12.2.4 Example Loaded Object

Let’s suppose we wanted to write a new GNU make function that would create a temporary file and return its name. We would like our function to take a prefix as an argument. First we can write the function in a file mk_temp.c:

#include <stdlib.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>

#include <gnumake.h>

int plugin_is_GPL_compatible;

char *
gen_tmpfile(const char *nm, int argc, char **argv)
  int fd;

  /* Compute the size of the filename and allocate space for it.  */
  int len = strlen (argv[0]) + 6 + 1;
  char *buf = gmk_alloc (len);

  strcpy (buf, argv[0]);
  strcat (buf, "XXXXXX");

  fd = mkstemp(buf);
  if (fd >= 0)
      /* Don't leak the file descriptor.  */
      close (fd);
      return buf;

  /* Failure.  */
  fprintf (stderr, "mkstemp(%s) failed: %s\n", buf, strerror (errno));
  gmk_free (buf);
  return NULL;

mk_temp_gmk_setup ()
  /* Register the function with make name "mk-temp".  */
  gmk_add_function ("mk-temp", gen_tmpfile, 1, 1, 1);
  return 1;

Next, we will write a makefile that can build this shared object, load it, and use it:

        @echo Temporary file: $(mk-temp tmpfile.)

load mk_temp.so

mk_temp.so: mk_temp.c
        $(CC) -shared -fPIC -o $ $<

On MS-Windows, due to peculiarities of how shared objects are produced, the compiler needs to scan the import library produced when building make, typically called libgnumake-version.dll.a, where version is the version of the load object API. So the recipe to produce a shared object will look on Windows like this (assuming the API version is 1):

mk_temp.dll: mk_temp.c
        $(CC) -shared -o $ $< -lgnumake-1

Now when you run make you’ll see something like:

$ make
cc -shared -fPIC -o mk_temp.so mk_temp.c
Temporary filename: tmpfile.A7JEwd