Using C header and implementation files in Xcode iOS project

I'm trying to use a separate C header and implementation file in Xcode iOS/Objective-C project.
I want to use the method I implemented in main.m but I get these errors:

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I've included user.h in main.m

Note that Target Membership is selected in user.c for HelloWorld. When I deselect this the errors are gone. But when I try to run the app, I get these errors at compile time:
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When I implement the struct and method in main.m it compiles and runs just fine. But I don't get it why I can't use this particular code in a separate file?

Source Code:
user.h

#ifndef HelloWorld_user_h
#define HelloWorld_user_h

typedef struct {
    char *name;
    int age;
    char sex;
} User; //sizeof(User) = 16 bytes

void CreateAndDisplay(User *usr, char *name, int age, char sex);

#endif

user.c

#include <stdio.h>
#include <stdlib.h>

void CreateAndDisplay(User *usr, char *name, int age, char sex) {
    usr->name = name;
    usr->age = age;
    usr->sex = sex;

    printf("User address -> value:\n");
    printf("Name:\t%u\t->\t%s\n", (uint)usr, *&usr->name);
    printf("Age:\t%u\t->\t%i\n", (uint)&usr->age, *&usr->age);
    printf("Sex:\t%u\t->\t%c\n\n", (uint)&usr->sex, *&usr->sex);

    printf("User has a size of %li bytes in memory", sizeof(*usr));
}

main.m

#import <UIKit/UIKit.h>

#import "HelloWorldAppDelegate.h"

#include <stdio.h>
#include <stdlib.h>

#include "user.h"

int main(int argc, char *argv[])
{
    User user1;
    CreateAndDisplay(&user1, "John Doe", 24, 'm');

    @autoreleasepool {
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([HelloWorldAppDelegate class]));
    }
}