# Proof: If d|a and d|b, then d|(a-b)

If you’re cheating on your homework, and found this through a websearch, then know your teacher can also find this. Do your own proof.

This is exercise 3.2 of Elementary Number Theory (pdf) by W. Edwin Clark.

We know that $d|a$ and $d|b$ so we write $a = kd$ and $b = ld$.

Now $a-b = kd - ld$ which we can also write as $d(k - l)$. It is now obvious that $a-b = d(k - l)$, q.e.d.

# PostgreSQL: Retroactively Adding Binary I/O to a Type

Here we go through the motions af adding binary input/output functions to an already existing type. We start by introducing a program that tests binary I/O for the given types, then show how to implement the binary I/O in C and finally how to fix the PostgreSQL catalog tables for the new functionality. We also include the changes in the installation script for normal installation of the new version.

# Building and Deploying 64bit FastCGI Applications in C on Windows 10

Here we go through the steps needed to build and run 64bit FastCGI applications with Visual Studio 2019.

## Building the FastCGI SDK

First, get the FastCGI SDK from the Github archive. Second, open an x64 Native Tools Command Prompt for VS 2019.

Then extract the archive. 7z can do this in two steps. Here we build in `C:\build` but any convenient place works.

```cd c:\build
7z x fcgi-2.4.1-SNAP-0910052249.tar
```

We can delete the temporay .tar file now.

```del fcgi-2.4.1-SNAP-0910052249.tar
```

Now we can try to build with the provided NT Makefile.

```cd fcgi-2.4.1-SNAP-0910052249
nmake -f Makefile.nt
```

And we get a fatal error.

```fatal error LNK1112: module machine type 'x64' conflicts with target machine type 'x86'
```

To figure out what’s wrong, we first take a look at the Makefile.

```all: "include\fcgi_config.h"
cd ".\libfcgi"
\$(MAKE) \$(MAKEFLAGS) /NOLOGO /F libfcgi.mak     CFG=\$(CFG) \$@

cd ".\..\cgi-fcgi"
\$(MAKE) \$(MAKEFLAGS) /NOLOGO /F cgi-fcgi.mak    CFG=\$(CFG) \$@

cd ".\..\examples"
\$(MAKE) \$(MAKEFLAGS) /NOLOGO /F authorizer.mak  CFG=\$(CFG) \$@
\$(MAKE) \$(MAKEFLAGS) /NOLOGO /F echo.mak        CFG=\$(CFG) \$@
\$(MAKE) \$(MAKEFLAGS) /NOLOGO /F echox.mak       CFG=\$(CFG) \$@
\$(MAKE) \$(MAKEFLAGS) /NOLOGO /F size.mak        CFG=\$(CFG) \$@
\$(MAKE) \$(MAKEFLAGS) /NOLOGO /F echo-cpp.mak    CFG=\$(CFG) \$@
```

We see that the Makefile is calling other Makefiles, so we take a look at the first one, `libfcgi.mak`, and there we find this line:

```LINK32_FLAGS=Ws2_32.lib /nologo /dll /pdb:none /machine:I386 /out:"\$(OUTDIR)\libfcgi.dll" /implib:"\$(OUTDIR)\libfcgi.lib"
```

where we try to remove the `/machine:I386` and rebuild.

```nmake -f Makefile.nt
```

We get a little further this time, and fail again when building `cgi-fcgi.mak`. In that file we have

```LINK32_FLAGS=libfcgi.lib /nologo /pdb:none /machine:IX86 /out:"\$(OUTDIR)\cgi-fcgi.exe" /libpath:"..\libfcgi\Release"
```

where we again remove the `/machine:IX86` and try again.

We repeat this process for `authorizer.mak`, `echo.mak`, `echox.mak`, `size.mak`, and `echo-cpp.mak`.

Now we have a successfully built FastCGI SDK, and can try to deploy the examples.

## Deploying the FastGCI Examples on Apache 2.4

### Installing the FastCGI Module

Here we assume that the Apache binaries have been downloaded from the Apache Lounge. It is outside the scope of this tutorial to include how to configure Apache to start and stop with Windows. We’ll only mention that the server can be run in a console with `httpd` in the `bin` subdirectory.

