go-bindata

README

bindata

This tool converts any file into managable Go source code. Useful for embedding binary data into a go program. The file data is optionally gzip compressed before being converted to a raw byte slice.

Usage

The simplest invocation is to pass it only the input file name. The output file and code settings are inferred from this automatically.

$ go-bindata testdata/gophercolor.png
[w] No output file specified. Using 'testdata/gophercolor.png.go'.
[w] No package name specified. Using 'main'.
[w] No function name specified. Using 'testdata_gophercolor_png'.

This creates the testdata/gophercolor.png.go file which has a package declaration with name main and one function named testdata_gophercolor_png with the following signature:

func testdata_gophercolor_png() []byte

You can now simply include the new .go file in your program and call testdata_gophercolor_png() to get the (uncompressed) image data. The function panics if something went wrong during decompression. See the testdata directory for example input and output files for various modes.

Aternatively, you can pipe the input file data into stdin. go-bindata will then spit out the generated Go code to stdout. This does require explicitly naming the desired function name, as it can not be inferred from the input data. The package name will still default to 'main'.

 $ cat testdata/gophercolor.png | go-bindata -f gophercolor_png | gofmt

Invoke the program with the -h flag for more options.

In order to strip off a part of the generated function name, we can use the -prefix flag. In the above example, the input file testdata/gophercolor.png yields a function named testdata_gophercolor_png. If we want the testdata component to be left out, we invoke the program as follows:

$ go-bindata -prefix "testdata/" testdata/gophercolor.png

Lower memory footprint

Using the -nomemcopy flag, will alter the way the output file is generated. It will employ a hack that allows us to read the file data directly from the compiled program's .rodata section. This ensures that when we call call our generated function, we omit unnecessary memcopies.

The downside of this, is that it requires dependencies on the reflect and unsafe packages. These may be restricted on platforms like AppEngine and thus prevent you from using this mode.

Another disadvantage is that the byte slice we create, is strictly read-only. For most use-cases this is not a problem, but if you ever try to alter the returned byte slice, a runtime panic is thrown. Use this mode only on target platforms where memory constraints are an issue.

The default behaviour is to use the old code generation method. This prevents the two previously mentioned issues, but will employ at least one extra memcopy and thus increase memory requirements.

For instance, consider the following two examples:

This would be the default mode, using an extra memcopy but gives a safe implementation without dependencies on reflect and unsafe:

func myfile() []byte {
    return []byte{0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a}
}

Here is the same functionality, but uses the .rodata hack. The byte slice returned from this example can not be written to without generating a runtime error.

var _myfile = "\x89\x50\x4e\x47\x0d\x0a\x1a"

func myfile() []byte {
    var empty [0]byte
    sx := (*reflect.StringHeader)(unsafe.Pointer(&_myfile))
    b := empty[:]
    bx := (*reflect.SliceHeader)(unsafe.Pointer(&b))
    bx.Data = sx.Data
    bx.Len = len(_myfile)
    bx.Cap = bx.Len
    return b
}

Optional compression

When the -uncompressed flag is given, the supplied resource is not GZIP compressed before being turned into Go code. The data should still be accessed through a function call, so nothing changes in the usage of the generated file.

This feature is useful if you do not care for compression, or the supplied resource is already compressed. Doing it again would not add any value and may even increase the size of the data.

The default behaviour of the program is to use compression.

Table of Contents

With the -toc flag, we can have go-bindata create a table of contents for all the files which have been generated by the tool. It does this by first generating a new file named bindata-toc.go. This contains a global map of type map[string] func() []byte. It uses the input filename as the key and the data function as the value. We can use this to fetch all data for our files, matching a given pattern.

It then appands an init function to each generated file, which simply makes the data function append itself to the global bindata map.

Once you have compiled your program with all these new files and run it, the map will be populated by all generated data files.

Note: The bindata-toc.go file will not be created when we run in pipe mode. The reason being, that the tool does not write any files at all, as it has no idea where to save them. The data file is written to stdout instead after all.

Table of Contents keys

The keys used in the go_bindata map, are the same as the input file name passed to go-bindata. This includes the fully qualified (absolute) path. In most cases, this is not desireable, as it puts potentially sensitive information in your code base. For this purpose, the tool supplies another command line flag -prefix. This accepts a portion of a path name, which should be stripped off from the map keys and function names.

For example, running without the -prefix flag, we get:

$ go-bindata /path/to/templates/foo.html

go_bindata["/path/to/templates/foo.html"] = path_to_templates_foo_html

Running with the -prefix flag, we get:

$ go-bindata -prefix "/path/to/" /path/to/templates/foo.html

go_bindata["templates/foo.html"] = templates_foo_html

bindata-toc.go

The bindata-toc.go file is very simple and looks as follows:

package $PACKAGENAME

// Global Table of Contents map. Generated by go-bindata.
// After startup of the program, all generated data files will
// put themselves in this map. The key is the full filename, as
// supplied to go-bindata.
var go_bindata = make(map[string] func() []byte)

Build tags

With the optional -tags flag, you can specify any go build tags that must be fulfilled for the output file to be included in a build. This is useful for including binary data in multiple formats, where the desired format is specified at build time with the appropriate tag(s).

The tags are appended to a // +build line in the beginning of the output file and must follow the build tags syntax specified by the go tool.