diff --git a/libc/src/stdio/printf_core/CMakeLists.txt b/libc/src/stdio/printf_core/CMakeLists.txt
--- a/libc/src/stdio/printf_core/CMakeLists.txt
+++ b/libc/src/stdio/printf_core/CMakeLists.txt
@@ -19,3 +19,21 @@
     libc.src.__support.CPP.bit
 
 )
+
+add_header_library(
+  string_writer
+  HDRS
+    string_writer.h
+  DEPENDS
+  libc.src.string.memory_utils.memcpy_implementation
+)
+
+add_object_library(
+  writer
+  SRCS
+    writer.cpp
+  HDRS
+    writer.h
+  DEPENDS
+    libc.src.string.memory_utils.memset_implementation
+)
diff --git a/libc/src/stdio/printf_core/string_writer.h b/libc/src/stdio/printf_core/string_writer.h
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/printf_core/string_writer.h
@@ -0,0 +1,37 @@
+//===-- String Writer function for printf -----------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STDIO_PRINTF_CORE_STRING_WRITER_H
+#define LLVM_LIBC_SRC_STDIO_PRINTF_CORE_STRING_WRITER_H
+
+#include "src/string/memory_utils/memcpy_implementations.h"
+#include <stddef.h>
+
+namespace __llvm_libc {
+namespace printf_core {
+
+// write_to_string treats raw_pointer as a char** so that it can increment the
+// inner pointer. This means that the variable raw_pointer points to should be a
+// copy of the original string passed to the call to sprintf. Additionally, this
+// function can't tell when the end of the string is, and as such cannot
+// properly null terminate the string. The copy of the original string should,
+// after the call to printf_main, point to the end of the string, and it should
+// be set to '\0' by the higher level call.
+void write_to_string(void *raw_pointer, const char *__restrict to_write,
+                     size_t len) {
+  // the string is stored with a double pointer so that it can be incremeneted
+  // without needing an additional variable.
+  char **string_dp = reinterpret_cast<char **>(raw_pointer);
+  inline_memcpy(*string_dp, to_write, len);
+  *string_dp += len;
+}
+
+} // namespace printf_core
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STDIO_PRINTF_CORE_STRING_WRITER_H
diff --git a/libc/src/stdio/printf_core/writer.h b/libc/src/stdio/printf_core/writer.h
--- a/libc/src/stdio/printf_core/writer.h
+++ b/libc/src/stdio/printf_core/writer.h
@@ -1,4 +1,4 @@
-//===-- String writer for printf --------------------------------*- C++ -*-===//
+//===-- Writer definition for printf ----------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -26,11 +26,17 @@
   // onto the end of output.
   WriteFunc raw_write;
 
+  // The max_length for the writer needs to be 1 less than the size of the
+  // buffer it's writing to. This is because it doesn't handle the null
+  // terminator.
   size_t max_length;
-  size_t chars_written;
+  size_t chars_written = 0;
 
 public:
-  Writer(void *output, WriteFunc raw_write, size_t max_length);
+  Writer(void *init_output, WriteFunc init_raw_write,
+         size_t init_max_length = ~size_t(0))
+      : output(init_output), raw_write(init_raw_write),
+        max_length(init_max_length) {}
 
   // write will copy length bytes from new_string into output using
   // raw_write, unless that would cause more bytes than max_length to be
@@ -42,7 +48,7 @@
   // increments chars_written by length.
   void write_chars(char new_char, size_t length);
 
-  size_t get_chars_written();
+  size_t get_chars_written() { return chars_written; }
 };
 
 } // namespace printf_core
diff --git a/libc/src/stdio/printf_core/writer.cpp b/libc/src/stdio/printf_core/writer.cpp
new file mode 100644
--- /dev/null
+++ b/libc/src/stdio/printf_core/writer.cpp
@@ -0,0 +1,42 @@
+//===-- Writer definition for printf ----------------------------*- 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 "writer.h"
+#include "src/string/memory_utils/memset_implementations.h"
+#include <stddef.h>
+
+namespace __llvm_libc {
+namespace printf_core {
+
+void Writer::write(const char *new_string, size_t length) {
+  // It's important that if max_length is 0, raw_write is never called. This is
+  // because for snprintf, it is valid to set the output string pointer to null
+  // if the max length is set to 0.
+  if (chars_written + length < max_length)
+    raw_write(output, new_string, length);
+  else if (chars_written < max_length) // chars_written + length >= max_length
+    raw_write(output, new_string, max_length - chars_written);
+
+  // chars_written tracks the number of chars that would have been written
+  // regardless of max_length.
+  chars_written += length;
+}
+
+void Writer::write_chars(char new_char, size_t length) {
+  constexpr size_t BUFF_SIZE = 8;
+  char buff[BUFF_SIZE];
+  inline_memset(buff, new_char, BUFF_SIZE);
+  while (length > BUFF_SIZE) {
+    write(buff, BUFF_SIZE);
+    length -= BUFF_SIZE;
+  }
+  write(buff, length);
+}
+
+} // namespace printf_core
+} // namespace __llvm_libc
diff --git a/libc/test/src/stdio/printf_core/CMakeLists.txt b/libc/test/src/stdio/printf_core/CMakeLists.txt
--- a/libc/test/src/stdio/printf_core/CMakeLists.txt
+++ b/libc/test/src/stdio/printf_core/CMakeLists.txt
@@ -10,3 +10,14 @@
 )
 