For our purposes, we’ll assume Apache has been installed in `C:\opt`

First, extract the module in a convenient location,

```7z x %homepath%\downloads\mod_fcgid-2.3.10-win64-vs16.zip
```

and copy the module into Apache’s root,

```copy mod_fcgid-2.3.10\mod_fcgid.so C:\opt\Apache24\modules\
```

and then edit `C:\opt\Apache24\conf\httpd.conf` and add

```LoadModule fcgid_module modules/mod_fcgid.so
```

and finally restart Apache. If you’re running it in a console, you can ctrl+c, wait for a few seconds while it shuts down, and then restart with `httpd`

### Deploying the Examples

First, we prepare the document root for FastCGI applications. On Windows, it’s customary to put DLL files directly where the executable is, or in the system’s `PATH`. For our purposes, putting the `libfcgi.dll` file in our path seems like an overkill, so we will put it in our document root along with the executables, but we don’t want people to be able to load the DLL files. Therefore we change the document root default access to a denial.

Given

```DocumentRoot "\${SRVROOT}/htdocs"
<Directory "\${SRVROOT}/htdocs">
AllowOverride None
Require all granted
</Directory>
```

we change the `Require` line to read

```    Require all denied
```

```    <Files "*.exe">
Options +ExecCGI
SetHandler fcgid-script
Require all granted
</Files>
```

So our complete document root directory config is now

```DocumentRoot "\${SRVROOT}/htdocs"
<Directory "\${SRVROOT}/htdocs">
AllowOverride None
Require all denied

<Files "*.exe">
Options +ExecCGI
SetHandler fcgid-script
Require all granted
</Files>
</Directory>
```

At this time we are ready to copy the executables we built earlier into our document root. In the x64 native tools command prompt we used earlier to build the SDK, copy the files into the document root we just prepared.

```copy examples\echo\Release\echo.exe c:\opt\Apache24\htdocs
copy examples\echo-x\Release\echo-x.exe c:\opt\Apache24\htdocs
copy examples\echo-cpp\Release\echo-cpp.exe c:\opt\Apache24\htdocs
copy examples\size\Release\size.exe c:\opt\Apache24\htdocs
copy libfcgi\Release\libfcgi.dll c:\opt\Apache24\htdocs
```

And now we can try the various samples in our browser with echo.exe, echo-x.exe, echo-cpp.exe, size.exe, and finally, we make sure that downloading the DLL doesn’t work with libfcgi.dll.

If there are any failures, check the Apache error log.

## Building Our Own FastCGI Application in C

We can now demonstrate how to build and deploy our very own FastCGI application, written in C.

The code is simple.

```#include <fcgiapp.h>

int main( int argc, char *argv[] ) {
FCGX_Stream *in, *out, *err;
FCGX_ParamArray envp;

while ( FCGX_Accept( &in, &out, &err, &envp ) >= 0 ) {
FCGX_FPrintF( out,
"Content-type: text/plain\r\n"
"\r\n"
"Hello, world.\r\n" );
}
}
```

This hello world example loops continuously and accepts FastCGI connections, and then just displays a plan text `Hello, world` message. It is just a bare bones example.

And the Makefile is also very simple. You will have to adjust the compiler path to your own installation, and the `-I` and `-L` flags. The `-m64` means we’ll build a 64bit executable, the `-g` means we’ll include debug symbols, and the `-Wall -Wextra` flags tells the compiler to issue warnings when we do something dangerous or unexpected. The link flag `-llibfcgi` tells us to link against the DLL we just built and installed.

```CC="C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\Llvm\8.0.0\bin\clang.exe"

CFLAGS=-m64 -g -Wall -Wextra -IC:\build\fcgi-2.4.1-SNAP-0910052249\include
LDFLAGS=-LC:\build\fcgi-2.4.1-SNAP-0910052249\libfcgi\Release -llibfcgi

hello.exe: hello.c
\$(CC) \$(CFLAGS) hello.c -ohello.exe \$(LDFLAGS)

install: hello.exe
copy hello.exe C:\opt\Apache24\htdocs
```

Here, the installation target doesn’t copy the DLL file, we assume that’s been taken care of beforehand.

