feat: add ffi c

__ffi/c2/Makefile

@@ -0,0 +1,16 @@
+.PHONY: clean
+
+libcfunctions.so: cfunctions.o
+	${CC} --shared -o $@ $<
+
+cfunctions.o: cfunctions.h cfunctions.c
+	${CC} -fPIC -o $@ -c cfunctions.c -I.
+
+something: something.rs libcfunctions.so
+	rustc -L. -lcfunctions something.rs -C link-arg='-Wl,-rpath,${PWD}'
+	@# The command below can be used if the library to link with is
+	@# specified in something.rs.
+	@#rustc -L. something.rs -C link-arg='-Wl,-rpath,${PWD}'
+clean: 
+	${RM} libcfunctions.so cfunctions.o something
+

__ffi/c2/README.md

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+
+url
+https://github.com/danbev/learning-rust/tree/master/ffi
+
+

__ffi/c2/cfunctions.c

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+#include "cfunctions.h"
+#include "stdio.h"
+#include <stdlib.h>
+
+void doit(int nr) {
+  printf("Do something. nr: %d\n", nr);
+  printf("Going to call exit\n");
+  exit(1);
+  printf("After calling exit\n");
+}
+
+void print_string(char* s) {
+  printf("print_string. s: %s\n", s);
+}
+

__ffi/c2/cfunctions.h

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+void doit(int x);
+void print_string(char* s);

__ffi/c2/something.rs

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+use std::ffi::CString;
+use std::os::raw::c_char;
+
+// The below can be left out if the library is specified to 
+// rustc as an option.
+//#[link(name = "cfunctions", kind="dylib")]
+extern "C" {
+  fn doit(nr: i32) -> ();
+  fn print_string(s: *const c_char) -> ();
+}
+
+fn main() {
+  println!("Example of calling a c library.");
+  let s = CString::new("bajja").expect("CString::new failed");
+  unsafe {
+    doit(18);
+    print_string(s.as_ptr());
+  }
+}