Redundant size of union compiling with -m32 on OS X

Question

I have the following tagged union in my code: https://github.com/EarlGray/SECD/blob/f2e364f84d194aea5cef9257630bf931e9f88cab/secd.h#L217

When I compile it on 64 bit Linux or OS X using gcc or clang, size of cell_t is always 32 bytes (4 * sizeof(long), as expected).

When I compile it on Linux (Ubuntu 14.04, gcc 4.8) using -m32 switch, the size is 16 bytes (as expected).

When I compile the same code on OS X (10.9.5) using either gcc (4.8) or clang (Apple 6.0) with -m32, the size is 20 bytes. I tried to debug the program and to see if there are any union cases that might use the fifth word, haven't found any. It does not depend on levels of optimization and debug information presence.

Any ideas why sizeof(cell_t) is 20 bytes?

@DavidHeffernan : it uses the first byte for `type`, three bytes for `nref`, union uses 12 bytes as expected. The last 4 bytes are zeros (from what I saw, if I haven't missed anything). — Dmytro Sirenko, Sep 26 '14 at 21:37

score 2 · Accepted Answer · answered Sep 26 '14 at 22:06

2

On OS X:

sizeof(off_t) == 8

On Linux:

sizeof(off_t) == 4

You use that type in string_t. There may be other occasions of this, but that was the first that I came across.

answered Sep 26 '14 at 22:06

Bill Lynch

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Thank you. Changed it to `ptrdiff_t`, `off_t` is actually for files. – Dmytro Sirenko Sep 26 '14 at 22:09

Redundant size of union compiling with -m32 on OS X

1 Answers1