Explicit use of a DLL in a C application

The following example shows explicit use of a DLL in a C application.

#include <dll.h>
#include <stdio.h>
#include <string.h>

#ifdef __cplusplus
  extern "C" {
#endif

  typedef int (DLL_FN)(void);

#ifdef __cplusplus
  }
#endif

#define FUNCTION        "FUNCTION"
#define VARIABLE        "VARIABLE"

static void Syntax(const char* progName) {
  fprintf(stderr, "Syntax: %s <DLL-name> <type> <identifier>\n"
                  "  where\n"
                  "  <DLL-name> is the DLL to load,\n"
                  "  <type> can be one of FUNCTION or VARIABLE\n"
                  "  and <identifier> is the function or variable\n"
                  "  to reference\n", progName);
  return;
}

main(int argc, char* argv[]) {
  int value;
  int* varPtr;
  char* dll;
  char* type;
  char* id;
  dllhandle* dllHandle;

  if (argc != 4) {
    Syntax(argv[0]);
    return(4);
  }
  dll  = argv[1];
  type = argv[2];
  id   = argv[3];

  dllHandle = dllload(dll);
  if (dllHandle == NULL) {
    perror("DLL-Load");
    fprintf(stderr, "Load of DLL %s failed\n", dll);
    return(8);
  }

  if (strcmp(type, FUNCTION)) {
    if (strcmp(type, VARIABLE)) {
      fprintf(stderr,
        "Type specified was not " FUNCTION " or " VARIABLE "\n");
      Syntax(argv[0]);
      return(8);
    }
    /*
     * variable request, so get address of variable
     */
    varPtr = (int*)(dllqueryvar(dllHandle, id));
    if (varPtr == NULL) {
      perror("DLL-Query-Var");
      fprintf(stderr, "Variable %s not exported from %s\n", id, dll);
      return(8);
    }
    value = *varPtr;
    printf("Variable %s has a value of %d\n", id, value);
  }
  else {
    /*
     * function request, so get function descriptor and call it
     */
    DLL_FN* fn = (DLL_FN*) (dllqueryfn(dllHandle, id));
    if (fn == NULL) {
      perror("DLL-Query-Fn");
      fprintf(stderr, "Function %s() not exported from %s\n", id, dll);
      return(8);
    }
    value = fn();
    printf("Result of call to %s() is %d\n", id, value);
  }
  dllfree(dllHandle);

  return(0);
}

The following example shows explicit use of a DLL in an application using the dlopen() family of functions.

  #define _UNIX03_SOURCE

  #include <dlfcn.h>
  #include <stdio.h>
  #include <string.h>

  #ifdef __cplusplus
    extern "C" {
  #endif

    typedef int (DLL_FN)(void);

  #ifdef __cplusplus
    }
  #endif

  #define FUNCTION        "FUNCTION"
  #define VARIABLE        "VARIABLE"

  static void Syntax(const char* progName) {
    fprintf(stderr, "Syntax: %s <DLL-name> <type> <identifier>\n"
                    "  where\n"
                    "  <DLL-name> is the DLL to open,\n"
                    "  <type> can be one of FUNCTION or VARIABLE,\n"
                    "  and <identifier> is the symbol to reference\n"
                    "  (either a function or variable, as determined by"
                    " <type>)\n", progName);
    return;
  }

main(int argc, char* argv[]) {
    int value;
    void* symPtr;
    char* dll;
    char* type;
    char* id;
    void* dllHandle;
    if (argc != 4) {
      Syntax(argv[0]);
      return(4);
    }
    dll  = argv[1];
    type = argv[2];
    id   = argv[3];

    dllHandle = dlopen(dll, 0);
    if (dllHandle == NULL) {
      fprintf(stderr, "dlopen() of DLL %s failed: %s\n", dll, dlerror());
      return(8);
    }

    /*
     * get address of symbol (may be either function or variable)
     */
    symPtr = (int*)(dlsym(dllHandle, id));
    if (symPtr == NULL) {
      fprintf(stderr, "dlsym() error: symbol %s not exported from %s: %s\n"
                    , id, dll, dlerror());
      return(8);
    }
    if (strcmp(type, FUNCTION)) {
      if (strcmp(type, VARIABLE)) {
        fprintf(stderr,
          "Type specified was not " FUNCTION " or " VARIABLE "\n");
        Syntax(argv[0]);
        return(8);
      }
      /*
       * variable request, so display its value
       */
      value = *(int *)symPtr;
      printf("Variable %s has a value of %d\n", id, value);
    }
    else {
      /*
       * function request, so call it and display its return value
       */
      value = ((DLL_FN *)symPtr)();
      printf("Result of call to %s() is %d\n", id, value);
    }
    dlclose(dllHandle);

    return(0);
  }