initial

.gitignore

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+*.so
+*.o
+plugin

Makefile

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+#!/bin/make -f
+
+PREFIX := .
+
+CP := cp -f
+
+all: plug/libplug.so src/plugin
+
+plug/libplug.so:
+	make -C plug
+	$(CP) $@ $(PREFIX)
+
+src/plugin:
+	make -C src
+	$(CP) $@ $(PREFIX)
+
+.PHONY: all plug/libplug.so src/plugin

README

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+plug/ contains the dynamically loaded library,
+and of course, src/ has the core program.
+
+You'll have to use LD_LIBRARY_PATH='.' ./plugin
+This could be remedied with a little messing with
+get/setenv.
+
+There's a good reason for the fragmentation and
+recursion of the Makefiles, as how the files are
+compiled vary at compliation and link time
+(link-time not so applicable in this case, but
+ it is effected by CFLAGS anyways).
+
+Copy as you wish, credit not.

plug/Makefile

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+include ../root.mk
+
+CFLAGS += -shared -fPIE
+
+libplug.so: plug.o
+	${LINK.c} $+ -o $@

plug/plug.c

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+#include <stdio.h>
+
+void
+plug(void)
+{
+  printf("Plugin works.\n");
+}

plug/plug.h

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+#ifndef PLUG_H_
+
+#define BIND(lib, func, func_name) *(void **) (&func) = dlsym(lib, func_name)
+
+typedef void (*plug_t)(void);
+
+static plug_t plug;
+
+#define PLUG_H_
+#endif

root.mk

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+CFLAGS   := -std=c89 -Wall -Wextra -Wpedantic -Wshadow -Wundef
+CPPFLAGS := -D_FORTIFY_SOURCE=2
+LDLIBS   := -lplug
+
+.c.o:
+	${COMPILE.c} $< -o $@

src/Makefile

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+include ../root.mk
+
+CPPFLAGS += -I../plug -L../plug
+
+plugin: main.o
+	${LINK.c} $+ -o $@ ${LDLIBS}

src/main.c

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+#include <stdio.h>
+#include <unistd.h>
+#include <dlfcn.h>
+
+#include "plug.h"
+
+int
+main (void)
+{
+  int x = 0;
+  char * libplug_fn = "libplug.so";
+  void * libplug = NULL;
+  while (1)
+  {
+    if (libplug) { dlclose(libplug); }
+    libplug = dlopen(libplug_fn, RTLD_NOW);
+    BIND(libplug, plug, "plug");
+    if (!libplug)
+    { fprintf(stderr, "ERR: Cannot load dynamic library %s: %s\n", libplug_fn, dlerror()); }
+    else
+    {
+      *(void **) (&plug) = dlsym(libplug, "plug");
+      printf("frame %d: ", x++);
+      if (plug)
+      { plug(); }
+      else
+      { fprintf(stderr, "ERR: Cannot find %s symbol in %s: %s\n", "plug", libplug_fn, dlerror()); }
+    }
+    sleep(2);
+  }
+}