// projdata.h
int something;
char filename[255];
...
// projlib.c
#include "projdata.h"

int somefunc() {
   ...
   strupr( filename );
   ...
   return 0;
}

void someotherfunc ( int n ) {
    ...
    something = n;
    ...
    return;
}

##</code><code>##

// projdata.h
struct {
    int something;
    char filename[255];
    ...
} projdata, *pprojdata;

// projlib.c
#include "projdata.h"

int somefunc( pprojdata p ) {
   ...
   strcat( p->filename, ... );
   ...
   return 0;
}

void someotherfunc ( pprojdata p, int n ) {
    ...
    p->something = n;
    ...
    return;
}

// main.c

int main ( ... ) 
    projdata pd;
    int n;
    ...
    n = somefunc( &pd );
    ...
    someotherfunc( &pd, n );
    ...
}