Technical Guide to System Calls: Implementation and Signal Handling in Modern OS

Summary

Table of Contents Introduction System calls are fundamental interfaces between user applications and the operating system kernel. They allow programs to request services from the operating system, such as file operations, process control, and network access. One important distinction in system calls is between “fast” and “slow” system calls, which affects how they interact with signal handling, process scheduling, and overall system performance. In this article, we’ll explore: The core differences between fast and slow system calls The mechanism behind how signals can wake up blocked system calls Practical examples with code demonstrations How the kernel handles these different types of system calls Core Definitions: Fast vs. Slow System Calls Fast System Calls Fast system calls are operations that can be completed immediately without requiring the kernel to wait for external events. They typically: Return quickly, usually within microseconds Don’t require the calling process to block or sleep Complete with just CPU processing time Don’t need to release the CPU to other processes while executing Common examples include: getpid(): Retrieves the process ID gettimeofday(): Gets the current time getuid(), setuid(): Retrieves or sets user IDs Simple memory operations like brk() Slow System Calls Slow system calls are operations that may need to wait for external events or resources, potentially for an indefinite period. They typically: May block the calling process for an unpredictable amount of time Often involve waiting for I/O operations or other processes Cause the kernel to suspend the calling process until the operation completes Allow the CPU to be allocated to other processes while waiting Common examples include: read() from a pipe, socket, or terminal (when data isn’t immediately available) accept() waiting for network connections wait() waiting for a child process to terminate sleep() deliberately pausing execution The Signal Interaction Mechanism One of the m...