[libc]: Implement strfromf() and shared utilities (#85438)

Fixes #84244.

Implements the function `strfromf()` introduced in C23, and adds shared
utilities for implementation of other `strfrom*()` functions, including
`strfromd()` and `strfroml()`.

1 files changed, 42 insertions, 0 deletions

diff --git a/libc/src/stdlib/strfromf.cpp b/libc/src/stdlib/strfromf.cpp
new file mode 100644
index 000000000000..40eff87eb454
--- /dev/null
+++ b/libc/src/stdlib/strfromf.cpp
@@ -0,0 +1,42 @@
+//===-- Implementation of strfromf ------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdlib/strfromf.h"
+#include "src/stdlib/str_from_util.h"
+
+#include <stdarg.h>
+#include <stddef.h>
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(int, strfromf,
+                   (char *__restrict s, size_t n, const char *__restrict format,
+                    float fp)) {
+  LIBC_ASSERT(s != nullptr);
+
+  printf_core::FormatSection section =
+      internal::parse_format_string(format, fp);
+  printf_core::WriteBuffer wb(s, (n > 0 ? n - 1 : 0));
+  printf_core::Writer writer(&wb);
+
+  int result = 0;
+  if (section.has_conv)
+    result = internal::strfromfloat_convert<float>(&writer, section);
+  else
+    result = writer.write(section.raw_string);
+
+  if (result < 0)
+    return result;
+
+  if (n > 0)
+    wb.buff[wb.buff_cur] = '\0';
+
+  return writer.get_chars_written();
+}
+
+} // namespace LIBC_NAMESPACE