Hi,

On Thu, 13 Mar 2008, Junio C Hamano wrote:

quoted text
> Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
> 
> > diff --git a/builtin-merge-file.c b/builtin-merge-file.c
> > index adce6d4..cde4b2c 100644
> > --- a/builtin-merge-file.c
> > +++ b/builtin-merge-file.c
> > @@ -57,7 +57,8 @@ int cmd_merge_file(int argc, const char **argv, const char *prefix)
> >  
> >  		if (!f)
> >  			ret = error("Could not open %s for writing", filename);
> > -		else if (fwrite(result.ptr, result.size, 1, f) != 1)
> > +		else if (result.size &&
> > +				fwrite(result.ptr, result.size, 1, f) != 1)
> >  			ret = error("Could not write to %s", filename);
> >  		else if (fclose(f))
> >  			ret = error("Could not close %s", filename);
> 
> Lol.  We are dealing with N-byte quantity so we send one record of length
> N and make sure we processed one record, and it does not work when N is
> zero.
> 
> We could instead send N records of size 1 and make sure we processed N
> records to lose the conditional instead, but the patch avoids unnecessary
> call to fread/fwrite so that is good.  Thanks.
> 
> It felt funny because my current bedtime reading happens to be "Zero: The
> Biography of a Dangerous Idea (ISBN 0140296476)".

Heh.

quoted text
> > diff --git a/xdiff-interface.c b/xdiff-interface.c
> > index bba2364..61dc5c5 100644
> > --- a/xdiff-interface.c
> > +++ b/xdiff-interface.c
> > @@ -152,8 +152,8 @@ int read_mmfile(mmfile_t *ptr, const char *filename)
> >  	if ((f = fopen(filename, "rb")) == NULL)
> >  		return error("Could not open %s", filename);
> >  	sz = xsize_t(st.st_size);
> > -	ptr->ptr = xmalloc(sz);
> > -	if (fread(ptr->ptr, sz, 1, f) != 1)
> > +	ptr->ptr = xmalloc(sz ? sz : 1);
> > +	if (sz && fread(ptr->ptr, sz, 1, f) != 1)
> >  		return error("Could not read %s", filename);
> >  	fclose(f);
> >  	ptr->size = sz;
> 
> Do you need to actually allocate ptr->ptr when sz is zero, instead of
> setting it to NULL, like:
> 
> 	sz = xsize_t(st.st_size);
> 	ptr->size = sz;
>         if (!sz)
>         	ptr->ptr = NULL;
> 	else {
>         	ptr->ptr = xmalloc(sz);
> 		if (fread(ptr->ptr, 1, sz, f) != sz)
> 			return error("Could not read %s", filename);
> 	}
> 	fclose(f);

I was going for the safe option.  In theory, you are right, but I cannot 
really be sure that e.g. a memcpy() with size 0 will not access the source 
pointer at all.

Ciao,
Dscho

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