multiple definition and making sure function is correctly written - c

Question

This is my .h file
#ifndef _test_h_
#define _test_h_
void new_cmd(void);
void open_cmd(void);
void close_cmd(void);
void close_all_cmd(void);
void save_cmd(void);
void save_as_cmd(void);
void save_all_cmd(void);
void print_cmd(void);
void exit_cmd(void);
void call(char *cmd_name);
struct {
char *cmd_name;
void (*cmd_pointer)(void);
} file_cmd[] =
{
{"new", new_cmd},
{"open", open_cmd},
{"close", close_cmd},
{"save", save_cmd},
{"save as", save_as_cmd},
{"save all", save_all_cmd},
{"print", print_cmd},
{"exit", exit_cmd}};
#endif
This is my function file
void call(char *cmd_name){
int i;
scanf("Enter command: %c\n",cmd_name);
for(i = 0; i < sizeof(file_cmd); i++){
if(strcmp(cmd_name, file_cmd[i].cmd_name) == 0){
file_cmd[i].cmd_pointer();
}
}
I just deleted all file_cmd functions because it seems for other people
and this is my main file
#include <stdio.h>
#include <string.h>
#include "test.h"
int main(void){
char cmd;
call(cmd);
return 0;
}
More I want to know is that my function void call is correctly written.
when I try to compile, it says
expected 'char*' but argument is of type 'char'
multiple definition of 'file_cmd'
first define here.
I am so confused how to fix it.
Please help
thank you.

Answer 1

You have two very different errors that you really should have asked about in two different question, but I'll be nice and answer both anyway.
In the main function you have a variable named cmd. It is of type char. You pass it to the call function, which expects an argument of type char * (i.e. a null-terminated string).
To fix this you first need to make sure that cmd is the correct type (i.e. char * instead of plain char). Secondly you need to make sure this pointer is actually pointing to a valid null-terminated byte string.
In other words, something like this:
char *cmd = "open";
call(cmd);
C really only cares about translation units. A translation unit is a single source files with all included header files.
The problem here is that variables can only be defined in a single translation unit. And since you define the variable file_cmd in a header file that is included in multiple source files, it will be defined in both translation units created from those two source files.
The solution here is to only declare the variable in the header file, and define it in a single source file.
For example, do something like this in the header file
struct command_struct
{
char *cmd_name;
void (*cmd_pointer)(void);
};
extern struct command_struct file_cmd[];
Then in one source file (like your "function file")
struct command_struct file_cmd[] = {
// Your initialization...
};
There are also other problems with your code, some of them will lead to undefined behavior.
For example the sizeof operator returns the size in bytes, not the number of elements in an array. To get the number of elements in an array you need to divide the byte-size of the full array with the byte-size of a single element. For example sizeof file_cmd / file_cmd[0].
Do note that the division only works for real arrays, not pointers, and that arrays very easily decays to pointers (to their first element).