Add C11 API with type-safe formatting (#4663) (#4671)

src/fmt-c.cc

@@ -0,0 +1,55 @@
+#include "fmt/fmt-c.h"
+
+#include <fmt/core.h>
+
+#include <cassert>
+
+extern "C" {
+
+using format_arg = fmt::basic_format_arg<fmt::format_context>;
+
+static bool populate_store(format_arg* out, const fmt_arg* c_args,
+                           size_t arg_count) {
+  if (arg_count > fmt_c_max_args) {
+    return false;
+  }
+
+  for (size_t i = 0; i < arg_count; ++i) {
+    switch (c_args[i].type) {
+    case fmt_int:         out[i] = c_args[i].value.i64; break;
+    case fmt_uint:        out[i] = c_args[i].value.u64; break;
+    case fmt_float:       out[i] = c_args[i].value.f32; break;
+    case fmt_double:      out[i] = c_args[i].value.f64; break;
+    case fmt_long_double: out[i] = c_args[i].value.f128; break;
+    case fmt_ptr:         out[i] = c_args[i].value.ptr; break;
+    case fmt_char:        out[i] = c_args[i].value.char_val; break;
+    case fmt_bool:        out[i] = c_args[i].value.bool_val; break;
+    case fmt_string:      out[i] = c_args[i].value.str; break;
+    default:              return false;
+    }
+  }
+  return true;
+}
+
+int fmt_vformat(char* buffer, size_t capacity, const char* format_str,
+                const fmt_arg* args, size_t arg_count) {
+  assert(format_str);
+
+  format_arg format_args[fmt_c_max_args];
+
+  if (arg_count > 0) {
+    assert(args);
+    if (!populate_store(format_args, args, arg_count)) {
+      return fmt_err_invalid_arg;
+    }
+  }
+
+  auto format_args_view = fmt::basic_format_args<fmt::format_context>(
+      format_args, static_cast<int>(arg_count));
+
+  auto result =
+      fmt::vformat_to_n(buffer, capacity, format_str, format_args_view);
+  return static_cast<int>(result.size);
+}
+
+}  // extern "C"