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
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Licensed under the GNU Free Documentation License.
https://www.gnu.org/software/make/manual/html_node/Loaded-Object-Example.html