error using MacPorts zlib, not present with system zlib?
Hi
Not sure of the correct place to discuss this but in tracking down a
build issue with a port I've found an interesting issue with MacPorts
zlib.
This simple test code illustrates the problem:
[ram@mimir tmp]$ cat test.c
#include stdio.h
#include stdlib.h
#include zlib.h
int main(void)
{
void *file;
int c;
file = gzopen(test.dat.gz, r);
if (file == Z_NULL)
{
fprintf(stderr, unable to open file\n);
exit(1);
}
c = gzgetc(file);
fprintf(stdout, c = %c\n, c);
gzclose(file);
}
[ram@mimir tmp]$ zcat test.dat.gz
test
file
If I compile this against the system zlib then it works as expected:
[ram@mimir tmp]$ clang -o test test.c -lz
[ram@mimir tmp]$ ./test
c = t
But not against the MacPorts zlib:
[ram@mimir tmp]$ clang -o test test.c -I/opt/local/include -L/opt/local/lib -lz
test.c:17:7: error: member reference base type 'void' is not a structure or
union
c = gzgetc(file);
^~~~
/opt/local/include/zlib.h:1655:11: note: instantiated from:
((g)-have ? ((g)-have--, (g)-pos++, *((g)-next)++) : gzgetc_(g))
~~~ ^
test.c:17:7: error: member reference base type 'void' is not a structure or
union
c = gzgetc(file);
^~~~
/opt/local/include/zlib.h:1655:24: note: instantiated from:
((g)-have ? ((g)-have--, (g)-pos++, *((g)-next)++) : gzgetc_(g))
~~~ ^
test.c:17:7: error: member reference base type 'void' is not a structure or
union
c = gzgetc(file);
^~~~
/opt/local/include/zlib.h:1655:37: note: instantiated from:
((g)-have ? ((g)-have--, (g)-pos++, *((g)-next)++) : gzgetc_(g))
~~~ ^
test.c:17:7: error: member reference base type 'void' is not a structure or
union
c = gzgetc(file);
^~~~
/opt/local/include/zlib.h:1655:51: note: instantiated from:
((g)-have ? ((g)-have--, (g)-pos++, *((g)-next)++) : gzgetc_(g))
~~~ ^
4 errors generated.
[ram@mimir tmp]$
Does anyone know why MacPorts zlib treats this as an error but the
system version doesn't?
Cheers
Adam
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Re: error using MacPorts zlib, not present with system zlib?
On 22.02.2012 15:32, Adam Mercer wrote:
Hi
Not sure of the correct place to discuss this but in tracking down a
build issue with a port I've found an interesting issue with MacPorts
zlib.
This simple test code illustrates the problem:
[ram@mimir tmp]$ cat test.c
#include stdio.h
#include stdlib.h
#include zlib.h
int main(void)
{
void *file;
snip
c = gzgetc(file);
This code is wrong. The zlib documentation says gzgetc takes a gzFile*
not a void*.
Does anyone know why MacPorts zlib treats this as an error but the
system version doesn't?
I'm not near a Mac right now, but I believe the system version of zlib
is pretty ancient. In version 1.2.4 there was a Wholesale replacement
of gz* functions with faster versions according to zlib.net, one of
which I suspect was the replacement of the gzgetc() function with a
macro.
C treats void* very unsafely, silently converting it to and from other
pointer types whenever it knows how. So converting the gzFile* gzopen()
returned to the void* 'file' is legal. Then, if gzgetc() is an actual
function declared as 'char gzgetc(gzFile*)' the compiler will allow the
void* be passed to the function, and within the function it is treated
as a gzFile*. But if gzgetc is just a macro the compiler doesn't know
to convert 'file' to a gzFile*, so you get errors about trying to treat
void as struct/union. If the API said gzgetc took a void* they would
fix this by casting g to a gzFile* inside the macro that replaced the
function, but the API doesn't say this, and so the zlib authors don't...
The current version is actually both faster and safer since it catches
this type of void* abuse.
So tldr answer: patch the port using libz to use a gzFile* or to cast
the void* gack to a gzFile* before calling functions expecting a
gzFile*.
Thanks,
Eric
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Re: error using MacPorts zlib, not present with system zlib?
On Wed, Feb 22, 2012 at 15:50, Eric Cronin ecro...@macports.org wrote: I'm not near a Mac right now, but I believe the system version of zlib is pretty ancient. In version 1.2.4 there was a Wholesale replacement of gz* functions with faster versions according to zlib.net, one of which I suspect was the replacement of the gzgetc() function with a macro. Looks like it, Lion seems to have zlib 1.2.5 which has a standard fucntion prototype for the gzgetc() function whereas 1.2.6 in MacPorts gzgetc() is indeed a macro. C treats void* very unsafely, silently converting it to and from other pointer types whenever it knows how. So converting the gzFile* gzopen() returned to the void* 'file' is legal. Then, if gzgetc() is an actual function declared as 'char gzgetc(gzFile*)' the compiler will allow the void* be passed to the function, and within the function it is treated as a gzFile*. But if gzgetc is just a macro the compiler doesn't know to convert 'file' to a gzFile*, so you get errors about trying to treat void as struct/union. If the API said gzgetc took a void* they would fix this by casting g to a gzFile* inside the macro that replaced the function, but the API doesn't say this, and so the zlib authors don't... The current version is actually both faster and safer since it catches this type of void* abuse. So tldr answer: patch the port using libz to use a gzFile* or to cast the void* gack to a gzFile* before calling functions expecting a gzFile*. I ended up applying a patch to the port that casts the void* pointer to gzFile and that allows it to build, but the upstream developer didn't like that approach. But this analysis is interesting and imagine this build error will be seen on other platforms that have zlib-1.2.6. Thanks for the help. Cheers Adam ___ macports-users mailing list macports-users@lists.macosforge.org http://lists.macosforge.org/mailman/listinfo.cgi/macports-users
Re: error using MacPorts zlib, not present with system zlib?
--On February 22, 2012 4:50:52 PM -0500 Eric Cronin ecro...@macports.org wrote: This code is wrong. The zlib documentation says gzgetc takes a gzFile* not a void*. Your analysis of this is correct except for one minor point. gzopen returns and gzgetc (and others) take a gzFile not a gzFile*. The definition of gzFile is typedef struct gzFile_s *gzFile; so it's defined as a pointer. Some compilers at some warning levels get annoyed if you use a gzFile* instead of gzFile for these parameters. Mike ___ macports-users mailing list macports-users@lists.macosforge.org http://lists.macosforge.org/mailman/listinfo.cgi/macports-users
Re: error using MacPorts zlib, not present with system zlib?
On Wed, Feb 22, 2012 at 21:36, Mike Alexander m...@umich.edu wrote: --On February 22, 2012 4:50:52 PM -0500 Eric Cronin ecro...@macports.org wrote: This code is wrong. The zlib documentation says gzgetc takes a gzFile* not a void*. typedef struct gzFile_s *gzFile; so it's defined as a pointer. Some compilers at some warning levels get annoyed if you use a gzFile* instead of gzFile for these parameters. As they well should. -- brandon s allbery allber...@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms ___ macports-users mailing list macports-users@lists.macosforge.org http://lists.macosforge.org/mailman/listinfo.cgi/macports-users
