Map ROS char into C signed char

[gerkey]

The C standard leaves it to the compiler implementor to decide whether a char is signed or unsigned:

• https://en.wikipedia.org/wiki/C_data_types#Basic_types

As a result, it's not portable to assign a negative value to a char and then test the result. This came up when building on aarch64:

• http://ci.ros2.org/job/ci_linux-aarch64/31/warnings22Result/
• http://ci.ros2.org/job/ci_linux-aarch64/31/testReport/(root)/projectroot/test_interfaces_c/

This PR changes the mapping of the ROS char to be a signed char in C. Requires ros2/rmw_fastrtps#99.

I'll also do a PR to update the design doc to specify the new mapping, but these code changes can be reviewed independently of that.

For a smaller test case, consider this program:

#include <stdio.h>

int main(void)
{
  char c1 = 66;
  char c2 = -66;

  if (c1 == 66)
    printf("positive pass\n");
  else
    printf("positive fail\n");

  if (c2 == -66)
    printf("negative pass\n");
  else
    printf("negative fail\n");
  return 0;
}

On linux/x86 it looks good:

$ gcc -Wtype-limits -o foo foo.c
$ ./foo
positive pass
negative pass

But on linux/aarch64 not so much:

$ gcc -Wall -Wtype-limits -o foo foo.c
foo.c: In function ‘main’:
foo.c:13:10: warning: comparison is always false due to limited range of data type [-Wtype-limits]
   if (c2 == -66)
          ^
$ ./foo
positive pass
negative fail

[gerkey]

• http://ci.ros2.org/job/ci_linux/2395/
• http://ci.ros2.org/job/ci_osx/1849/
• http://ci.ros2.org/job/ci_windows/2514/

[mikaelarguedas]

This looks to me like a good reason to revive the char/byte usefulness in our IDL format.

The design doc defines:

char:
an integer value in the following interval [-128, 127]

I'm not convinced that changing the tests to stop testing the entire range of the char type is a long term solution. Maybe reviving #190 and the associated Pull Requests will be more adequate.

If we are looking for a short term solution I think that we should explicitly assign this to a signed char to comply with our spec and compiler on ARM.

[mikaelarguedas]

Thanks for iterating on this. Can c6e45ca be reverted now that signed char is used ?

[dirk-thomas]

Please also create a corresponding PR for the design article which specifies the type mapping to be in sync.

[dirk-thomas]

This should be reverted.

Same below for test.

[gerkey]

00901e5

[dirk-thomas]

signed char is not a valid type name. Valid type names are only the keys of the MSG_TYPE_TO_C dict.

[gerkey]

00901e5

[gerkey]

http://ci.ros2.org/job/ci_linux-aarch64/38

[gerkey]

http://ci.ros2.org/job/ci_linux-aarch64/39/

[gerkey]

http://ci.ros2.org/job/ci_linux-aarch64/41/

[gerkey]

Now that aarch64 is happy, here's full CI:

• Linux:
  • 
• Linux aarch64:
  • 
• macOS:
  • 
• Windows:
  • 

[mikaelarguedas]

lgtm once CI passed