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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;
     }
     
     int
     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:

     all:
             @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