I’m having a problem with a global file pointer in C with Visual Studio 2022 in a solution involving a large number of files that are compiled and linked using the “Build” command.

Based on the link at https://codeforwin.org/c-programming/global-variables-c#google_vignette I’ve created a file called globals.c with the code:

 #include <stdio.h>
 FILE* logFp;

Then in main.c I have the following code:

 #include "main.h"
 extern FILE* logFp;

 int main(void) {
   errno_t fileErr = 0;
   logFp = NULL;
   char logFn[FNAME_LEN] = { 0 };
   printf("Type the output log filename:                                 ");
    scanf_s("%s", logFn, FNAME_LEN);
    fileErr = fopen_s(&logFp, logFn, "w");
    if (fileErr) {
        printf("*** There is an error when attempting to open the log file ***n");
        return 1;
    }
    int x = 5;
    UINT bits = 255;
    gprint(logFp, 161, "d b bn", x, bits, 2 * bits);
    gprint(logFp, 1, "d b bn", x, bits, 2 * bits);
    fclose(logFp);
    return 0;
 }

This is a test part in main() in a large solution with many other files involving the gprint() functions which I know works when logFp is defined locally.

When I attempt to compile this I get a link error that logFp is an unresolved symbol, which I get with other files in the solution containing logFp. Incidentally, main.h is my header file which includes many function prototypes, various defines and macros, as well as a number of includes. The solution works correctly until I try using a global file pointer. Some advice would be most appreciated.