 target_link_libraries(libc.test.src.stdio.printf_core.parser_test PRIVATE LibcPrintfHelpers)
+
+add_libc_unittest(
+  string_writer_test
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    string_writer_test.cpp
+  DEPENDS
+    libc.src.stdio.printf_core.writer
+    libc.src.stdio.printf_core.string_writer
+)
diff --git a/libc/test/src/stdio/printf_core/string_writer_test.cpp b/libc/test/src/stdio/printf_core/string_writer_test.cpp
new file mode 100644
--- /dev/null
+++ b/libc/test/src/stdio/printf_core/string_writer_test.cpp
@@ -0,0 +1,186 @@
+//===-- Unittests for the printf String Writer ----------------------------===//
+//
+// 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/stdio/printf_core/string_writer.h"
+#include "src/stdio/printf_core/writer.h"
+
+#include "utils/UnitTest/Test.h"
+
+TEST(LlvmLibcPrintfStringWriterTest, Constructor) {
+  char str[10];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string);
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, Write) {
+  char str[4] = {'D', 'E', 'F', 'G'};
+  char *str_ptr = str; // This is neccessary because the pointer passed to the
+                       // writer will be modified.
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string);
+  writer.write("abc", 3);
+
+  EXPECT_EQ(str[3], 'G');
+  // This null terminates the string. The writer has no indication when the
+  // string is done, so it relies on the user to null terminate the string using
+  // the pointer that was passed to it.
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("abc", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(3));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, WriteMultipleTimes) {
+  char str[10];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string);
+  writer.write("abc", 3);
+  writer.write("DEF", 3);
+  writer.write("1234", 3);
+
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("abcDEF123", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(9));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, WriteChars) {
+  char str[4] = {'D', 'E', 'F', 'G'};
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string);
+  writer.write_chars('a', 3);
+
+  EXPECT_EQ(str[3], 'G');
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("aaa", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(3));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, WriteCharsMultipleTimes) {
+  char str[10];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string);
+  writer.write_chars('a', 3);
+  writer.write_chars('D', 3);
+  writer.write_chars('1', 3);
+
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("aaaDDD111", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(9));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, WriteManyChars) {
+  char str[100];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string);
+  writer.write_chars('Z', 99);
+
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZZ"
+               "ZZZZZZZZZ",
+               str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(99));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, MixedWrites) {
+  char str[13];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string);
+  writer.write_chars('a', 3);
+  writer.write("DEF", 3);
+  writer.write_chars('1', 3);
+  writer.write("456", 3);
+
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("aaaDEF111456", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(12));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, WriteWithMaxLength) {
+  char str[11];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string, 10);
+  writer.write("abcDEF123456", 12);
+
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("abcDEF1234", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(12));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, WriteCharsWithMaxLength) {
+  char str[11];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string, 10);
+
+  writer.write_chars('1', 15);
+
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("1111111111", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(15));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, MixedWriteWithMaxLength) {
+  char str[11];
+  char *str_ptr = str;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string, 10);
+  writer.write_chars('a', 3);
+  writer.write("DEF", 3);
+  writer.write_chars('1', 3);
+  writer.write("456", 3);
+
+  *str_ptr = '\0';
+
+  ASSERT_STREQ("aaaDEF1114", str);
+  ASSERT_EQ(writer.get_chars_written(), size_t(12));
+}
+
+TEST(LlvmLibcPrintfStringWriterTest, NullStringWithZeroMaxLength) {
+  char *str_ptr = nullptr;
+  __llvm_libc::printf_core::Writer writer(
+      reinterpret_cast<void *>(&str_ptr),
+      __llvm_libc::printf_core::write_to_string, 0);
+  writer.write_chars('a', 3);
+  writer.write("DEF", 3);
+  writer.write_chars('1', 3);
+  writer.write("456", 3);
+
+  ASSERT_EQ(writer.get_chars_written(), size_t(12));
+}