On 6/10/19 5:54 AM, Andrew Haley wrote: > On 6/10/19 8:24 AM, Martin Liška wrote: > >> I've just noticed that we have couple of alloca (0) in libiberty: >> >> #ifndef REGEX_MALLOC >> # ifdef C_ALLOCA >> alloca (0); >> # endif >> #endif >> >> If I'm correct the value 0 has a special meaning that tells a C library >> to clean up all previous alloca allocations. >> >> man alloca does not document the behavior > > I'm sure it doesn't. This alloca(0) behaviour is a libiberty-internal > thing. It's not really libiberty-internal, the C-alloca implementation on top of malloc was commonly copied into various free software packages in the 90s.
In that implementation alloca(0) had special meaning -- it asked for a cleanup which was useful in certain circumstances when the C implementation couldn't detect you'd previously left a scope and thus wasn't aware it could clean up stuff. We used to have alloca(0) sprinkled all over the GCC sources to deal with these issues. > >> Question is how legacy is alloca call from a standard library? > > alloca(3) is implemented by GCC as a builtin. It's fast, simple, and > widely used. These days you might use variable-length arrays instead, > which are at least portable. > > Is there some problem that you want to solve? Certainly the GCC builtin is better as are VLAs compared to the old C implementation on top of malloc. jeff
