Creating a read-only SharedMemoryObject

[tobiasblass]

Required information

Operating system:
Ubuntu 20.04.3 LTS

Compiler version:
gcc 8.4

Observed result or behaviour:

The produced error is

[ 22 ]  Invalid argument
Unable to create shared memory with the following properties [ name = readOnlyShmMem, access mode = AccessMode::READ_ONLY, open mode = OpenMode::PURGE_AND_CREATE, mode = 0000, sizeInBytes = 100 ]
Requested Shared Memory is larger then the maximum file size or the filedescriptor does not belong to a regular file.

Expected result or behaviour:

The error message is incorrect: both the size and the file are all right, the problem is that creating the segment involves truncate, which requires write permission.

The error should idenfity the READ_ONLY AccessMode as the problem, or the call should not accept AccessMode in the first place.

Conditions where it occurred / Performed steps:

    auto sut = iox::posix::SharedMemoryObjectBuilder()
                   .name("readOnlyShmMem")
                   .memorySizeInBytes(100)
                   .accessMode(iox::posix::AccessMode::READ_ONLY)
                   .openMode(iox::posix::OpenMode::PURGE_AND_CREATE)
                   .create();