[libc] add strerror

[michaelrj]

Summary

Strerror maps error numbers to strings. Additionally, a utility for
mapping errors to strings was added so that it could be reused for
perror and similar.

[sivachandra]

libc/src/__support/error_to_string.h

2

Almost everything in this file should live in a .cpp file. The only thing declared in the .h file should something like:

cpp::string_view error_to_string(int error);

IMO, that single function should take care of unknown error numbers also. Do you have a use case where max_buff_size and get_unknown_str are required?

A more fundamental question - is there any reason why this should live in __support at all?

[michaelrj]

libc/src/__support/error_to_string.h

2

I did make a .cpp file to contain the functions for the class, but was running into compile errors where the linker couldn't find the implementations, so I moved it into the header for now.

As for the other parts:
The reason max_buff_size and get_unknown_str exist is that when an error without a specified string mapping is passed, we need to generate a new string that says Unknown error [number]. Since this string will be passed back to the caller code, it can't be on the stack, so we need a buffer to hold it. In strerror.cpp I define a global buffer of size max_buff_size to be that buffer. This is specifically allowed by the standard: "The strerror function returns a pointer to the string, the contents of which are localespecific. The array pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the strerror function."

I placed this in support because both strerror which is in strings.h and perror which is in stdio.h use the same functionality. I can move it into strings, but generally I try to avoid including files between headers when possible.

[sivachandra]

libc/src/__support/error_to_string.h

2

I did make a .cpp file to contain the functions for the class, but was running into compile errors where the linker couldn't find the implementations, so I moved it into the header for now.

That shouldn't happen - we have lots of examples where in we store tables in .cpp files, especially in the math directory.

As for the other parts:
The reason max_buff_size and get_unknown_str exist is that when an error without a specified string mapping is passed, we need to generate a new string that says Unknown error [number]. Since this string will be passed back to the caller code, it can't be on the stack, so we need a buffer to hold it. In strerror.cpp I define a global buffer of size max_buff_size to be that buffer. This is specifically allowed by the standard: "The strerror function returns a pointer to the string, the contents of which are localespecific. The array pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the strerror function."

My question was more about, why should, say the strerror implementation, provide its own error buffer. Instead, why can't all these details be hidden behind a single error_to_string. Also, since errno is a thread local value, the error string buffer should also be thread local.

I placed this in support because both strerror which is in strings.h and perror which is in stdio.h use the same functionality. I can move it into strings, but generally I try to avoid including files between headers when possible.

SGTM

[michaelrj]

avoid global array of global pointers
moved implementation details into internal namespace

[sivachandra]

libc/src/__support/error_to_string.cpp

28

Add a comment somewhere explaining why this is present at all.

230

Can we use https://github.com/llvm/llvm-project/blob/main/libc/src/__support/CPP/stringstream.h to simply this?

[michaelrj]

move to stringstream for string construction

[sivachandra]

libc/src/__support/error_to_string.cpp

226

There is no point making this method a constexpr and introducing the const_convert (that would in a way be an antipattern) method - the argument err_num is not going to be a constexpr so this function is always evaluated at runtime.

[michaelrj]

remove constexpr from parts that don't need it

libc/config/linux/x86_64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strspn
     libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
 
     # string.h entrypoints that depend on malloc
     libc.src.string.strdup
     libc.src.string.strndup
+    libc.src.string.strerror
 
     # inttypes.h entrypoints
     libc.src.inttypes.imaxabs
     libc.src.inttypes.imaxdiv
     libc.src.inttypes.strtoimax
     libc.src.inttypes.strtoumax
 
     # stdlib.h entrypoints

libc/src/__support/CMakeLists.txt

DEPENDS
     .uint128
     libc.include.errno
     libc.src.__support.CPP.limits
     libc.src.__support.FPUtil.fp_bits
     libc.src.__support.builtin_wrappers
     libc.src.errno.errno
 )
 
+
+add_object_library(
+  error_to_string
+  HDRS
+    error_to_string.h
+  SRCS
+    error_to_string.cpp
+  DEPENDS
+  libc.include.errno
+  libc.src.__support.CPP.string_view
+  libc.src.__support.CPP.stringstream
+)
+
 add_header_library(
   integer_operations
   HDRS
     integer_operations.h
 )
 
 add_header_library(
   arg_list

libc/src/__support/CPP/stringstream.h

 // This class is to be used to write simple strings into a user provided buffer
 // without any dynamic memory allocation. There is no requirement to mimic the
 // C++ standard library class std::stringstream.
 class StringStream {
   span<char> data;
   size_t write_ptr = 0; // The current write pointer
   bool err = false;     // If an error occurs while writing
 
