Fix base64 decode error on ARM platforms

[meyerj]

I had the same problem as reported in #1768 when compiling the released version 1.14.3 of ros_comm on a Raspberry Pi 4 with gcc. Also here char seems to be an unsigned type and base64 decoding was broken. I assume this is the case on all ARM platforms.

I found this patch in the upstream bugtracker at https://sourceforge.net/p/libb64/bugs/2/ created in 2012 by Jakub Wilk. The last proposed patch that I applied here was submitted by Jonathan Wakely in 2016.

Only when I wanted to open a pull request here I realized that #1769 already addressed the topic. Jonathan's patch leaves the API as-is (char) and eliminates the implicit conversions between signed and unsigned types. For example, static const char decoding[] should not have negative values on implementations where char is unsigned.

The current patch (#1769) also solves the problem, but still produces sign conversion warnings on Raspbian 10 with gcc 8.3.0 and command line, while the new patch does not:

pi@raspberrypi:/home/pi/ros_ws/src/ros_comm/xmlrpcpp/libb64/src $ lsb_release -a
No LSB modules are available.
Distributor ID:	Raspbian
Description:	Raspbian GNU/Linux 10 (buster)
Release:	10
Codename:	buster
pi@raspberrypi:/home/pi/ros_ws/src/ros_comm/xmlrpcpp/libb64/src $ gcc --version
gcc (Raspbian 8.3.0-6+rpi1) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

pi@raspberrypi:/home/pi/ros_ws/src/ros_comm/xmlrpcpp/libb64/src $ gcc -Wall -Wextra -Werror -Wsign-conversion -pedantic -I../include -c cdecode.c
cdecode.c: In function ‘base64_decode_value’:
cdecode.c:12:37: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                     ^
cdecode.c:12:40: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                        ^
cdecode.c:12:43: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                           ^
cdecode.c:12:79: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                               ^
cdecode.c:12:82: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                  ^
cdecode.c:12:85: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                     ^
cdecode.c:12:88: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-2’ to ‘254’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                        ^
cdecode.c:12:91: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                           ^
cdecode.c:12:94: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                              ^
cdecode.c:12:97: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                                 ^
cdecode.c:12:168: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                                                                                                        ^
cdecode.c:12:171: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                                                                                                           ^
cdecode.c:12:174: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                                                                                                              ^
cdecode.c:12:177: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                                                                                                                 ^
cdecode.c:12:180: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                                                                                                                    ^
cdecode.c:12:183: error: unsigned conversion from ‘int’ to ‘char’ changes value from ‘-1’ to ‘255’ [-Werror=sign-conversion]
  static const char decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
                                                                                                                                                                                       ^
cdecode.c: In function ‘base64_decode_block’:
cdecode.c:49:42: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);
                                          ^~~~~~~~~~~
cdecode.c:49:16: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);
                ^
cdecode.c:61:42: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);
                                          ^~~~~~~~~~~
cdecode.c:61:16: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);
                ^
cdecode.c:74:42: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);
                                          ^~~~~~~~~~~
cdecode.c:74:16: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);
                ^
cdecode.c:87:42: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);
                                          ^~~~~~~~~~~
cdecode.c:87:16: error: conversion to ‘signed char’ from ‘char’ may change the sign of the result [-Werror=sign-conversion]
     fragment = (char)base64_decode_value(*codechar++);

I leave it to the maintainers to decide whether to keep the current patch by @randoms or apply this patch provided by Jakub Wilk and Jonathan Wakely instead.

[dirk-thomas]

Thanks for the patch.