Kernel, User Mode, and System Calls

# A library write() is not a system call. It appends to a user-space
# buffer, and only a full buffer (or a flush) traps into the kernel.
BUF = 16

buffer = ''
syscalls = 0

def flush():
    global buffer, syscalls
    if buffer:
        syscalls += 1          # this is the only place we enter the kernel
        buffer = ''

def write(line):
    # the library call
    global buffer
    if len(buffer) + len(line) > BUF:
        flush()
    buffer += line

lines = ['one\n', 'two\n', 'three\n', 'four\n', 'five\n', 'six\n']
for line in lines:
    write(line)
flush()                        # fclose/exit flushes what is left

print(f'{len(lines)} library calls -> {syscalls} system calls')

#include <iostream>
#include <string>
#include <vector>
using namespace std;
// A library write() is not a system call. It appends to a user-space
// buffer, and only a full buffer (or a flush) traps into the kernel.
const size_t BUF = 16;
string buffer;
int syscalls = 0;
void flush() {
    if (!buffer.empty()) {
        syscalls++; // the only place we enter the kernel
        buffer.clear();
    }
}
void write(const string& line) {   // the library call
    if (buffer.size() + line.size() > BUF) flush();
    buffer += line;
}
int main() {
    vector<string> lines = {"one\n", "two\n", "three\n", "four\n", "five\n", "six\n"};
    for (const string& line : lines) write(line);
    flush(); // fclose/exit flushes what is left
    cout << lines.size() << " library calls -> " << syscalls << " system calls\n";
}

import java.util.List;
class Main {
    // A library write() is not a system call. It appends to a user-space
    // buffer, and only a full buffer (or a flush) traps into the kernel.
    static final int BUF = 16;
    static StringBuilder buffer = new StringBuilder();
    static int syscalls = 0;
    static void flush() {
        if (buffer.length() > 0) {
            syscalls++; // the only place we enter the kernel
            buffer.setLength(0);
        }
    }
    static void write(String line) {   // the library call
        if (buffer.length() + line.length() > BUF) flush();
        buffer.append(line);
    }
    public static void main(String[] args) {
        List<String> lines = List.of("one\n", "two\n", "three\n", "four\n", "five\n", "six\n");
        for (String line : lines) write(line);
        flush(); // fclose/exit flushes what is left
        System.out.println(lines.size() + " library calls -> " + syscalls + " system calls");
    }
}