Make serd_strtod API const-correct

This is an API breakage, but a minor one (particularly since NULL is
allowed) that avoids the flaw in the standard C API.

4 files changed, 12 insertions, 8 deletions

diff --git a/NEWS b/NEWS
index 86f0e2e2..2e0f3ccb 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,7 @@
 serd (1.0.1) unstable;
 
   * Add SerdBuffer for mutable buffers to keep SerdChunk const-correct
+  * Make serd_strtod API const-correct
   * Remove useless character counting from API
   * Rename SerdChunk to SerdStringView
   * Use char* for strings in public API
diff --git a/serd/serd.h b/serd/serd.h
index b3ecb850..1963ff6b 100644
--- a/serd/serd.h
+++ b/serd/serd.h
@@ -320,13 +320,15 @@ serd_strlen(const char* str, SerdNodeFlags* flags);
 /**
    Parse a string to a double.
 
-   The API of this function is identical to the standard C strtod function,
+   The API of this function is similar to the standard C strtod function,
    except this function is locale-independent and always matches the lexical
-   format used in the Turtle grammar (the decimal point is always ".").
+   format used in the Turtle grammar (the decimal point is always ".").  The
+   end parameter is an offset from the start of `str` to avoid the
+   const-correctness issues of the strtod API.
 */
 SERD_API
 double
-serd_strtod(const char* str, char** endptr);
+serd_strtod(const char* str, size_t* end);
 
 /**
    Decode a base64 string.
diff --git a/src/string.c b/src/string.c
index 3f068a1d..1e7e9348 100644
--- a/src/string.c
+++ b/src/string.c
@@ -109,7 +109,7 @@ read_sign(const char** sptr)
 }
 
 double
-serd_strtod(const char* str, char** endptr)
+serd_strtod(const char* str, size_t* end)
 {
 	double result = 0.0;
 
@@ -145,8 +145,8 @@ serd_strtod(const char* str, char** endptr)
 		result *= pow(10, expt * expt_sign);
 	}
 
-	if (endptr) {
-		*endptr = (char*)s;
+	if (end) {
+		*end = s - str;
 	}
 
 	return result * sign;
diff --git a/tests/serd_test.c b/tests/serd_test.c
index ab0a0896..a8853969 100644
--- a/tests/serd_test.c
+++ b/tests/serd_test.c
@@ -40,11 +40,12 @@ test_strtod(double dbl, double max_delta)
 	char buf[1024];
 	snprintf(buf, sizeof(buf), "%f", dbl);
 
-	char* endptr = NULL;
-	const double out = serd_strtod(buf, &endptr);
+	size_t       end = 0;
+	const double out = serd_strtod(buf, &end);
 
 	const double diff = fabs(out - dbl);
 	assert(diff <= max_delta);
+	assert(end == strlen(buf));
 }
 
 typedef struct {