-  void write(const char *bytes, size_t size) {
+  constexpr void write(const char *bytes, size_t size) {
     size_t i = 0;
     const size_t data_size = data.size();
     for (; write_ptr < data_size && i < size; ++i, ++write_ptr)
       data[write_ptr] = bytes[i];
     if (i < size) {
       // If some of the characters couldn't be written, set error.
       err = true;
     }
   }
 
 public:
   static constexpr char ENDS = '\0';
 
   // Create a string stream which will write into |buf|.
   constexpr StringStream(const span<char> &buf) : data(buf) {}
 
   // Return a string_view to the current characters in the stream. If a
   // null terminator was not explicitly written, then the return value
   // will not include one. In order to produce a string_view to a null
   // terminated string, write ENDS explicitly.
-  string_view str() const { return string_view(data.data(), write_ptr); }
+  constexpr string_view str() const {
+    return string_view(data.data(), write_ptr);
+  }
 
   // Write the characters from |str| to the stream.
-  StringStream &operator<<(string_view str) {
+  constexpr StringStream &operator<<(string_view str) {
     write(str.data(), str.size());
     return *this;
   }
 
   // Write the |val| as string.
   template <typename T, enable_if_t<is_integral_v<T>, int> = 0>
-  StringStream &operator<<(T val) {
+  constexpr StringStream &operator<<(T val) {
     char buffer[IntegerToString::dec_bufsize<T>()];
-    auto int_to_str = IntegerToString::dec(val, buffer);
-    if (int_to_str)
-      return operator<<(*int_to_str);
-    return *this;
+    auto int_to_str = IntegerToString::const_convert<10, int>(val, buffer);
+    return operator<<(int_to_str);
   }
 
   template <typename T, enable_if_t<is_floating_point_v<T>, int> = 0>
-  StringStream &operator<<(T) {
+  constexpr StringStream &operator<<(T) {
     // If this specialization gets activated, then the static_assert will
     // trigger a compile error about missing floating point number support.
     static_assert(!is_floating_point_v<T>,
                   "Writing floating point numbers is not yet supported");
     return *this;
   }
 
   // Write a null-terminated string. The terminating null character is not
   // written to allow stremaing to continue.
-  StringStream &operator<<(const char *str) {
+  constexpr StringStream &operator<<(const char *str) {
     return operator<<(string_view(str));
   }
 
   // Write a single character.
-  StringStream &operator<<(char a) {
+  constexpr StringStream &operator<<(char a) {
     write(&a, 1);
     return *this;
   }
 
   // Return true if any write operation(s) failed due to insufficient size.
-  bool overflow() const { return err; }
+  constexpr bool overflow() const { return err; }
 
-  size_t bufsize() const { return data.size(); }
+  constexpr size_t bufsize() const { return data.size(); }
 };
 
 } // namespace cpp
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_SRC_SUPPORT_CPP_STRINGSTREAM_H

libc/src/__support/error_to_string.h

+//===-- Definition of a class for mapping errors to strings -----*- 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/__support/CPP/string_view.h"
+#include "src/__support/integer_to_string.h"
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_ERROR_TO_STRING
+#define LLVM_LIBC_SRC_SUPPORT_ERROR_TO_STRING
+
+namespace __llvm_libc {
+
+cpp::string_view get_error_string(int err_num);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SUPPORT_ERROR_TO_STRING

