[libc-commits] [libc] [libc]: Implement strfrom* functions and utils (PR #85438)

[Michael Jones via libc-commits]

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@@ -0,0 +1,134 @@
+//===-- Implementation header for strfromx() utilitites -------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// According to the C23 standard, any input character sequences except a
+// precision specifier and the usual floating point formats, namely
+// %{a,A,e,E,f,F,g,G}, are not allowed and any code that does otherwise results
+// in undefined behaviour; which in this case is that the buffer string is
+// simply populated with the format string. The case of the value of buffer size
+// being 0 or the input being NULL should be handled in the calling function
+// (strfromf, strfromd, strfroml) itself.
+
+#ifndef LLVM_LIBC_SRC_STDLIB_STRFROM_UTIL_H
+#define LLVM_LIBC_SRC_STDLIB_STRFROM_UTIL_H
+
+#include "src/__support/str_to_integer.h"
+#include "src/stdio/printf_core/converter_atlas.h"
+#include "src/stdio/printf_core/core_structs.h"
+#include "src/stdio/printf_core/writer.h"
+
+#include <stddef.h>
+
+namespace LIBC_NAMESPACE::internal {
+
+template <typename T> struct type_of {
+  using type = T;
+};
+
+template <> struct type_of<float> {
+  using type = fputil::FPBits<float>::StorageType;
+};
+template <> struct type_of<double> {
+  using type = fputil::FPBits<double>::StorageType;
+};
+template <> struct type_of<long double> {
+  using type = fputil::FPBits<long double>::StorageType;
+};
+
+template <typename T> using type_of_v = typename type_of<T>::type;
+
+template <typename T>
+printf_core::FormatSection parse_format_string(const char *__restrict format,
+                                               T fp) {
+  printf_core::FormatSection section;
+  size_t cur_pos = 0;
+
+  if (format[cur_pos] == '%') {
+    section.has_conv = true;
+    ++cur_pos;
+
+    // handle precision
+    section.precision = -1;
+    if (format[cur_pos] == '.') {
+      ++cur_pos;
+      section.precision = 0;
+
+      // The standard does not allow the '*' (asterisk) operator for strfromx()
+      // functions
+      if (internal::isdigit(format[cur_pos])) {
+        auto result = internal::strtointeger<int>(format + cur_pos, 10);
+        section.precision += result.value;
+        cur_pos += result.parsed_len;
+      }
+    }
+
+    section.conv_name = format[cur_pos];
+    switch (format[cur_pos]) {
+    case '%':
----------------
michaelrj-google wrote:

we shouldn't provide the `%%` conversion here. It's not defined for this function, and the result isn't intuitive. Here's the doc describing how to deal with this sort of undefined behavior: https://libc.llvm.org/dev/undefined_behavior.html

https://github.com/llvm/llvm-project/pull/85438