Header file not working (source file not compiled) c language

Question

In my header file file.h I have:

#ifdef FILE_H
extern "C" {
#endif

int size;
int array[3];
    
void saveToFile();
void loadFromFile();

#ifdef FILE_H
}
#endif

After I try compiling it, table with "Source file not compiled" pops up and I also get warning like this command line option '-std=c99' is valid for C/ObjC but not for C++.

In my file.c I have:

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

void saveToFile(){
    FILE *load=fopen("1uzd.txt", "w");
    int array[3];
    fprintf(load, "%d ", 3);
    array[0]=1;
    array[1]=2;
    array[2]=3;
    for(int i=0; i<3; i++){
        fprintf(load, "%d ", array[i]);
    }
    fclose(load);
}
void loadFromFile(){
    FILE *load=fopen("1uzd.txt", "r");
    int size;
    fscanf(load, "%d", &size);
    int array[size];
    for(int i=0; i<size; i++){
        fscanf(load, "%d", &array[i]);
    }
    printf("%d %d %d", size, array[0], array[1]);
}

And undefined reference to `WinMain' with ld returned 1 exit status And in main.c:

#include <stdlib.h>
#include <stdio.h>
#include "file.h"
int main(void){
    saveToFile();
    loadFromFile();
    return 0;
}

With undefined reference to saveToFile', undefined reference to loadFromFile' and ld returned 1 exit status.

Answer 1

In my header file file.h I have:

#ifdef FILE_H
extern "C" {
#endif

int size;
int array[3];
    
void saveToFile();
void loadFromFile();

#ifdef FILE_H
}
#endif

Your multi-inclusion guards have the wrong sense and scope. Also, if your intention is for this header to be usable in both C and C++, then the extern "C" declaration is incorrectly protected. Perhaps you want this:

#ifndef FILE_H
#define FILE_H

#ifdef __cplusplus
extern "C" {
#endif

int size;
int array[3];
    
void saveToFile();
void loadFromFile();

#ifdef __cplusplus
}
#endif

#endif

That provides for

• the contents of the header to be considered only the first time the header is included in any translation unit, and
• the extern "C" declaration to be ignored unless the header is being processed by a C++ compiler.

After I try compiling it, table with "Source file not compiled" pops up and I also get warning like this command line option '-std=c99' is valid for C/ObjC but not for C++.

One does not compile headers directly. One uses #include directives to incorporate their contents into other sources. The source files you present contain such #include directives, so you should not need to do anything further in that regard.

In my file.c I have [...] undefined reference to `WinMain'

You are trying to compile file.c as a complete program, but that doesn't work because it does not have a main() function.

And in main.c [...] undefined reference to saveToFile', undefined reference to loadFromFile'

You are trying to compile main.c as a complete program, but it does not contain the source of a complete program because it calls functions that are defined in a different source file.

There are two main approaches you could take:

1. compile each .c file to an object file and then link them together into a whole program in a separate step, or

2. compile both .c files in a single compilation command.

Details of both options depend on the compiler and build system used. Some of your diagnostic messages suggest GCC to me, and with GCC running in command-line mode, the latter would be something like

gcc -std=c99 -o my_program.exe main.c file.c

. The "my_program.exe" is the name you want your executable to have. The -std=c99 is necessary with your code for some versions of GCC, and the diagnostics suggest that it appears in the command-line arguments you are actually using. You may include other command-line options, too, if you wish.

Note also that you must use a C compiler to compile C code, not a C++ compiler. Your diagnostics suggest that you may be trying to use the latter. C and C++ are different languages. Although they have a shared subset, neither is a superset of the other.