Ruby's source code is written assuming the following. It is considered to be extremely difficult to port to an environment not satisfying (any of) them.

• Your C compiler complies ISO/IEC 9899:1999 at least for features in "Known supported features" of https://bugs.ruby-lang.org/projects/ruby-trunk/wiki/C99.
• The "execution character set" of your C environment (cf: the standard) must be ASCII, not something like EBCDIC.
• Your compiler should not limit user (re-)definitions of identifiers listed in the C standard library.
• (If you choose to opt-in instruction unification feature of the VM) Your C compiler must accept switch statement with more than 256 branches.
• Your C compiler must accept string literals of at least 7,000 characters.

• char must either be identical to signed char or identical to unsigned char and nothing else.
• char must be 8 bits long. There are a lot of places where 8 appear as magic numbers.
• int must be 32 bits long.
• long must either be 32 bits long or 64 bits long.
• Fixnum's width is that of long, not that of pointers. This is true even when long is smaller than VALUE.
• time_t must be an integer type (but signedness might vary).
• Unsigned 32 bits integer is required for character handlings due to due to GB18030. Signed 32 bits is insufficient.
• There must either be intptr_t, or an integer type that can be casted from/to void * without loss.
• Although the standard says enum is a signed int, there are parts in the source code where enum and long are assumed convertible without loss.
• sizeof(size_t) == sizeof(void*) holds.
• sizeof(VALUE) == sizeof(void*) holds.

• Either long or long long (_int64) must be convertible with pointers.
• void * must exist.
• Function pointers and void * must be convertible without loss.
• Arbitrary function pointers must be convertible via cast to each other and can be called via casting.
• Machine stack must exist; that is, automatic variables should be arranged in a high-order or low-order specific position of some memory address, not distributed.
• Least significant 2 bits of pointer type values ​​must always be 0.

• Infinity and NaN must exist somewhere in the double type.