using setvbuf to make memory buffer act like FILE*

Question

I need a cross platform way of treating memory buffer as FILE* . I have seen other questions which point out that there is no portable way to do this (fmemopen in linux is what I need but it fails on Windows platform).

I have tried using the setvbuf and it seems to work. Can anyone please point out the exact problem of using setvbuf function?

Also , I have seen the C standard draft WG14/N1256 and 7.19.5.6 says:

the contents of array at any time are indeterminate.

I don't understand if I use my own buffer how can its contents be indeterminate?

EDIT : Thanks for all the answers. Not using this method anymore.

Answer 1

No really, there's no portable way to do this.

Using setvbuf may appear to work but you're really invoking undefined behavior, and it will fail in unexpected ways at unexpected times. The GNU C library does have fmemopen(3) as an extension, as you mentioned, but it's not portable to non-GNU systems.

If you're using some library that requires a FILE* pointer and you only have the required data in memory, you'll just have to write it out to a temporary file and pass in a handle to that file. Ideally, your library should provide an alternative function that takes a memory pointer instead of a file pointer, but if not, you're out of luck (and you should complain to the library writer about that deficiency).

Answer 2

Function setvbuf() is used to tell the FILE the memory to be used as buffer, but it does not specify how this memory will be used: that's up to the implementation.

Thus, the contents of the buffer are indeterminate at any time, and if it happens to work for you, it is just by chance.

Answer 3

It depends on what you want to do with the buffer/FILE*. You can certainly perform simple operations and get away with them, but you cannot guarantee that all of the FILE* operations will perform as expected on your memory buffer.

Sorry, there is simply no cross-platform one-liner to get full FILE* characteristics, I've tried myself many times haha

what you can try:

• #define-wrapped OS-specific logic
• Look further into the interface you are trying to interact with. At some point it just plays with a buffer anyway. Then splice in your buffer. This is what I did.
• Your technique + faith.