Different size of long with C and C++ on x86

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saprykin
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Different size of long with C and C++ on x86

Post by saprykin » Tue May 14, 2024 5:23 pm

Hello,

Thanks for providing community images of OpenVMS for x86! I'm trying to port a library which works on OpenVMS 8.4 (Alpha, IA64) to OpenVMS 9.2 (x86_64) using the latest community VMDK image. Right now I have a strange problem: when I compile a simple program with C compiler (basically, printing the size of long type), it shows that the size of long is 4 bytes, as on Alpha/IA64. However, the same program compiled with C++ compiler, outputs 8 bytes. How is that possible? Are there special compile options required to make C++ programs compatible with C?

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arne_v
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Re: Different size of long with C and C++ on x86

Post by arne_v » Sat May 18, 2024 7:15 pm

Indeed. Very clever.
$ type cwhat.cpp
#include <stdio.h>

int main()
{
if(sizeof(' ') == sizeof(int))
{
puts("C compiler");
}
else if(sizeof(' ') == sizeof(char))
{
puts("C++ compiler");
}
else
{
puts("Houston we have a problem");
}
return 0;
}
$ clang cwhat.cpp
$ link cwhat
$ run cwhat
C++ compiler
$ copy cwhat.cpp *.c
$ clang cwhat.c
$ link cwhat
$ run cwhat
C compiler
Arne
arne@vajhoej.dk
VMS user since 1986


jreagan
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Re: Different size of long with C and C++ on x86

Post by jreagan » Mon May 20, 2024 4:17 pm

mberryman wrote:
Fri May 17, 2024 5:46 pm
I have worked on a few packages that mix C and C++ code and which also share structs that include elements defined using long, size_t, etc. Compiling with the C and CXX compilers produced the obvious size mismatches. I solved this problem by simply compiling everything with clang. Since clang is both a C and C++ compiler, this also eliminates the need to make one's C code C++ clean.
sure, until you have to call a system/RTL routine that was compiled with our C compiler. Do a "scanf" into a C++ "long" and you'll only get 1/2 the value. And if you printf a "long" with "%ld", you'll only print 1/2 the value. The CRTL believes that "long" is 32-bits even when called from clang. We'll fix that.

Code: Select all

$ type scanf.cxx
#include <stdio.h>
#include <string>
int main () {
std::string inputstring = "1024 1024";
long l1;
long l2;

l1 = -1;
l2 = -1;
sscanf(inputstring.c_str(),"%ld %lld",&l1,&l2);
printf("l1 = %16llx, l2 = %16llx\n",l1,l2);
}
$ cxx scanf
DKA300:[jreagan]SCANF.CXX;7:10:43: warning: format specifies type 'long long *' but the argument has type 'long *' [-Wformat]
sscanf(inputstring.c_str(),"%ld %lld",&l1,&l2);
                                ~~~~      ^~~
                                %ld
DKA300:[jreagan]SCANF.CXX;7:11:37: warning: format specifies type 'unsigned long long' but the argument has type 'long' [-Wformat]
printf("l1 = %16llx, l2 = %16llx\n",l1,l2);
             ~~~~~~                 ^~
             %16lx
DKA300:[jreagan]SCANF.CXX;7:11:40: warning: format specifies type 'unsigned long long' but the argument has type 'long' [-Wformat]
printf("l1 = %16llx, l2 = %16llx\n",l1,l2);
                          ~~~~~~       ^~
                          %16lx
3 warnings generated.
$ link scanf
$ run scanf
l1 = ffffffff00000400, l2 =              400


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arne_v
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Re: Different size of long with C and C++ on x86

Post by arne_v » Mon May 20, 2024 5:27 pm

Little gotcha.

But that is also a C++ problem not just a "clang for C" problem.

I believe the C++ standard requires stdio.

Even though I believe the correct syntax nowadays are:

#include <cstdio>

instead of:

#include <stdio.h>
Arne
arne@vajhoej.dk
VMS user since 1986

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