libc/src/__support/error_to_string.cpp

+//===-- Implementation of a class for mapping errors to strings -----------===//
+//
+// 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/__support/error_to_string.h"
+#include "src/__support/CPP/string_view.h"
+#include "src/__support/CPP/stringstream.h"
+#include <errno.h>
+#include <stddef.h>
+
+namespace __llvm_libc {
+namespace internal {
+
+constexpr size_t max_buff_size() {
+  constexpr size_t unknown_str_len = sizeof("Unknown error");
+  constexpr size_t max_num_len =
+      __llvm_libc::IntegerToString::dec_bufsize<int>();
+  // the buffer should be able to hold "Unknown error" + ' ' + num_str
+  return (unknown_str_len + 1 + max_num_len) * sizeof(char);
+}
+
+// This is to hold error strings that have to be custom built. It may be
+// rewritten on every call to strerror (or other error to string function).
+constexpr size_t BUFFER_SIZE = max_buff_size();
+thread_local char error_buffer[BUFFER_SIZE];
+
+struct ErrMsgMapping {
+  int err_num;
+  cpp::string_view err_msg;
+
+public:
+  constexpr ErrMsgMapping(int num, const char *msg)
+      : err_num(num), err_msg(msg) {
+    ;
+  }
+};
+
+constexpr ErrMsgMapping raw_err_array[] = {
+    ErrMsgMapping(0, "Success"),
+    ErrMsgMapping(EPERM, "Operation not permitted"),
+    ErrMsgMapping(ENOENT, "No such file or directory"),
+    ErrMsgMapping(ESRCH, "No such process"),
+    ErrMsgMapping(EINTR, "Interrupted system call"),
+    ErrMsgMapping(EIO, "Input/output error"),
+    ErrMsgMapping(ENXIO, "No such device or address"),
+    ErrMsgMapping(E2BIG, "Argument list too long"),
+    ErrMsgMapping(ENOEXEC, "Exec format error"),
+    ErrMsgMapping(EBADF, "Bad file descriptor"),
+    ErrMsgMapping(ECHILD, "No child processes"),
+    ErrMsgMapping(EAGAIN, "Resource temporarily unavailable"),
+    ErrMsgMapping(ENOMEM, "Cannot allocate memory"),
+    ErrMsgMapping(EACCES, "Permission denied"),
+    ErrMsgMapping(EFAULT, "Bad address"),
+    ErrMsgMapping(ENOTBLK, "Block device required"),
+    ErrMsgMapping(EBUSY, "Device or resource busy"),
+    ErrMsgMapping(EEXIST, "File exists"),
+    ErrMsgMapping(EXDEV, "Invalid cross-device link"),
+    ErrMsgMapping(ENODEV, "No such device"),
+    ErrMsgMapping(ENOTDIR, "Not a directory"),
+    ErrMsgMapping(EISDIR, "Is a directory"),
+    ErrMsgMapping(EINVAL, "Invalid argument"),
+    ErrMsgMapping(ENFILE, "Too many open files in system"),
+    ErrMsgMapping(EMFILE, "Too many open files"),
+    ErrMsgMapping(ENOTTY, "Inappropriate ioctl for device"),
+    ErrMsgMapping(ETXTBSY, "Text file busy"),
+    ErrMsgMapping(EFBIG, "File too large"),
+    ErrMsgMapping(ENOSPC, "No space left on device"),
+    ErrMsgMapping(ESPIPE, "Illegal seek"),
+    ErrMsgMapping(EROFS, "Read-only file system"),
+    ErrMsgMapping(EMLINK, "Too many links"),
+    ErrMsgMapping(EPIPE, "Broken pipe"),
+    ErrMsgMapping(EDOM, "Numerical argument out of domain"),
+    ErrMsgMapping(ERANGE, "Numerical result out of range"),
+    ErrMsgMapping(EDEADLK, "Resource deadlock avoided"),
+    ErrMsgMapping(ENAMETOOLONG, "File name too long"),
+    ErrMsgMapping(ENOLCK, "No locks available"),
+    ErrMsgMapping(ENOSYS, "Function not implemented"),
+    ErrMsgMapping(ENOTEMPTY, "Directory not empty"),
+    ErrMsgMapping(ELOOP, "Too many levels of symbolic links"),
+    // No error for 41. Would be EWOULDBLOCK
+    ErrMsgMapping(ENOMSG, "No message of desired type"),
+    ErrMsgMapping(EIDRM, "Identifier removed"),
+    ErrMsgMapping(ECHRNG, "Channel number out of range"),
+    ErrMsgMapping(EL2NSYNC, "Level 2 not synchronized"),
+    ErrMsgMapping(EL3HLT, "Level 3 halted"),
+    ErrMsgMapping(EL3RST, "Level 3 reset"),
+    ErrMsgMapping(ELNRNG, "Link number out of range"),
+    ErrMsgMapping(EUNATCH, "Protocol driver not attached"),
