--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/python/pydc/examples/callback.py	Tue Feb 02 20:42:02 2021 +0100
@@ -0,0 +1,58 @@
+# callback of python function to qsort(3) some numbers - this is just a example
+# using an existing libc function that uses a callback; it's not practical for
+# real world use as it comes with a huge overhead:
+# - sorting requires many calls of the comparison function
+# - each such callback back into python comes with a lot of overhead
+# - on top of that, for this example, 2 memcpy(3)s are needed to access the
+#   data to compare, further adding to the overhead
+
+from pydc import *
+import sys
+import platform
+import struct
+
+if sys.platform == "win32":
+  libc = load("msvcrt")
+elif sys.platform == "darwin":
+  libc = load("/usr/lib/libc.dylib")
+elif "bsd" in sys.platform:
+  #libc = load("/usr/lib/libc.so")
+  libc = load("/lib/libc.so.7")
+elif platform.architecture()[0] == "64bit":
+  libc = load("/lib64/libc.so.6")
+else:
+  libc = load("/lib/libc.so.6")
+
+
+
+fp_qsort  = find(libc,"qsort")  # void qsort(void *base, size_t nmemb, size_t size, int (*compar)(const void *, const void *));
+fp_memcpy = find(libc,"memcpy") # void * memcpy(void *dst, const void *src, size_t len);
+
+
+
+nums = bytearray(struct.pack("i"*8, 12, 3, 5, 99, 3, -9, -9, 0))
+es = int(len(nums)/8)  # element size
+
+
+def compar(a, b):
+    ba = bytearray(es)
+    call(fp_memcpy,"ppi)p", ba, a, es)
+    a = struct.unpack("i", ba)[0]
+    call(fp_memcpy,"ppi)p", ba, b, es)
+    b = struct.unpack("i", ba)[0]
+    return a - b
+
+cb = new_callback("pp)i", compar)
+
+# --------
+
+print(*struct.unpack("i"*8, nums))
+
+print('... qsort ...')
+call(fp_qsort,"piip)v", nums, 8, es, cb)
+
+print(*struct.unpack("i"*8, nums))
+
+
+free_callback(cb)
+