Date: Fri, 24 Feb 2023 09:07:41 -0500 From: Rich Felker <dalias@...c.org> To: NRK <nrk@...root.org> Cc: musl@...ts.openwall.com, Tamir Duberstein <tamird@...gle.com> Subject: Re: undefined behavior in fread.c On Fri, Feb 24, 2023 at 07:55:11PM +0600, NRK wrote: > On Fri, Feb 24, 2023 at 08:34:14AM -0500, Rich Felker wrote: > > Is there any indication that passing NULL as the first argument to > > fread is not itself undefined? > > C99 says the following: > > | If size or nmemb is zero, fread returns zero and the contents of the > | array and the state of the stream remain unchanged. > > It doesn't explicitly mention weather stream can be NULL or not in case > of 0 size/nmemb - but regardless, is there any actual reason for not > returning early? > > The following should be OK as far as I see: > > - if (!size) nmemb = 0; > + if (!size || !nmemb) return 0; > > or `if (!len) return 0;` could also work if multiplication overflow is > not a concern. stdio functions are required (by POSIX) to behave as if they take a mutex on the FILE. If fread with a length of zero makes forward progress when another thread holds the lock, this is non-conforming. Rich
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