C89 - Initialising struct with flexible char array and prototypes

Question

I'm new to C and had some questions around struct instantiation. I have two files :

• Index.c : which instantiates a new Server struct
• server/Server.c which defines the Server struct, the new_Server() constructor and the kill_Server() deconstructor

Content of Index.c :

/* Standard libraries */
#include <stdio.h>
#include <string.h>

/* Project's Custom classes */
#include "./server/Server.c"

/* Configuration value */
#define HOST "127.0.0.1"
#define PORT 80

int main (void) {
    unsigned short port = PORT;
    unsigned char host[255] = HOST;
    Server* server = new_Server(port, host);
    return 0;
}

Content of server/Server.c :

#include <stdlib.h>

/* HOST_NAME_MAX will have to be changed into getconf HOST_NAME_MAX */
#define HOST_NAME_MAX 255

typedef struct {
    unsigned short port;
    unsigned char host[HOST_NAME_MAX];
} Server;

Server* new_Server (unsigned short port, unsigned char host[]) {
    Server* server = malloc(sizeof(Server));
    server->port = port;
    server->host = host;
    return server;
}

void kill_Server (Server* server) {
    free(server);
}

When I compile the program, I get the following output :

In file included from src/index.c:6:
src/./server/Server.c:11:9: warning: no previous prototype for function 'new_Server' [-Wmissing-prototypes]
Server* new_Server (unsigned short port, unsigned char host[]) {
        ^
src/./server/Server.c:14:15: error: array type 'unsigned char [255]' is not assignable
        server->host = host;
        ~~~~~~~~~~~~ ^
src/./server/Server.c:18:6: warning: no previous prototype for function 'kill_Server' [-Wmissing-prototypes]
void kill_Server (Server* server) {
     ^
2 warnings and 1 error generated.

(I just left out the "server" variable not used warning.)

So here are my questions :

• Why am I getting the "missing prototype" warning since I actually did specify the output and method argument types ?

• How can I initialise a struct with it's "host" key being an array of chars ?

• Is what I'm doing efficient ? Steps :

  1. configure values in a #define
  2. create the corresponding variables
  3. passing them as constructor parameters
  4. initialise a struct instance
  5. assign the value to it's associated key

I read that to get the maximum hostname size you should do "getconf HOST_NAME_MAX". In the shell it works of course and I get 255, but I'd like to store the value in a variable in my C program.

• How can I achieve this ?

I'm compiling with the following GCC flags :

gcc -g -O0 -Wall -Wextra -std=c89 -pedantic -Wmissing-prototypes -Wstrict-prototypes -Wmissing-declarations -Wold-style-definition -Wdeclaration-after-statement

I know this is (unnecessarily) strict, but I'd like to learn C the hard way and really understand it's ins-and-outs. And I think that stuff like warnings are great for this.

EDIT: I did already read this question How to initialize a structure with flexible array member, but I didn't really understand the answer. It also leaves out the issue about method prototypes.

Answer 1

Well, you should avoid using #include to include a .c file. The .c files are supposed to be compiled separately (with common definitions in a .h file that both include), and then you link the compiled objects together.

The two warnings is bogus, there is no code problem. You could suppress the warning by adding a prototype (just copy line 11 but put a ; on the end). Same for the second one

The error: server->host = host; is illegal. Arrays are second-class citizens in C, you cannot copy them using the = operator. You need to either go memcpy(&server->host, &host, sizeof server->host);, or strcpy(server->host, host);.

I think it'd be overkill to have your C program execute getconf and see what it got; instead, look up what the max value is that getconf will ever give back to you.