Execvp linux: trying to make my shell work in C

Question

I'm trying to make simple shell, but with a specific condition, I have to use the following structure:

typedef  struct cmd_struct{
  char cmd[80];
  char args[10][80];
  int nargs;
} cmd_type;

Inside cmd I will save de main command and arguments in args.

Then I read from a file, different commands, and save them into and array of cmd_type. My program or fake shell, ask for a number and should take it from this array.

My function that executes the command looks like:

void execCmd(cmd_type* cmds_arg, int idxCmd){
  pid_t pid;
    printf("Father: my pid is %d\n", getpid());
    char* buff;
    pid = fork();
    if (pid == 0) {  
        printf("Child process: My pid is %d\n", getpid());
           printf("-------------- Child doing exec: %s\n", cmds_arg[idxCmd].cmd);
           execvp(cmds_arg[idxCmd].cmd,&cmds_arg[idxCmd].args);
           _exit(2);
        _exit(1);
    } 
    printf("Father: Gonna wait for Child\n");

    int status;
    wait(&status);
    printf("-------------- Father: Child finished\n");

    // WIFEXITED, WEXITSTATUS Macro of the gnu lib POSIX standard to recover end status
    if ( WIFEXITED(status) ) {   
        const int es = WEXITSTATUS(status);
        printf("Father: Child Complete with exit status %d\n", es);
        if(es == 1) printf("Father: Child didn't execute any command\n");
        else if(es == 2) printf("Father: Child command was not found\n");
    }
}

As you can see, when I'm calling the execvp() system call, I'm doing it wrong. First argument I think that it is right, second one it's totally wrong.

First of all, I have a conversion problem right there, and the second problem is that the array should contain "main command", "arg1", "arg2" ... and mine only has the arguments. Am I wrong?

Is there a way to add the "main command" using services like sscanf ()? And the most important part, do I have any chance to make it work this way?

Answer 1

In the same spirit of the answer from @pts, you can copy the parameters for execvp() in a dynamically allocated table:

void execCmd(cmd_type* cmds_arg, int idxCmd){
  pid_t pid;
  printf("Father: my pid is %d\n", getpid());
  char* buff;
  pid = fork();
  if (pid == 0) {

    int i;
    char **args = (char **)malloc((1 + cmds_arg[idxCmd].nargs + 1) * sizeof(char *));

    args[0] = cmds_arg[idxCmd].cmd;
    for (i = 1; i < (cmds_arg[idxCmd].nargs + 1); i ++) {
      args[i] = cmds_arg[idxCmd].args[i - 1];
    }
    args[i] = NULL;

    printf("Child process: My pid is %d\n", getpid());
    printf("-------------- Child doing exec: %s\n", cmds_arg[idxCmd].cmd);
    execvp(cmds_arg[idxCmd].cmd, args);
    _exit(2);
  } 
  printf("Father: Gonna wait for Child\n");

  int status;
  wait(&status);
  printf("-------------- Father: Child finished\n");

  // WIFEXITED, WEXITSTATUS Macro of the gnu lib POSIX standard to recover end status
  if ( WIFEXITED(status) ) {   
    const int es = WEXITSTATUS(status);
    printf("Father: Child Complete with exit status %d\n", es);
    if(es == 1) printf("Father: Child didn't execute any command\n");
    else if(es == 2) printf("Father: Child command was not found\n");
  }
}