Happy coding FastCGI applications.

## Contact

The author can be reached at johann@myrkraverk.com.

On Windows and as of 11.5, PostgreSQL has a bug when trying to `copy` from a huge file. According to stack overflow, this does not manifest on earlier releases like PostgreSQL 10.

For example, given a table

```create table pwned( passwd sha1 not null, count int4 not null );
```

and when trying to copy from the pwned password list which is about 24GB uncompressed,

```copy pwned( passwd, count ) from 'c:\src\postgres\pwned\pwned-passwords-sha1-ordered-by-hash-v5.txt' with ( format text, delimiter ':' );
```

fails and the message follows.

```ERROR:  could not stat file "c:\src\postgres\pwned\pwned-passwords-sha1-ordered-by-hash-v5.txt": Unknown error
```

To work around this, we can pipe the file through a command;

```copy pwned( passwd, count ) from program 'cmd /c "type c:\src\postgres\pwned\pwned-passwords-sha1-ordered-by-hash-v5.txt"' with ( format text, delimiter ':' );
```

which results in

```COPY 555278657
```

and we can happily create an index and query the table.

The trick here is that we run `cmd` in sigle command mode with the `/c` flag, and tell it to `type` out the file we want to `copy from`.

## Bug Report

Update. There is now a bug report.

## Contact

The author can be reached at johann@myrkraverk.com.

# Porting a PostgreSQL Extension from Unix to Windows 10

2ndQuadrant has written before about how to build your own extension for Windows. This time we take a look at what it takes to port one written by someone else. I am building this for PostgreSQL 11 on Windows using Visual Studio 2019 and clang.

Here we are working with the hashtypes extension. Unfortunately, at the time of writing, version 0.1.5 has not been released with the support for PostgreSQL 11. So we build one from git.

```git clone https://github.com/adjust/hashtypes.git
```

The first thing we do is to take a look at the C source files. There are only four of them, `common.c`, `crc32.c`, `md5.c`, and `sha.c` so we can be reasonably sure the porting effort will be quick.
Continue reading Porting a PostgreSQL Extension from Unix to Windows 10

# Simple SHA256 Hashing with NSS in C

Here we present a program that calculates SHA256 of the string `"abc"` as a demonstration of how to use Mozilla’s Network Security Services library. We assume the reader has compiled a working library, or installed one through a package manager. The build instruction in this tutorial assume Windows and Visual Studio 2019 with `clang`.

My comments are overly verbose, and this should make it easier for beginners to follow along.
Continue reading Simple SHA256 Hashing with NSS in C

# Building Mozilla NSS on Windows 10

If you’re like me you may have searched the web on how to build the Mozilla Network Security Services on Windows 10. You may have found this obsolete project on Github. You may also have found the obsolete directions for Pidgin too.

On Windows the NSS build requriements are the same as for Firefox, so go look through the build instructions for Firefox on Windows. Install Visual Studio 2019 as directed, or add the packages you need with the Visual Studio Installer. And install the Mozilla Build package in the default directory.

Now that’s done, we have one more requirement left, and that’s Gyp. For Gyp, we first need Git. If you don’t have it already, install it now. At this point I’m not certain Git comes with Visual Studio 2019. It could be, but I have a separate install anyway.

All the instructions now assume you have opened an x64 Native Tools Command Prompt for VS 2019, so open one if you don’t have already.

