# ImageMagick MagickInfo Structure

The MagickInfo structure is used by ImageMagick to register support for an Image format. The MagickInfo structure is allocated with default parameters by calling SetMagickInfo(). Image formats are registered by calling RegisterMagickInfo() which adds the initial structure to a linked list (at which point it is owned by the list). A pointer to the structure describing a format may be obtained by calling GetMagickInfo(). Pass the argument NULL to obtain the first member of this list. A human-readable list of registered image formats may be printed to a file descriptor by calling ListMagickInfo().

Support for formats may be provided as a module which is part of the ImageMagick library, provided by a module which is loaded dynamically at run-time, or directly by the linked program. Users of ImageMagick will normally want to create a loadable-module, or support encode/decode of an image format directly from within their program.

### Sample Module:

The following shows sample code for a module called "GIF" (gif.c). Note that the names of the Register and Unregister call-back routines are calculated at run-time, and therefore must follow the rigid naming scheme `RegisterFORMATImage` and `UnregisterFORMATImage`, respectively, where FORMAT is the upper-cased name of the module file:

/* Read image */
`Image *ReadGIFImage(const ImageInfo *image_info)`
`{`
`[ decode the image ... ]`
`}`

/* Write image */
`unsigned int WriteGIFImage(const ImageInfo *image_info,Image *image)`
`{`
`[ encode the image ... ]`
`}`

`/* Module call-back to register support for formats */`
`void RegisterGIFImage(void)`
`{`
`MagickInfo`
`*entry;`

`entry=SetMagickInfo("GIF");`
`entry->decoder=ReadGIFImage;`
`entry->encoder=WriteGIFImage;`
`entry->description=AllocateString("CompuServe graphics interchange format");`
`entry->module=AllocateString("GIF");`
`RegisterMagickInfo(entry);`
`entry=SetMagickInfo("GIF87");`
`entry->decoder=ReadGIFImage;`
`entry->encoder=WriteGIFImage;`
`entry->adjoin=False;`
`entry->description=`
`AllocateString("CompuServe graphics interchange format (version 87a)");`
`entry->module=AllocateString("GIF");`
`RegisterMagickInfo(entry);`
`}`

`/* Module call-back to unregister support for formats */`
`Export void UnregisterGIFImage(void)`
`{`
`UnregisterMagickInfo("GIF");`
`UnregisterMagickInfo("GIF87");`
`}`

### Sample Application Code

Image format support provided within the user's application does not need to implement the RegisterFORMATImage and UnregisterFORMATImage call-back routines. Instead, the application takes responsibility for the registration itself. An example follows:

/* Read image */
`Image *ReadGIFImage(const ImageInfo *image_info)`
`{`
`[ decode the image ... ]`
`}`

/* Write image */
`unsigned int WriteGIFImage(const ImageInfo *image_info,Image *image)`
`{`
`[ encode the image ... ]`
`}`

`#include <stdio.h>`
`int main( void )`
`{`
`struct MagickInfo* info;`

`info = SetMagickInfo("GIF");`

`if ( info == (MagickInfo*)NULL )`
`exit(1);`

`info->decoder = ReadGIFImage;`
`info->encoder = WriteGIFImage;`
`info->adjoin = False;`
`info->description = AllocateString("CompuServe graphics interchange format");`
`/* Add MagickInfo structure to list */`
`RegisterMagickInfo(info);`

`info = GetMagickInfo("GIF");`
`[ do something with info ... ]`

`ListMagickInfo( stdout );`
`return;`
`}`

### MagickInfo Structure Definition

The members of the MagickInfo structure are shown in the following table:

MagickInfo Structure Members
Member | Type | Description
tag | const char * | Magick string (e.g. "GIF") to call this format.
decoder | Image *(*decoder)(const ImageInfo *) | Function to decode image data and return ImageMagick Image.
encoder | unsigned int (*encoder)(const ImageInfo *, Image *) | Function to encode image data with options passed via ImageInfo and image represented by Image.
adjoin | unsigned int | Set to non-zero (True) if this file format supports multi-frame images.
blob_support | unsigned int | Set to non-zero (True) if the encoder and decoder for this format supports operating arbitrary BLOBs (rather than only disk files).
raw | unsigned int | Image format does not contain size (must be specified in ImageInfo).
description | char * | Long form image format description (e.g. "CompuServe graphics interchange format").
module | char * | Name of module (e.g. "GIF") which registered this format. Set to NULL if format is not registered by a module.
data | void * | User specified data. A way to pass any sort of data structure to the endoder/decoder. To set this, GetMagickInfo() must be called to first obtain a pointer to the registered structure since it can not be set via a RegisterMagickInfo() parameter.
previous | MagickInfo | Previous MagickInfo struct in linked-list. NULL if none.
next | MagickInfo | Next MagickInfo struct in linked-list. NULL if none.

Image manipulation software that works like magic.