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resource map

Resource File Format Definition of resource file structure.

Writing programs that modify resource files directly requires that you know

exactly what a resource file consists of.

The resource header is a 16-byte block that gives the offsets to and lengths of

the resource-data and resource-map parts of the file. It contains: 4 bytes that tell

the offset from the beginning of the resource file to the resource data; 4 bytes that

indicate the offset from the beginning of the resource file to the resource map; 4

bytes that tell the length of the resource data; and 4 bytes that list the length of the

resource map.

The next 112 bytes are reserved for system use and the 128 bytes that come next

are reserved for the Resource Manager. These 128 bytes are labeled " available

for application data" in Inside Macintosh Volume 1, but Macintosh Technical Note

#62 states that they are reserved by the Resource Manager.

Next in the resource header come 4 bytes that tell the length of the resource

data, which follows immediately and whose size is variable.

The final component of the resource header is the resource map, which starts

out with three reserved areas: 16 bytes for a copy of the resource header; 4 bytes

for a handle to the next resource map; and 2 bytes for a file reference

number. The resource map also contains 2 bytes describing the resource file's

attributes; 2 bytes to show the offset from the beginning of the resource map

to the type list; and 2 bytes to indicate the offset to the resource name list

(see diagram below).

Once the Resource Manager reads the resource map, it uses the reserved

areas at the beginning of the header to store the indicated information from the

map.

The type list has a 2-byte block that shows the total number of resource

types in the map, minus 1. It also contains 4 bytes to describe the resource type,

2 bytes showing the number of resources of this type (minus 1) that the map

contains; and 2 bytes to show the offset from the beginning of the type list to the

reference list for this type of resource.

The reference list holds the resource references for all resources of that type .

Reference lists are placed one after an other, in their order of appearance on the

type lists. Each reference list consists of: 2 bytes of Resource ID; 2 bytes

showing the offset from the beginning of the resource name list to the length of

the resource name (-1 if none); 1 byte showing resource attributes; 3 bytes

with information on the offset from the beginning of the resource data to the length

of the data for this resource; and 4 bytes of reserved space for a handle to the

resource.

Lastly, the resource file contains a resource name list that has a 1-byte indicator

showing the length of the resource name and a variable number of bytes holding the

characters of the resource's name. The following diagram shows the relation of

resource file elements.

Remember that parts of the system ( including the File Manager and the Finder)

assume that if the resource fork exists, it will contain valid Resource Manager

information. For this reason, it is important to avoid using the resource fork for

non-resource data. Apple provides the call PBOpenRF to let you copy the resource

fork of a file without it being interpreted by the Resource Manager, but this

function is not intended to be used to open "another data fork". The File Manager

assumes that the first block of the file's resource fork will be part of the resource

header, and puts information there to aid in scavenging. This means that one copy of

a resource file opened with PBOpenRF may not be exactly the same as the original.

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