NSIS's built-in string functions are limited to NSIS_MAX_STRLEN (typically 1024 bytes). When reading configuration files, JSON, or INI files larger than this limit, the string is silently truncated.

nsParser solves this by reading only the specific value requested, without loading the entire file into a string.

Build

Prerequisites

Visual Studio 2022 (with "Desktop development with C++" workload)
Python 3.x

Build script

python build_plugin.py

Build options

python build_plugin.py --configs x86-unicode      # Single architecture (x86-ansi|x86-unicode|x64-unicode|all)
python build_plugin.py --vs-version 2026          # Specific toolset (auto|2022|2026)
python build_plugin.py --clean                    # Clean dist/ before build
python build_plugin.py --rebuild                  # Force full rebuild
python build_plugin.py --verbosity detailed       # Extended MSBuild output
python build_plugin.py --install-dir "C:\NSIS\Plugins"  # Copy to additional NSIS directory
python build_plugin.py --list                     # List available configurations

Usage in NSIS

!addplugindir "plugins\x86-unicode"

Section
  nsParser::Extract "C:\config.ini" "key" $0
  DetailPrint "Value: $0"
SectionEnd

API

`nsParser::Extract`

nsParser::Extract "FilePath" "Key" $OutputVar

Parameters:

Parameter	Description
`FilePath`	Full path to the file to parse
`Key`	Key to search for (e.g. `version`, `path`)
`$OutputVar`	Variable to receive the extracted value

Search logic:

The plugin searches for the pattern Key=Value or Key = Value in the file. If found, returns the value trimmed of leading/trailing whitespace. If not found, returns an empty string.

Buffer:

The value is extracted with a buffer of 32768 bytes, well beyond the NSIS_MAX_STRLEN limit. This allows reading long paths and embedded JSON strings.

Advantages

No dependency on the NSIS string size limit
Reads only the requested value (efficient on large files)
Handles key=value and key = value formats
Thread-safe (no global state)
Small binary size

Technical Implementation

The plugin is written in C and uses a direct file reader via ReadFile (Win32). The algorithm:

Opens the file with CreateFile (read-only, shared)
Reads the file in 8192-byte blocks
Scans each block for the pattern Key= or Key =
Extracts the value up to the first newline (\n or \r\n)
Trims leading/trailing whitespace
Returns the result via the NSIS stack

Visual Studio configurations

Configuration	Architecture	Output
Release-x86-ansi	x86 ANSI	`plugins/x86-ansi/nsParser.dll`
Release-x86-unicode	x86 Unicode	`plugins/x86-unicode/nsParser.dll`
Release-amd64-unicode	x64 Unicode	`plugins/amd64-unicode/nsParser.dll`

File Structure

nsParser/
 build_plugin.py          # Unified build script
 src/
    nsParser.c           # Plugin source code
    nsParser.vcxproj     # VS2022 project (x86-ansi, x86-unicode, amd64-unicode)
    nsParser.sln         # VS2022 solution
 dist/
     x86-ansi/
     x86-unicode/
     amd64-unicode/

See README_IT.md for the Italian version.