Introduction to System Calls
System calls are essential building blocks in Linux, acting as the main mechanism through which user programs interact with the operating system kernel. When you write a program in C that needs to access hardware resources—such as files, devices, or network connections—it cannot do so directly. Instead, your program must ask the kernel to perform these operations on its behalf. This request is made through a system call.
A system call is a controlled entry point into the kernel, designed to protect the system from accidental or malicious interference. By enforcing strict boundaries between user programs and kernel operations, system calls ensure system stability and security. In Linux, nearly all operations that require privileged access—such as reading from or writing to a file, creating new processes, or allocating memory—are performed via system calls.
The Linux kernel exposes a set of predefined system calls, each identified by a unique number. User programs typically do not invoke these calls directly; instead, they use higher-level library functions, such as those provided by the C standard library (libc). However, it is possible to bypass these wrappers and invoke system calls directly, which is sometimes necessary for low-level programming or systems development.
main.c
12345678#include <unistd.h> #include <sys/syscall.h> int main() { const char msg[] = "Hello from a direct system call!\n"; syscall(SYS_write, 1, msg, sizeof(msg) - 1); return 0; }
In the program above, you see a direct use of the Linux write system call with the syscall function. Here, SYS_write identifies the write system call, 1 specifies the file descriptor for standard output, msg is a pointer to the message to print, and sizeof(msg) - 1 gives the number of bytes to write. When you invoke syscall, your program switches from user mode to kernel mode so the kernel can process the request. This is a low-level interface, bypassing the standard C library's buffered I/O functions such as printf or puts.
The difference between a system call and a regular library function is crucial. System calls provide a secure, controlled way to access kernel services, while library functions are just code running in user space. Library functions like printf may internally use system calls (for example, to actually write data to a file or terminal), but they can also perform additional work such as formatting or buffering. By calling a system call directly, you interact with the kernel without any intermediate layers, giving you more control but also more responsibility for handling low-level details.
Bedankt voor je feedback!
Vraag AI
Vraag AI
Vraag wat u wilt of probeer een van de voorgestelde vragen om onze chat te starten.
Geweldig!
Completion tarief verbeterd naar 7.69Introduction to System Calls
Veeg om het menu te tonen
System calls are essential building blocks in Linux, acting as the main mechanism through which user programs interact with the operating system kernel. When you write a program in C that needs to access hardware resources—such as files, devices, or network connections—it cannot do so directly. Instead, your program must ask the kernel to perform these operations on its behalf. This request is made through a system call.
A system call is a controlled entry point into the kernel, designed to protect the system from accidental or malicious interference. By enforcing strict boundaries between user programs and kernel operations, system calls ensure system stability and security. In Linux, nearly all operations that require privileged access—such as reading from or writing to a file, creating new processes, or allocating memory—are performed via system calls.
The Linux kernel exposes a set of predefined system calls, each identified by a unique number. User programs typically do not invoke these calls directly; instead, they use higher-level library functions, such as those provided by the C standard library (libc). However, it is possible to bypass these wrappers and invoke system calls directly, which is sometimes necessary for low-level programming or systems development.
main.c
12345678#include <unistd.h> #include <sys/syscall.h> int main() { const char msg[] = "Hello from a direct system call!\n"; syscall(SYS_write, 1, msg, sizeof(msg) - 1); return 0; }
In the program above, you see a direct use of the Linux write system call with the syscall function. Here, SYS_write identifies the write system call, 1 specifies the file descriptor for standard output, msg is a pointer to the message to print, and sizeof(msg) - 1 gives the number of bytes to write. When you invoke syscall, your program switches from user mode to kernel mode so the kernel can process the request. This is a low-level interface, bypassing the standard C library's buffered I/O functions such as printf or puts.
The difference between a system call and a regular library function is crucial. System calls provide a secure, controlled way to access kernel services, while library functions are just code running in user space. Library functions like printf may internally use system calls (for example, to actually write data to a file or terminal), but they can also perform additional work such as formatting or buffering. By calling a system call directly, you interact with the kernel without any intermediate layers, giving you more control but also more responsibility for handling low-level details.
Bedankt voor je feedback!
