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# codecgen tool
Generate is given a list of *.go files to parse, and an output file (fout),
codecgen will create an output file __file.go__ which
contains `codec.Selfer` implementations for the named types found
in the files parsed.
Using codecgen is very straightforward.
**Download and install the tool**
`go get -u github.com/ugorji/go/codec/codecgen`
**Run the tool on your files**
The command line format is:
`codecgen [options] (-o outfile) (infile ...)`
```sh
% codecgen -?
Usage of codecgen:
-c="github.com/ugorji/go/codec": codec path
-o="": out file
-d="": random identifier for use in generated code (help reduce changes when files are regenerated)
-nx=false: do not support extensions (use when you know no extensions are used)
-r=".*": regex for type name to match
-nr="": regex for type name to exclude
-rt="": tags for go run
-st="codec,json": struct tag keys to introspect
-t="": build tag to put in file
-u=false: Use unsafe, e.g. to avoid unnecessary allocation on []byte->string
-x=false: keep temp file
% codecgen -o values_codecgen.go values.go values2.go moretypedefs.go
```
**Limitations**
codecgen caveats:
- Canonical option.
If Canonical=true, codecgen'ed code will delegate encoding maps to reflection-based code.
This is due to the runtime work needed to marshal a map in canonical mode.
- CheckCircularRef option.
When encoding a struct, a circular reference can lead to a stack overflow.
If CheckCircularRef=true, codecgen'ed code will delegate encoding structs to reflection-based code.
- MissingFielder implementation.
If a type implements MissingFielder, a Selfer is not generated (with a warning message).
Statically reproducing the runtime work needed to extract the missing fields and marshal them along with the struct fields,
while handling the Canonical=true special case, was onerous to implement.
**More Information**
Please see the [blog article](http://ugorji.net/blog/go-codecgen)
for more information on how to use the tool.
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