arsh/main.c

#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <stdint.h>
#include <stdio.h>
#include <sys/wait.h>
#include <signal.h>
#include <setjmp.h>

static sigjmp_buf restart_jmp_buf;
static const int restart_value = 69;
static volatile sig_atomic_t jump_active = 0;

void sigint_handler(int signo)
{
  if(!jump_active)
  {
    return;
  }
  siglongjmp(restart_jmp_buf, restart_value);
}
static constexpr size_t max_line = 1024;
static char string_buffer[max_line];
char* readline(char* print)
{
  if(print)
  {
    printf("%s", print);
  }
  if((fgets(string_buffer, max_line, stdin) == NULL) && ferror(stdin))
  {
    perror("fgets error");
  }
  
  size_t length = strlen(string_buffer);
  if(length == 0)
  {
    return NULL;
  }
  //Remove trailing \n
  if (length > 0 && string_buffer[length - 1] == '\n')
  {
    string_buffer[length - 1] = '\0';
  }
  return string_buffer;
}

char** split_input(char* input)
{
  const size_t alloc_size = ((strlen(input) + 1) / 2) + 1;
  char** command = malloc(alloc_size * sizeof(char*));
  if (command == NULL)
  {
    perror("malloc failed");
    exit(1);
  }
  const char* separator = " ";
  char* parsed;
  uint32_t index = 0;
  parsed = strtok(input, separator);
  while (parsed != NULL)
  {
    command[index] = parsed;
    index++;
    parsed = strtok(NULL, separator);
  }
  command[index] = NULL;
  return command;
}

int cd(char* path)
{
  return chdir(path);
}

int main()
{
  struct sigaction s;
  s.sa_handler = sigint_handler;
  sigemptyset(&s.sa_mask);
  s.sa_flags = SA_RESTART;
  struct sigaction s_old;
  sigaction(SIGINT, &s, &s_old);

  int stat_loc;

  if(sigsetjmp(restart_jmp_buf, 1) == restart_value)
  {
    printf("\n");
  }
  jump_active = 1;
  
  while(true)
  {
    char* input = readline("arsh> ");
    if(input == NULL) //CTRL + D
    {
      printf("\nexit\n");
      exit(0);
    }
    char** command = split_input(input);
    if(command[0] == NULL)
    {
      goto cleanup;
    }
    
    if(strcmp(command[0], "cd") == 0)
    {
      if (cd(command[1]) < 0)
      {
	perror(command[1]);
      }

      goto cleanup; //skip fork()
    }
    if(strcmp(command[0], "exit") == 0)
    {
      int32_t status_code = command[1] != NULL ? atoi(command[1]) : 0;
      exit(status_code);
    }
    
    pid_t child_pid = fork();
    if (child_pid < 0) 
    {
      perror("Fork failed");
      exit(1);
    }
    if (child_pid == 0)
    {
      sigaction(SIGINT, &s_old, NULL);
      //never returns if call is successful
      if(execvp(command[0], command) < 0)
      {
	perror(command[0]);
	exit(1);
      }
    }
    else
    {
      waitpid(child_pid, &stat_loc, WUNTRACED);
    }

cleanup:

    free(command);
  }
}