Understanding the 7-zip plugin system ​

Before we kick off ​

In this documentation we will examine the 23.01 version of the 7-Zip plugin system used in File Manager. It may not work with the older versions. So if you would like develop the plugins for the older versions you will not find the solution in this repository.

Once we have all of the declarations in place we can create a mock definitions for the required API. In scope of my research I used the CMake install command to install the plugin .dll to the Formats directory on the 7-zip root. At the same time I used VSCode launch configuration to launch the the 7-Zip File Manager to debug and test the sample plugins:

{
    "name": "Debug 7zFM",
    "type": "cppvsdbg",
    "request": "launch",
    "program": "C:\\Program Files\\7-Zip\\7zFM.exe",
    "cwd": "${fileDirname}",
    "console": "internalConsole"
}

{
    "name": "Debug 7zFM",
    "type": "cppvsdbg",
    "request": "launch",
    "program": "C:\\Program Files\\7-Zip\\7zFM.exe",
    "cwd": "${fileDirname}",
    "console": "internalConsole"
}

This way you can debug the plugin system of the 7-Zip and also test the sample plugins developed in this repository.

In-place Plugins ​

The plugins infrastructure plays important role in 7-Zip. Looking through the source code we can see that 7z.dll by itself is a plugin that is loaded by the File Manager or command line interface to provide implementation for supported archive formats. The formats entry points are called Handlers and are located under the CPP/7zip/Archive folder. For example, this peace of code is from end of the GzHandler.cpp:

REGISTER_ARC_IO(
  "gzip", "gz gzip tgz tpz apk", "* * .tar .tar .tar", 0xEF,
  k_Signature, 0,
    NArcInfoFlags::kKeepName
  | NArcInfoFlags::kMTime
  | NArcInfoFlags::kMTime_Default
  , TIME_PREC_TO_ARC_FLAGS_MASK (NFileTimeType::kUnix)
  | TIME_PREC_TO_ARC_FLAGS_TIME_DEFAULT (NFileTimeType::kUnix)
  , IsArc_Gz)

REGISTER_ARC_IO(
  "gzip", "gz gzip tgz tpz apk", "* * .tar .tar .tar", 0xEF,
  k_Signature, 0,
    NArcInfoFlags::kKeepName
  | NArcInfoFlags::kMTime
  | NArcInfoFlags::kMTime_Default
  , TIME_PREC_TO_ARC_FLAGS_MASK (NFileTimeType::kUnix)
  | TIME_PREC_TO_ARC_FLAGS_TIME_DEFAULT (NFileTimeType::kUnix)
  , IsArc_Gz)

It is evident that macro REGISTER_ARC_IO is used to register the implementation that supports gzip formats like gz, gzip, tgz, tpz, and apk. It's worth mentioning that there are multiple REGISTER_ARC_* macros that are designed to tune how Handlers are registered. We won't go into the details of what all these parameters mean because we won't be creating a Handler and registering it inside the 7-Zip source code. Instead, we'll use a separate dynamic library to register and implement Handlers. It is also possible to add a handler into the 7-Zip source code, but that would require compiling 7-Zip binaries and shipping a custom version of 7-Zip, which is not acceptable.

In the end, the REGISTER_ARC_* macro expands to the definition of the struct and the static variables. These static variables are later aggregated into a collection by invoking the constructor of a static class during the initialization phase. When the 7-Zip File Manager tries to open a file, it searches for the matching handler for that file within the internal collection and uses it to open the archive. For all of this to work, the REGISTER_ARC_* macros must be defined in a unique namespace.

Dynamic Plugins ​

The dynamic plugins are DLLs that export functions defined in the Archive2.def file. It's worth noting that not all of the defined functions are required to be implemented. Usually, 7-Zip will fallback to default values for most of the callbacks.

• The Archive2.def function can be located in Initial Section.
• The function documentation can be found in Plugin Interface.