用户输入的字符串在c中运行特定的功能

Question

Guys so I'm working on the web service assignment and I have the server dishing out random stuff and reading the uri but now i want to have the server run a different function depending on what it reads in the uri. I understand that we can do this with function pointers but i'm not exactly sure how to read char* and assign it to a function pointer and have it invoke that function. Example of what I'm trying to do: http://pastebin.com/FadCVH0h

I could use a switch statement i believe but wondering if there's a better way.

Answer 1

For such a thing, you will need a table that maps char * strings to function pointers. The program segfaults when you assign a function pointer to string because technically, a function pointer is not a string.

Note: the following program is for demonstration purpose only. No bounds checking is involved, and it contains hard-coded values and magic numbers

Now:

void print1()
{
   printf("here");
}

void print2() 
{
   printf("Hello world");
}
struct Table {
  char ptr[100];
  void (*funcptr)(void)
}table[100] = {
{"here", print1},
{"hw", helloWorld}
};

int main(int argc, char *argv[])
{
   int i = 0;
   for(i = 0; i < 2; i++){
      if(!strcmp(argv[1],table[i].ptr) { table[i].funcptr(); return 0;}
   }
   return 0;
}

Answer 2

I'm gonna give you a quite simple example, that I think, is useful to understand how good can be functions pointers in C. (If for example you would like to make a shell)

For example if you had a struct like this:

typedef struct s_function_pointer
{
    char*      cmp_string;
    int        (*function)(char* line);
}              t_function_pointer;

Then, you could set up a t_function_pointer array which you'll browse:

int     ls_function(char* line)
{
      // do whatever you want with your ls function to parse line
      return 0;
}

int     echo_function(char* line)
{
      // do whatever you want with your echo function to parse line
      return 0;
}

void    treat_input(t_function_pointer* functions, char* line)
{
       int    counter;
       int    builtin_size;

       builtin_size = 0;
       counter = 0;
       while (functions[counter].cmp_string != NULL)
       {
             builtin_size = strlen(functions[counter].cmp_string);
             if (strncmp(functions[counter].cmp_string, line, builtin_size) == 0)
             {
                   if (functions[counter].function(line + builtin_size) < 0)
                          printf("An error has occured\n");
             }
             counter = counter + 1;
       }
}

int     main(void)
{
     t_function_pointer      functions[] = {{"ls", &ls_function},
                                            {"echo", &echo_function},
                                            {NULL, NULL}};
     // Of course i'm not gonna do the input treatment part, but just guess it was here, and you'd call treat_input with each line you receive.
     treat_input(functions, "ls -laR");
     treat_input(functions, "echo helloworld");
     return 0;
}

Hope this helps !