4 files changed, 77 insertions(+), 40 deletions(-)

M README.md
M astr.c
M astr.h
M astr_tests.c

@@ 7,8 7,11 @@ A C99 library for working with dynamically allocated strings.
 ## Overview
 
 All functions in *astr* begin with the `astr_` prefix.  Functions that return
-newly allocated strings will contain `alloc_` in their names; the caller is
+newly allocated strings will contain `_alloc` in their names; the caller is
 responsible for calling [`free()`][1] on the returned pointer.
+Functions with `_format` take a `printf()` style format string.
+Functions that end with the suffix `_args` take variable arguments as a 
+`va_list` object from `stdarg.h`.
 
 [1]: http://man7.org/linux/man-pages/man3/free.3.html
 


@@ 34,12 37,25 @@ Allocate a formatted string using a `va_list` object.
 
 Compare the bytes of two strings for equality; `s1` and `s2` may be `NULL`.
 
+    size_t
+    astr_len_format(char const *format, ...);
+
+Calculate the length in bytes needed for the formatted string.
+
+    size_t
+    astr_len_format_args(char const *format, va_list arguments);
+
+Calculate the length in bytes needed for the formatted string using a `va_list`
+object.
+
 ## Dependencies
-`astr` requires that `<stdio.h>` defines the [`vasprintf()`][2] function.
-Building from repository source requires [GNU Autotools][3].
+`astr` requires that `<stdio.h>` defines the [`vasprintf()`][2] and
+[`vsnprintf()`][3] functions.
+Building from repository source requires [GNU Autotools][4].
 
 [2]: http://man7.org/linux/man-pages/man3/asprintf.3.html
-[3]: https://www.gnu.org/software/automake/faq/autotools-faq.html
+[3]: http://man7.org/linux/man-pages/man3/printf.3.html
+[4]: https://www.gnu.org/software/automake/faq/autotools-faq.html
 
 ## Building from Repository Source
 

@@ 28,7 28,9 @@ astr_alloc_format_args(char const *format, va_list arguments)
     }
 
     char *s;
-    int chars_printed = vasprintf(&s, format, arguments);
+    va_list arguments_copy;
+    va_copy(arguments_copy, arguments);
+    int chars_printed = vasprintf(&s, format, arguments_copy);
     if (chars_printed < 0) return NULL;
     return s;
 }


@@ 37,3 39,29 @@ astr_alloc_format_args(char const *format, va_list arguments)
 extern bool
 astr_eq_bytes(char const *s1, char const *s2);
 
+
+size_t
+astr_len_format(char const *format, ...)
+{
+    va_list arguments;
+    va_start(arguments, format);
+    size_t length = astr_len_format_args(format, arguments);
+    va_end(arguments);
+    return length;
+}
+
+
+size_t
+astr_len_format_args(char const *format, va_list arguments)
+{
+    if (!format) {
+        errno = EINVAL;
+        return 0;
+    }
+
+    va_list arguments_copy;
+    va_copy(arguments_copy, arguments);
+    int length = vsnprintf(NULL, 0, format, arguments_copy);
+    return length < 0 ? 0 : length;
+}
+

@@ 27,6 27,12 @@ astr_eq_bytes(char const *s1, char const *s2)
     return 0 == strcmp(s1, s2);
 }
 
+size_t
+astr_len_format(char const *format, ...);
+
+size_t
+astr_len_format_args(char const *format, va_list arguments);
+
 
 #endif
 

@@ 36,39 36,6 @@ test_astr_alloc_format_for_NULL_format(void)
 }
 
 
-static char *
-helper_alloc_format(char const *format, ...)
-{
-    va_list arguments;
-    va_start(arguments, format);
-    char *s = astr_alloc_format_args(format, arguments);
-    va_end(arguments);
-    return s;
-}
-
-
-static void
-test_astr_alloc_format_args(void)
-{
-    char *s = helper_alloc_format("answer: %i", 42);
-    assert(s);
-    assert(10 == strlen(s));
-    assert(astr_eq_bytes("answer: 42", s));
-    free(s);
-}
-
-
-static void
-test_astr_alloc_format_args_for_NULL_format(void)
-{
-    errno = 0;
-    char *s = helper_alloc_format(NULL);
-    assert(!s);
-    assert(EINVAL == errno);
-    free(s);
-}
-
-
 static void
 test_astr_eq_bytes(void)
 {


@@ 94,16 61,36 @@ test_astr_eq_bytes_for_NULL_string(void)
 }
 
 
+static void
+test_astr_len_format(void)
+{
+    errno = 0;
+    size_t length = astr_len_format("answer: %i", 42);
+    assert(10 == length);
+    assert(0 == errno);
+}
+
+
+static void
+test_astr_len_format_for_NULL_format(void)
+{
+    errno = 0;
+    size_t length = astr_len_format(NULL);
+    assert(0 == length);
+    assert(EINVAL == errno);
+}
+
+
 int
 main(int argc, char *argv[])
 {
     test_astr_alloc_empty();
     test_astr_alloc_format();
     test_astr_alloc_format_for_NULL_format();
-    test_astr_alloc_format_args();
-    test_astr_alloc_format_args_for_NULL_format();
     test_astr_eq_bytes();
     test_astr_eq_bytes_for_NULL_string();
+    test_astr_len_format();
+    test_astr_len_format_for_NULL_format();
     return EXIT_SUCCESS;
 }