+    ErrMsgMapping(ENOCSI, "No CSI structure available"),
+    ErrMsgMapping(EL2HLT, "Level 2 halted"),
+    ErrMsgMapping(EBADE, "Invalid exchange"),
+    ErrMsgMapping(EBADR, "Invalid request descriptor"),
+    ErrMsgMapping(EXFULL, "Exchange full"),
+    ErrMsgMapping(ENOANO, "No anode"),
+    ErrMsgMapping(EBADRQC, "Invalid request code"),
+    ErrMsgMapping(EBADSLT, "Invalid slot"),
+    // No error for 58. Would be EDEADLOCK.
+    ErrMsgMapping(EBFONT, "Bad font file format"),
+    ErrMsgMapping(ENOSTR, "Device not a stream"),
+    ErrMsgMapping(ENODATA, "No data available"),
+    ErrMsgMapping(ETIME, "Timer expired"),
+    ErrMsgMapping(ENOSR, "Out of streams resources"),
+    ErrMsgMapping(ENONET, "Machine is not on the network"),
+    ErrMsgMapping(ENOPKG, "Package not installed"),
+    ErrMsgMapping(EREMOTE, "Object is remote"),
+    ErrMsgMapping(ENOLINK, "Link has been severed"),
+    ErrMsgMapping(EADV, "Advertise error"),
+    ErrMsgMapping(ESRMNT, "Srmount error"),
+    ErrMsgMapping(ECOMM, "Communication error on send"),
+    ErrMsgMapping(EPROTO, "Protocol error"),
+    ErrMsgMapping(EMULTIHOP, "Multihop attempted"),
+    ErrMsgMapping(EDOTDOT, "RFS specific error"),
+    ErrMsgMapping(EBADMSG, "Bad message"),
+    ErrMsgMapping(EOVERFLOW, "Value too large for defined data type"),
+    ErrMsgMapping(ENOTUNIQ, "Name not unique on network"),
+    ErrMsgMapping(EBADFD, "File descriptor in bad state"),
+    ErrMsgMapping(EREMCHG, "Remote address changed"),
+    ErrMsgMapping(ELIBACC, "Can not access a needed shared library"),
+    ErrMsgMapping(ELIBBAD, "Accessing a corrupted shared library"),
+    ErrMsgMapping(ELIBSCN, ".lib section in a.out corrupted"),
+    ErrMsgMapping(ELIBMAX, "Attempting to link in too many shared libraries"),
+    ErrMsgMapping(ELIBEXEC, "Cannot exec a shared library directly"),
+    ErrMsgMapping(EILSEQ, "Invalid or incomplete multibyte or wide character"),
+    ErrMsgMapping(ERESTART, "Interrupted system call should be restarted"),
+    ErrMsgMapping(ESTRPIPE, "Streams pipe error"),
+    ErrMsgMapping(EUSERS, "Too many users"),
+    ErrMsgMapping(ENOTSOCK, "Socket operation on non-socket"),
+    ErrMsgMapping(EDESTADDRREQ, "Destination address required"),
+    ErrMsgMapping(EMSGSIZE, "Message too long"),
+    ErrMsgMapping(EPROTOTYPE, "Protocol wrong type for socket"),
+    ErrMsgMapping(ENOPROTOOPT, "Protocol not available"),
+    ErrMsgMapping(EPROTONOSUPPORT, "Protocol not supported"),
+    ErrMsgMapping(ESOCKTNOSUPPORT, "Socket type not supported"),
+    ErrMsgMapping(ENOTSUP, "Operation not supported"),
+    ErrMsgMapping(EPFNOSUPPORT, "Protocol family not supported"),
+    ErrMsgMapping(EAFNOSUPPORT, "Address family not supported by protocol"),
+    ErrMsgMapping(EADDRINUSE, "Address already in use"),
+    ErrMsgMapping(EADDRNOTAVAIL, "Cannot assign requested address"),
+    ErrMsgMapping(ENETDOWN, "Network is down"),
+    ErrMsgMapping(ENETUNREACH, "Network is unreachable"),
+    ErrMsgMapping(ENETRESET, "Network dropped connection on reset"),
+    ErrMsgMapping(ECONNABORTED, "Software caused connection abort"),
+    ErrMsgMapping(ECONNRESET, "Connection reset by peer"),
+    ErrMsgMapping(ENOBUFS, "No buffer space available"),
+    ErrMsgMapping(EISCONN, "Transport endpoint is already connected"),
+    ErrMsgMapping(ENOTCONN, "Transport endpoint is not connected"),
+    ErrMsgMapping(ESHUTDOWN, "Cannot send after transport endpoint shutdown"),
+    ErrMsgMapping(ETOOMANYREFS, "Too many references: cannot splice"),
+    ErrMsgMapping(ETIMEDOUT, "Connection timed out"),
+    ErrMsgMapping(ECONNREFUSED, "Connection refused"),
+    ErrMsgMapping(EHOSTDOWN, "Host is down"),
+    ErrMsgMapping(EHOSTUNREACH, "No route to host"),
+    ErrMsgMapping(EALREADY, "Operation already in progress"),
+    ErrMsgMapping(EINPROGRESS, "Operation now in progress"),
