New subject: GHC FFI Return Type Bug

7 Aug 2001

      Sigbjorn

Am confused by your answer.

| > char fooble ( ... )
| > {
| >    return 'z';
| > }
| > 
| > on an x86, 'z' will be returned at the lowest 8 bits in %eax. What I

| > don't know is, is the C compiler obliged to clear the upper 24 bits
of 
| > %eax, or does that onus fall on the callee?
| 
| Yes, the callee is required to narrow <expr> in 'return 
| <expr>;' to fit that of the specified return type -- see 9.8 
| of Harbison and Steele. So, a C compiler that cause f() to 
| return 0x7fffffff for the following,
| 
| unsigned char g()
| {
|     return 0x7fffffff;
| }
| 
| unsigned int f()
| {
|     return g();
| }
| 
| is in the wrong. [Notice that narrowing for signed integral 
| types is undefined in ISO C, but most current-day compilers 
| implement such narrowing ops the same way, i.e., by masking 
| off excess bits.]

So we don't need to worry about doing the masking on the 
Haskell side, right?

Why would we need these unless we *did* need to worry about
masking on the Haskell side?

Pls clarify.

J

RE: GHC FFI Return Type Bug

Julian Seward (Intl Vendor)

Sigbjorn Finne

Sigbjorn Finne

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