simple open() and close() syscall interception via dlsym()

Makefile

+default:
+	gcc -Wall -fPIC -c -o nocache.o nocache.c
+	gcc -Wall -shared -Wl,-soname,nocache.so.1 -ldl -o nocache.so nocache.o

nocache.c

+#define _GNU_SOURCE
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <dlfcn.h>
+#include <stdio.h>
+
+int (*_original_open)(const char *pathname, int flags, mode_t mode);
+int (*_original_close)(int fd);
+
+void init(void) __attribute__((constructor));
+int open(const char *pathname, int flags, mode_t mode);
+int close(int fd);
+
+void init(void)
+{
+    _original_open = (int (*)(const char *, int, mode_t))
+        dlsym(RTLD_NEXT, "open");
+    _original_close = (int (*)(int)) dlsym(RTLD_NEXT, "close");
+}
+
+int open(const char *pathname, int flags, mode_t mode)
+{
+    fprintf(stderr, "I intercepted the open() call!\n");
+    return _original_open(pathname, flags, mode);
+}
+
+int close(int fd)
+{
+    fprintf(stderr, "I intercepted the close() call!\n");
+    return _original_close(fd);
+}