+    ErrMsgMapping(ESTALE, "Stale file handle"),
+    ErrMsgMapping(EUCLEAN, "Structure needs cleaning"),
+    ErrMsgMapping(ENOTNAM, "Not a XENIX named type file"),
+    ErrMsgMapping(ENAVAIL, "No XENIX semaphores available"),
+    ErrMsgMapping(EISNAM, "Is a named type file"),
+    ErrMsgMapping(EREMOTEIO, "Remote I/O error"),
+    ErrMsgMapping(EDQUOT, "Disk quota exceeded"),
+    ErrMsgMapping(ENOMEDIUM, "No medium found"),
+    ErrMsgMapping(EMEDIUMTYPE, "Wrong medium type"),
+    ErrMsgMapping(ECANCELED, "Operation canceled"),
+    ErrMsgMapping(ENOKEY, "Required key not available"),
+    ErrMsgMapping(EKEYEXPIRED, "Key has expired"),
+    ErrMsgMapping(EKEYREVOKED, "Key has been revoked"),
+    ErrMsgMapping(EKEYREJECTED, "Key was rejected by service"),
+    ErrMsgMapping(EOWNERDEAD, "Owner died"),
+    ErrMsgMapping(ENOTRECOVERABLE, "State not recoverable"),
+    ErrMsgMapping(ERFKILL, "Operation not possible due to RF-kill"),
+    ErrMsgMapping(EHWPOISON, "Memory page has hardware error"),
+};
+
+constexpr size_t total_str_len(const ErrMsgMapping *array, size_t len) {
+  size_t total = 0;
+  for (size_t i = 0; i < len; ++i) {
+    // add 1 for the null terminator.
+    total += array[i].err_msg.size() + 1;
+  }
+  return total;
+}
+
+// Since the StringMappings array is a map from error numbers to their
+// corresponding strings, we have to have an array large enough we can use the
+// error numbers as indexes. Thankfully there are 132 errors in the above list
+// (41 and 58 are skipped) and the highest number is 133. If other platforms use
+// different error numbers, then this number may need to be adjusted.
+// Also if negative numbers or particularly large numbers are used, then the
+// array should be turned into a proper hashmap.
+constexpr size_t ERR_ARRAY_SIZE = 134;
+
+class ErrorMapper {
+
+  // const char *StringMappings[ERR_ARRAY_SIZE] = {""};
+  int err_offsets[ERR_ARRAY_SIZE] = {-1};
+  char string_array[total_str_len(
+      raw_err_array, sizeof(raw_err_array) / sizeof(ErrMsgMapping))] = {'\0'};
+
+public:
+  constexpr ErrorMapper() {
+    cpp::string_view string_mappings[ERR_ARRAY_SIZE] = {""};
+    for (size_t i = 0; i < (sizeof(raw_err_array) / sizeof(ErrMsgMapping)); ++i)
+      string_mappings[raw_err_array[i].err_num] = raw_err_array[i].err_msg;
+
+    size_t string_array_index = 0;
+    for (size_t cur_err = 0; cur_err < ERR_ARRAY_SIZE; ++cur_err) {
+      if (string_mappings[cur_err].size() != 0) {
+        err_offsets[cur_err] = string_array_index;
+        // No need to replace with proper strcpy, this is evaluated at compile
+        // time.
+        for (size_t i = 0; i < string_mappings[cur_err].size() + 1;
+             ++i, ++string_array_index) {
+          string_array[string_array_index] = string_mappings[cur_err][i];
+        }
+      } else {
+        err_offsets[cur_err] = -1;
+      }
+    }
+  }
+
+  constexpr cpp::string_view get_str(int err_num) const {
+    if (err_num >= 0 && static_cast<size_t>(err_num) < ERR_ARRAY_SIZE &&
+        err_offsets[err_num] != -1) {
+      return const_cast<char *>(string_array + err_offsets[err_num]);
+    } else {
+      // if the buffer can't hold "Unknown error" + ' ' + num_str, then just
+      // return "Unknown error".
+      if (BUFFER_SIZE <
+          (sizeof("Unknown error") + 1 + IntegerToString::dec_bufsize<int>()))
+        return const_cast<char *>("Unknown error");
+
+      cpp::StringStream buffer_stream({error_buffer, BUFFER_SIZE});
+      buffer_stream << "Unknown error" << ' ' << err_num << '\0';
+      return buffer_stream.str();
+    }
+  }
+};
+
+static constexpr ErrorMapper error_mapper;
+
+} // namespace internal
+
+cpp::string_view get_error_string(int err_num) {
+  return internal::error_mapper.get_str(err_num);
+}
+} // namespace __llvm_libc

