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.

John Bollinger · Accepted Answer · 2020-12-25 16:43:19Z

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:

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

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