Mpf get str fixups

src/ffi.rs

@@ -1,7 +1,38 @@
 use mpz::*;
+use std::{mem, slice, str};
+use libc::{c_char, strlen};
+
+type AllocFunc = extern "C" fn(usize) -> *mut c_char;
+type ReallocFunc = extern "C" fn(*mut c_char, usize, usize) -> *mut c_char;
+type FreeFunc = extern "C" fn(*mut c_char, usize);
 
 #[link(name = "gmp")]
 extern "C" {
     pub fn __gmpz_fdiv_q(q: mpz_ptr, n: mpz_srcptr, d: mpz_srcptr);
     pub fn __gmpz_cdiv_q(q: mpz_ptr, n: mpz_srcptr, d: mpz_srcptr);
+    pub fn __gmp_get_memory_functions(alloc: *mut AllocFunc, relloc: *mut ReallocFunc, free: *mut FreeFunc);
+}
+
+pub struct GString(*mut u8, usize);
+
+impl GString {
+    pub unsafe fn from_raw(raw: *mut c_char) -> GString {
+        GString(mem::transmute(raw), strlen(raw) as usize + 1)
+    }
+
+    pub fn to_str(&self) -> Result<&str, str::Utf8Error> {
+        let bytes: &[u8] = unsafe { slice::from_raw_parts(self.0, self.1) };
+        str::from_utf8(&bytes[..bytes.len() - 1])
+    }
+}
+
+impl Drop for GString {
+    fn drop(&mut self) {
+        use std::ptr::null_mut;
+        unsafe {
+            let mut free_func: FreeFunc = mem::uninitialized();
+            __gmp_get_memory_functions(null_mut(), null_mut(), mem::transmute(&mut free_func));
+            free_func(mem::transmute(self.0), self.1);
+        }
+    }
 }

src/mpf.rs

@@ -10,6 +10,7 @@ use super::mpz::{Mpz, mpz_srcptr};
 use super::mpq::{Mpq, mpq_srcptr};
 use super::sign::Sign;
 use num_traits::{Zero, One};
+use ffi::GString;
 
 type mp_exp_t = c_long;
 
@@ -37,7 +38,7 @@ extern "C" {
 
     fn __gmpf_set_str(rop: mpf_ptr, str: *const c_char, base: c_int);
     fn __gmpf_set_si(rop: mpf_ptr, op: c_long);
-    fn __gmpf_get_str(str: *const c_char, expptr: *const mp_exp_t, base: i32, n_digits: i32, op: mpf_ptr) -> *mut c_char;
+    fn __gmpf_get_str(str: *const c_char, expptr: *const mp_exp_t, base: i32, n_digits: i32, op: mpf_srcptr) -> *mut c_char;
 
     fn __gmpf_cmp(op1: mpf_srcptr, op2: mpf_srcptr) -> c_int;
     fn __gmpf_cmp_d(op1: mpf_srcptr, op2: c_double) -> c_int;
@@ -119,13 +120,24 @@ impl Mpf {
         }
     }
 
-    pub fn get_str(&mut self, n_digits: i32, base: i32, exp: &mut c_long) -> String{
-        let c_str = CString::new("").unwrap();
-        let out;
-        unsafe{
-            out = CString::from_raw(__gmpf_get_str(c_str.into_raw(), exp, base, n_digits, &mut self.mpf));
+    pub fn get_str(&self, n_digits: i32, base: i32, exp: &mut c_long) -> String {
+        use std::ptr::null;
+        if n_digits == 0 {
+            // Maximal significant digits requested. Let GMP compute the length
+            // and allocate the space required.
+            unsafe {
+                let p = __gmpf_get_str(null(), exp, base, n_digits, &self.mpf);
+                // De-allocates the GMP string on drop.
+                GString::from_raw(p).to_str().unwrap().to_string()
+            }
+        } else {
+            // From mpf/get_str.c.
+            let c_str = CString::new(vec![37; n_digits as usize + 2]).unwrap();
+            unsafe {
+                let p = __gmpf_get_str(c_str.as_ptr(), exp, base, n_digits, &self.mpf);
+                CString::from_raw(p).to_str().unwrap().to_string()
+            }
         }
-        out.to_str().unwrap().to_string()
     }
 
     pub fn abs(&self) -> Mpf {