libc/src/__support/integer_to_string.h

 //===-- Utilities to convert integral values to string ----------*- 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_SUPPORT_INTEGER_TO_STRING_H
 #define LLVM_LIBC_SRC_SUPPORT_INTEGER_TO_STRING_H
 
 #include <stdint.h>
 
-#include "src/__support/CPP/string_view.h"
 #include "src/__support/CPP/optional.h"
 #include "src/__support/CPP/span.h"
+#include "src/__support/CPP/string_view.h"
 #include "src/__support/CPP/type_traits.h"
 
 namespace __llvm_libc {
 
 // Convert integer values to their string representation.
 //
 // Example usage:
 //   int a = 1234567;

 //   char binbuf[IntegerToString::bin_bufsize<int>(a)];
 //   auto str = IntegerToString::bin(a, binbuf);
 //
 //   // Convert to base 30 string:
 //   char b30buf[IntegerToString::bufsize<30, int>(a)];
 //   auto str = IntegerToString::convert<30>(a, b30buf);
 class IntegerToString {
   static cpp::string_view convert_uintmax(uintmax_t uval,
                                           cpp::span<char> &buffer,
                                           bool lowercase,
                                           const uint8_t conv_base) {
     const char a = lowercase ? 'a' : 'A';
 
     size_t len = 0;
 
     size_t buffptr = buffer.size();
     if (uval == 0) {
       buffer[buffptr - 1] = '0';
       --buffptr;
     } else {
       for (; uval > 0; --buffptr, uval /= conv_base) {
         uintmax_t digit = (uval % conv_base);
         buffer[buffptr - 1] = digit < 10 ? digit + '0' : digit + a - 10;
       }
     }
     len = buffer.size() - buffptr;
 
     return cpp::string_view(buffer.data() + buffer.size() - len, len);
   }
 
   static cpp::string_view convert_intmax(intmax_t val, cpp::span<char> &buffer,
                                          bool lowercase,
                                          const uint8_t conv_base) {
     if (val >= 0)
       return convert_uintmax(uintmax_t(val), buffer, lowercase, conv_base);
     uintmax_t uval = uintmax_t(-val);
     auto str_view = convert_uintmax(uval, buffer, lowercase, conv_base);
     size_t len = str_view.size();
     ++len;
     buffer[buffer.size() - len] = '-';
     return cpp::string_view(buffer.data() + buffer.size() - len, len);
public:
   template <typename T> static constexpr size_t bin_bufsize() {
     return bufsize<2, T>();
   }
 
   template <uint8_t BASE, typename T,
             cpp::enable_if_t<2 <= BASE && BASE <= 36 && cpp::is_integral_v<T>,
                              int> = 0>
   static cpp::optional<cpp::string_view> convert(T val, cpp::span<char> buffer,
                                                  bool lowercase = true) {
     if (buffer.size() < bufsize<BASE, T>())
       return cpp::optional<cpp::string_view>();
     if (cpp::is_signed_v<T>)
       return convert_intmax(intmax_t(val), buffer, lowercase, BASE);
     else
       return convert_uintmax(uintmax_t(val), buffer, lowercase, BASE);
   }
 
