What does fmemopen() do in file format readers?

[szhorvat]

If HAVE_FMEMOPEN is defined, the file format readers make use of fmemopen(). Example:

rigraph/src/rinterface_extra.c

Lines 6423 to 6427 in 323f1b4

	#if HAVE_FMEMOPEN == 1
	file=fmemopen(RAW(pvfile), Rf_xlength(pvfile), "r");
	#else
	file=fopen(CHAR(STRING_ELT(pvfile, 0)), "r");
	#endif

• Is this code correct at all? It seems to me that depending on this macro, pvfile is interpreter either as the contents of a file, or a file name. These are not interchangeable.
• If this code is present, why are we not making use of HAVE_FMEMOPEN?

This needs to be reviewed by someone familiar with R internals and R's facilities to treat strings as files. Can we make use of this and gain better performance when parsing contents of strings?

[szhorvat]

Related: f936f3c This references a "bug 604907" which I cannot find (@ntamas is this an old Launchpad bug?) The claim that fmemopen is too slow is very surprising ...

[szhorvat]

It was probably this bug: igraph/igraph#197

fmemopen is available on macOS now, and it is definitely much faster than creating a file and reading from it if the data is not too large. We do use fmemopen to improve fuzzing performance.

[szhorvat]

Supporting fast reading from strings would be a new feature that takes time to implement. It takes a lot more than just defining HAVE_FMEMOPEN.

For now, I would recommend removing all mentions of HAVE_FMEMOPEN, as this is broken and dead code that's just confusing, and blocks auto-generation of these functions.