Compiler component
- C course
- Expert
- Lesson 45/47
- 6 min
Write a small language with lexer and parser.
Compiler component
In the third lesson you add the compiler component. The parse_command function takes a command string and returns a code if it recognizes it. This is a simplified combination of a compiler's lexer and parser steps.
A real compiler first splits source code into tokens, then produces an abstract syntax tree (AST). Here we compare the command string with strcmp and return an integer: 1 for "help", 2 for "exit", 0 for unknown. This is the basis of command-driven interfaces. The previous lesson's solution is in the starting code.
Command recognition with strcmp
The strcmp(a, b) function compares two strings: it returns 0 if equal. For command recognition: if (strcmp(cmd, "help") == 0) return 1;. This pattern is the basis of command-driven interfaces.
For multiple commands, a chain of if statements is written: each command is checked with a separate strcmp. If no match, a default return (0) is given. This resembles a compiler's keyword recognition step. The string.h header provides strcmp. The parse_command result is used in main; for example, a switch processes different commands.
Practice Add the parse_command function
Below the existing definitions, write int parse_command(const char *cmd). If strcmp(cmd, "help") == 0 return 1;, if strcmp(cmd, "exit") == 0 return 2;, else return 0;. In main call printf("Komut: %d\n", parse_command("help")).
Starter code
#include <stdio.h>
#include <string.h>
void kernel_init(void);
int parse_command(const char *cmd);
int security_check(const char *buf, int len);
void kernel_init(void) {
printf("Çekirdek başladı\n");
}
int syscall(int num) {
return num * 2;
}
int main(void) {
printf("Proje başladı\n");
kernel_init();
return 0;
}
// parse_command fonksiyonunu yaz: "help" -> 1, "exit" -> 2, else 0
// main'de parse_command("help") çağır ve yazdırSample solution
#include <stdio.h>
#include <string.h>
void kernel_init(void);
int parse_command(const char *cmd);
int security_check(const char *buf, int len);
void kernel_init(void) {
printf("Çekirdek başladı\n");
}
int syscall(int num) {
return num * 2;
}
int parse_command(const char *cmd) {
if (strcmp(cmd, "help") == 0) return 1;
if (strcmp(cmd, "exit") == 0) return 2;
return 0;
}
int main(void) {
printf("Proje başladı\n");
kernel_init();
printf("Komut: %d\n", parse_command("help"));
return 0;
}Expected output
Proje başladı Çekirdek başladı Komut: 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match parser concept
| Term | Meaning |
|---|---|
strcmp |
String compare |
parse_command |
Command recognition |
"help" |
Return 1 |
"exit" |
Return 2 |
2 Complete the strcmp call
if (strcmp(cmd, ____) == 0) return 1;if (strcmp(cmd, "help") == 0) return 1;Explanation: strcmp(cmd, "help") == 0 means the 'help' command is recognized.
3 What does parse_command("help") return?
- 1 Correct answer
- 0
- 2
- -1
Explanation: parse_command returns 1 for "help".