+  // This is constexpr, which is useful, but the buffer MUST be large enough.
+  template <uint8_t BASE, typename T,
+            cpp::enable_if_t<2 <= BASE && BASE <= 36 && cpp::is_integral_v<T>,
+                             int> = 0>
+  static constexpr cpp::string_view const_convert(T val, cpp::span<char> buffer,
+                                                  bool lowercase = true) {
+    if (cpp::is_signed_v<T>)
+      return convert_intmax(intmax_t(val), buffer, lowercase, BASE);
+    else
+      return convert_uintmax(uintmax_t(val), buffer, lowercase, BASE);
+  }
+
   template <typename T, cpp::enable_if_t<cpp::is_integral_v<T>, int> = 0>
   static cpp::optional<cpp::string_view> dec(T val, cpp::span<char> buffer) {
     return convert<10>(val, buffer);
   }
 
   template <typename T, cpp::enable_if_t<cpp::is_integral_v<T>, int> = 0>
   static cpp::optional<cpp::string_view> hex(T val, cpp::span<char> buffer,
                                              bool lowercase = true) {
     return convert<16>(val, buffer, lowercase);
   }
 
   template <typename T, cpp::enable_if_t<cpp::is_integral_v<T>, int> = 0>
   static cpp::optional<cpp::string_view> oct(T val, cpp::span<char> buffer) {
     return convert<8>(val, buffer);
   }
 

libc/src/string/CMakeLists.txt

HDRS
     strdup.h
   DEPENDS
     .memory_utils.memcpy_implementation
     .string_utils
     libc.include.stdlib
 )
 
 add_entrypoint_object(
+  strerror
+  SRCS
+    strerror.cpp
+  HDRS
+    strerror.h
+  DEPENDS
+    libc.src.__support.error_to_string
+    libc.src.__support.CPP.span
+)
+
+add_entrypoint_object(
   strlcat
   SRCS
     strlcat.cpp
   HDRS
     strlcat.h
   DEPENDS
     .string_utils
     libc.include.string

libc/src/string/strerror.h

+//===-- Implementation header for strerror ----------------------*- 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_STRING_STRERROR_H
+#define LLVM_LIBC_SRC_STRING_STRERROR_H
+
+namespace __llvm_libc {
+
+char *strerror(int errnum);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRERROR_H

libc/src/string/strerror.cpp

+//===-- Implementation of strerror ----------------------------------------===//
+//
+// 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/string/strerror.h"
+#include "src/__support/CPP/span.h"
+#include "src/__support/common.h"
+#include "src/__support/error_to_string.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(char *, strerror, (int err_num)) {
+  return const_cast<char *>(get_error_string(err_num).data());
+}
+
+} // namespace __llvm_libc

libc/test/ErrnoSetterMatcher.h

 
 #include <errno.h>
 
 namespace __llvm_libc {
 namespace testing {
 
 namespace internal {
 
-extern "C" const char *strerror(int);
+extern "C" char *strerror(int);
 
 template <typename T> class ErrnoSetterMatcher : public Matcher<T> {
   T ExpectedReturn;
   T ActualReturn;
   int ExpectedErrno;
   int ActualErrno;
 
 public:

libc/test/src/string/CMakeLists.txt

add_libc_unittest(
   SRCS
     strdup_test.cpp
   DEPENDS
     libc.include.stdlib
     libc.src.string.strdup
 )
 
 add_libc_unittest(
+  strerror_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strerror_test.cpp
+  DEPENDS
+    libc.src.string.strerror
+)
+
+add_libc_unittest(
   strlcat_test
   SUITE
     libc_string_unittests
   SRCS
     strlcat_test.cpp
   DEPENDS
     libc.src.string.strlcat
 )