I prefer to keep my software I build in `C:\build`, so that’s what the directions will indicate.

```cd c:\build
```

Gyp is a Python program, so if you don’t have it already, add Python to your path. You’ll find it in `C:\mozilla-build\python` and also add gyp to your path, like so

```set path=%path%;C:\mozilla-build\python;C:\build\gyp
```

I use 7z for my command line examples.

```cd c:\build
```

That only creates the tar file, so we need to run 7z again.

```7z x nss-3.45-with-nspr-4.21.tar
```

You may need to add `vswhere` to your path, and you can do that now with

```set path=%path%;%ProgramFiles(x86)%\Microsoft Visual Studio\Installer
```

At this point we’ll need an msys shell. So still in your x64 Native Tools Command Prompt for VS 2019,

```cd C:\mozilla-build\msys
msys
```

In msys, cd to the place where you extracted NSS.

```cd /c/build/nss-3.45/nss
```

And build with

```./build.sh
```

And you should get a nicely compiled debug build in `C:\build\nss-3.45\dist`. Happy hacking with NSS.

## Contact

The author can be reached at johann@myrkraverk.com.

# The sizeof Operator in C and C++

Textbooks rarely make good use of the `sizeof` operator in C (and C++). The syntax is

```sizeof ( type )
```

and

```sizeof variable
```

That is, in order to get the size of a variable, including an array, the parenthesis are not necessary. I personally find them a visual distraction, unless of course the operand is a type, like `sizeof ( int )`.

Let’s look at a concrete example of using `sizeof` without parenthesis. In this example, we’re preparing an error message for display on screen.

```char buffer[ 1024 ]; /* Arbitrary size of the buffer. */
snprintf( buffer, sizeof buffer, "Unable to initialize SDL: %s", SDL_GetError() );
```

Because we’re using `sizeof buffer` in the `snprintf()` call we don’t have to worry about mistakes, or out of sync constants, and we don’t have to `#define BUFFER_SIZE 1024` to use the same size in both definition of the `buffer` and call to `snprintf()`.

Note to Windows programmers. The `snprintf()` function isn’t documented to always terminate the string with zero until Visual Studio 2015 and Windows 10. Programmers on the Windows platform might want to add an explicit zero termination to account for older compilers and systems. That can be done with `buffer[ sizeof buffer - 1 ] = '\0'`.

The trick here is that we defined `buffer` as an array. If we had instead used `malloc()` to allocate the buffer, we would have to add the size of the buffer explicitly, like so,

```char *buffer = malloc( 1024 );
if ( !buffer ) exit( 1 );
snprintf( buffer, 1024, "Unable to initialize SDL: %s", SDL_GetError() );
```

and we would have to explicitly check the return value from `malloc()` as is written about at length in why it is important to check what the malloc function returned. If we had instead used `sizeof buffer` here, we’d have gotten 4 on a 32bit system, and 8 on a 64bit system — which is totally not the value we need.

The `snprintf()` function returns how many characters have been or would be printed. In our case we don’t care if the message the user receives gets truncated, so we don’t check its return value.

It is worth noting that when a character literal is used, it matters whether the literal is declared as a pointer or array. That is, given

```  char *foo   = "Error string.",
bar[] = "Another error string.";
```

then `sizeof foo` will give us 4 on 32bit system, 8 on a 64bit system, while `sizeof bar` is 22 or the length of the string including the terminating zero byte.

## Literals

The `sizeof` operator applies to literals also without parenthesis, and it’s instructive to test it on some literal combinations on a given system. For example, this program

```#include <stdio.h>

int main( int argc, char *argv[] )
{
printf( "sizeof 0 = %zu\n",       sizeof 0 );     // int
printf( "sizeof 0l = %zu\n",      sizeof 0l );    // long
printf( "sizeof 0ll = %zu\n",     sizeof 0ll );   // long long
printf( "sizeof NULL = %zu\n",    sizeof NULL );  // pointer
printf( "sizeof 0.0 = %zu\n",     sizeof 0.0 );   // double
printf( "sizeof 0.0f = %zu\n",    sizeof 0.0f );  // float
printf( "sizeof \"foo\" = %zu\n", sizeof "foo" ); // string size including zero terminator
return 0;
}
```

run an a 64bit Linux and compiled with `clang`, gives

```sizeof 0 = 4
sizeof 0l = 8
sizeof 0ll = 8
sizeof NULL = 8
sizeof 0.0 = 8
sizeof 0.0f = 4
sizeof "foo" = 4
```

and none of that should be surprising.

## Contact

The author can be reached at johann@myrkraverk.com.