a key to a hash table of pointers to system call functions, which is stored in memory or in a register. The system call function is called by hashing the key and dereferencing the pointer.

4 System call implementation

The system call implementation is the code that performs the actual service requested by the user program. The system call implementation can be divided into two parts: the generic part and the specific part. The generic part is the code that is common to all system calls, such as validating the arguments, checking the permissions, handling the errors, and returning the results. The specific part is the code that is unique to each system call, such as accessing the file system, creating a process, sending a packet, or controlling a device.

5 System call optimization

The system call optimization is the process of improving the performance and the security of the system call mechanism and implementation. This can be done at the hardware level by providing features that reduce the overhead of system call invocation, such as fast system call instructions, dedicated registers, or privileged instructions. At the kernel level, features such as system call tables, system call wrappers, or system call filters can reduce the complexity of system call dispatching and implementation. Finally, the user program can provide features that reduce the frequency of system call invocation, such as caching, buffering, or batching.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?