I have .dat file that I m trying to read in python. File format is:
1.1 CDR description1
Field length(bytes) Offset
x 4 0
x1 2 4
x2 1 6
x3 1 7
......
......
......
x4 16 210
x5 4 226
x6 70 230
Total length of information 300
These are CDR records and I'm trying to read with the struct module but I can't understand how to use it with my specific file format... Any help?
You need to know what kind of information is stored in each field for the struct module to make sense of each field.
For example, the first field at offset 0 is 4 bytes long, which means it could be an int (ranges from −2,147,483,648 to +2,147,483,647) or it could be a unsigned int instead (ranges from 0 to 4,294,967,295). It could also be a single-precision floating point number.
You probably also need to figure out the endianness of your file format. If this is not explicitly named you need to experiment a little, or infer from context what it would be (a Windows file format is almost always little-endian, for example).
If you want to unpack the first 4 values only, you read the correct number of bytes (8 in your format) and pass this to the struct.unpack function together with a set of formatting characters to tell struct what types to expect. If we assume little-endian data, and the first four fields represent an unsigned int, an unsigned short and two unsigned chars, respectively, you'd use:
with open('something.cdr', 'rb') as data:
x, x1, x2, x3 = struct.unpack('<IH2B', data.read(8))
2 in front of the B means "expect 2 character bytes". If you want to parse just 3 fields, you need to adjust this to '<IHB' instead.