Add an as_bytes function for the C-ABI (#750)

Author: mitsuhiko

CHANGELOG.md

@@ -13,6 +13,8 @@ All notable changes to MiniJinja are documented here.
   merge feature, and fixed enumeration behavior when non-map objects
   are attempted to be merged.  #745
 - Added `mj_value_new_bytes` to the C-ABI.  #749
+- Added `mj_value_as_bytes` to the C-ABI to borrow from strings or
+  byte values.  #750
 
 ## 2.8.0
 

minijinja-cabi/example/hello.c

@@ -1,6 +1,7 @@
 #include <minijinja.h>
 #include <stdio.h>
 #include <assert.h>
+#include <string.h>
 
 int main()
 {
@@ -60,6 +61,14 @@ seq: {{ seq }}");
     mj_str_free(ervs);
     mj_value_decref(&erv);
 
+    // you can borrow utf-8 from strings, if they are strings.
+    mj_value bv = mj_value_new_string("Hello");
+    uintptr_t bvlen;
+    const char *bvstr = mj_value_as_bytes(bv, &bvlen);
+    assert(bvlen == 5);
+    assert(memcmp(bvstr, "Hello", 5) == 0);
+    mj_value_decref(&bv);
+
     mj_env_free(env);
 
     return 0;

minijinja-cabi/include/minijinja.h

@@ -252,6 +252,11 @@ MINIJINJA_API void mj_syntax_config_default(struct mj_syntax_config *syntax);
  */
 MINIJINJA_API bool mj_value_append(struct mj_value *slf, struct mj_value value);
 
+/*
+ If the value is a string or bytes, returns it the pointer
+ */
+MINIJINJA_API const char *mj_value_as_bytes(struct mj_value value, uintptr_t *len_out);
+
 /*
  Extracts a float from the value
  */

minijinja-cabi/src/utils.rs

@@ -21,6 +21,12 @@ impl<T> AbiResult for *mut T {
     }
 }
 
+impl<T> AbiResult for *const T {
+    fn err_value() -> Self {
+        ptr::null()
+    }
+}
+
 impl AbiResult for u64 {
     fn err_value() -> Self {
         0

minijinja-cabi/src/value.rs

@@ -2,6 +2,7 @@ use std::collections::BTreeMap;
 use std::ffi::{c_char, CStr, CString};
 use std::mem::{transmute, ManuallyDrop};
 use std::ops::Deref;
+use std::ptr;
 use std::sync::Arc;
 
 use minijinja::value::{Object, ValueIter, ValueKind};
@@ -284,6 +285,22 @@ ffi_fn! {
     }
 }
 
+ffi_fn! {
+    /// If the value is a string or bytes, returns it the pointer
+    /// to it, and the length.
+    ///
+    /// Note that strings are not null terminated.  If you need that, use
+    /// `mj_value_to_str` which will also stringify non string values.
+    unsafe fn mj_value_as_bytes(_scope, value: mj_value, len_out: &mut usize) -> *const c_char {
+        if let Some(bytes) = value.as_bytes() {
+            *len_out = bytes.len();
+            bytes.as_ptr() as *const c_char
+        } else {
+            ptr::null()
+        }
+    }
+}
+
 ffi_fn! {
     /// Extracts an integer from the value
     unsafe fn mj_value_as_i64(_scope, value: mj_value) -> i64 {