libc/test/src/string/strerror_test.cpp

+//===-- Unittests for strerror --------------------------------------------===//
+//
+// 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/string/strerror.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(LlvmLibcStrErrorTest, KnownErrors) {
+  ASSERT_STREQ(__llvm_libc::strerror(0), "Success");
+
+  const char *message_array[] = {
+      "Success",
+      "Operation not permitted",
+      "No such file or directory",
+      "No such process",
+      "Interrupted system call",
+      "Input/output error",
+      "No such device or address",
+      "Argument list too long",
+      "Exec format error",
+      "Bad file descriptor",
+      "No child processes",
+      "Resource temporarily unavailable",
+      "Cannot allocate memory",
+      "Permission denied",
+      "Bad address",
+      "Block device required",
+      "Device or resource busy",
+      "File exists",
+      "Invalid cross-device link",
+      "No such device",
+      "Not a directory",
+      "Is a directory",
+      "Invalid argument",
+      "Too many open files in system",
+      "Too many open files",
+      "Inappropriate ioctl for device",
+      "Text file busy",
+      "File too large",
+      "No space left on device",
+      "Illegal seek",
+      "Read-only file system",
+      "Too many links",
+      "Broken pipe",
+      "Numerical argument out of domain",
+      "Numerical result out of range",
+      "Resource deadlock avoided",
+      "File name too long",
+      "No locks available",
+      "Function not implemented",
+      "Directory not empty",
+      "Too many levels of symbolic links",
+      "Unknown error 41", // Unknown
+      "No message of desired type",
+      "Identifier removed",
+      "Channel number out of range",
+      "Level 2 not synchronized",
+      "Level 3 halted",
+      "Level 3 reset",
+      "Link number out of range",
+      "Protocol driver not attached",
+      "No CSI structure available",
+      "Level 2 halted",
+      "Invalid exchange",
+      "Invalid request descriptor",
+      "Exchange full",
+      "No anode",
+      "Invalid request code",
+      "Invalid slot",
+      "Unknown error 58", // Unknown
+      "Bad font file format",
+      "Device not a stream",
+      "No data available",
+      "Timer expired",
+      "Out of streams resources",
+      "Machine is not on the network",
+      "Package not installed",
+      "Object is remote",
+      "Link has been severed",
+      "Advertise error",
+      "Srmount error",
+      "Communication error on send",
+      "Protocol error",
+      "Multihop attempted",
+      "RFS specific error",
+      "Bad message",
+      "Value too large for defined data type",
+      "Name not unique on network",
+      "File descriptor in bad state",
+      "Remote address changed",
+      "Can not access a needed shared library",
+      "Accessing a corrupted shared library",
+      ".lib section in a.out corrupted",
+      "Attempting to link in too many shared libraries",
+      "Cannot exec a shared library directly",
+      "Invalid or incomplete multibyte or wide character",
+      "Interrupted system call should be restarted",
+      "Streams pipe error",
+      "Too many users",
+      "Socket operation on non-socket",
+      "Destination address required",
+      "Message too long",
+      "Protocol wrong type for socket",
+      "Protocol not available",
+      "Protocol not supported",
+      "Socket type not supported",
+      "Operation not supported",
+      "Protocol family not supported",
+      "Address family not supported by protocol",
+      "Address already in use",
+      "Cannot assign requested address",
+      "Network is down",
+      "Network is unreachable",
+      "Network dropped connection on reset",
+      "Software caused connection abort",
+      "Connection reset by peer",
+      "No buffer space available",
+      "Transport endpoint is already connected",
+      "Transport endpoint is not connected",
+      "Cannot send after transport endpoint shutdown",
+      "Too many references: cannot splice",
+      "Connection timed out",
+      "Connection refused",
+      "Host is down",
+      "No route to host",
+      "Operation already in progress",
+      "Operation now in progress",
+      "Stale file handle",
+      "Structure needs cleaning",
+      "Not a XENIX named type file",
+      "No XENIX semaphores available",
+      "Is a named type file",
+      "Remote I/O error",
+      "Disk quota exceeded",
+      "No medium found",
+      "Wrong medium type",
+      "Operation canceled",
+      "Required key not available",
+      "Key has expired",
+      "Key has been revoked",
+      "Key was rejected by service",
+      "Owner died",
+      "State not recoverable",
+      "Operation not possible due to RF-kill",
+      "Memory page has hardware error",
+  };
+
+  for (size_t i = 0; i < (sizeof(message_array) / sizeof(char *)); ++i) {
+    EXPECT_STREQ(__llvm_libc::strerror(i), message_array[i]);
+  }
+}
+
+TEST(LlvmLibcStrErrorTest, UnknownErrors) {
+  ASSERT_STREQ(__llvm_libc::strerror(-1), "Unknown error -1");
+  ASSERT_STREQ(__llvm_libc::strerror(134), "Unknown error 134");
+  ASSERT_STREQ(__llvm_libc::strerror(2147483647), "Unknown error 2147483647");
+  ASSERT_STREQ(__llvm_libc::strerror(-2147483648), "Unknown error -2147483648");
+}