OS 6 server.c

#include <errno.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/iomsg.h>
#include <sys/neutrino.h>
#include "httpd_pid.h"

const char HELP[ ] = "\
OS Lab 5 Server v1.1\n\
\n\
Available commands:\n\
  help\n\
  ls\n\
  cat\n\
  cp\n\
  rm\n\
  echo\n\
";

const char ERROR[ ] = "Invalid command; use 'help' for more information\n"

char buffer[65535];

short CheckCommand(const char cmd[ ]) {
    switch (cmd[0]) {
        case 'e': {
            return cmd[1] == 'c' && cmd[2] == 'h' && cmd[3] == 'o' && cmd[4] == ' ' ? 3 : 0;
        }

        case 'h': {
            return cmd[1] == 'e' && cmd[2] == 'l' && cmd[3] == 'p' && !cmd[4] ? 2 : 0;
        }

        case 'l': {
            return (short)(cmd[1] == 's' && (cmd[2] == ' ' || !cmd[2]));
        }

        case 'c': {
            if (cmd[1] == 'a')
                return (short)(cmd[2] == 't' && cmd[3] == ' ');
            if (cmd[1] == 'p')
                return (short)(cmd[2] == ' ');
            break;
        }

        case 'r': {
            return (short)(cmd[1] == 'm' && cmd[2] == ' ');
        }
    }
    return 0;
}

int main(void) {
    int channelId = ChannelCreate(0);
    FILE* f = fopen(HTTPD_PID, "w");
    if (!f) {
        fputs("Cannot open \"" HTTPD_PID "\"\n", stderr);
        return 1;
    }
    fprintf(f, "0/%d/%d\n", getpid(), channelId);
    fclose(f);
    fputs(HELP, stdout);
    while (true) {
        int receivedId = MsgReceive(channelId, buffer, sizeof buffer, NULL);
        const char* response = buffer;
        int responseSize = 0;
        switch (CheckCommand(buffer)) {
            case 0: {
                response = ERROR;
                responseSize = sizeof ERROR;
                break;
            }

            case 1: {
                f = popen(buffer, "r");
                responseSize = fread(buffer, sizeof(char), sizeof buffer / sizeof(char) - 1, f);
                buffer[responseSize++] = '\0';
                pclose(f);
                break;
            }

            case 2: {
                response = HELP;
                responseSize = sizeof HELP;
                break;
            }

            case 3: {
                puts(buffer + 5);
                *response = '\0';
                responseSize = sizeof(char);
            }
        }
        MsgReply(receivedId, EOK, response, responseSize);
    }
    return 0;
}