Read Write byte range/chunk of data from specific location in file


 
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Old 05-22-2009
Read Write byte range/chunk of data from specific location in file

I am new to Unix so will really appreciate if someone can guide me on this.
What I want to do is:

Step1: Read binary file - pick first 2 bytes, convert from hex to decimal. Read the next 3 bytes as well.

2 bytes will specify the number of bytes 'n' that I want to read and write (starting from location of 6th byte) only if the 3 bytes meet a specific pattern

Step2: If the 3 bytes meet condition/pattern then I want to read next 'n' bytes starting from location of 6th byte. Write these 'n' bytes to output file.

This step should move seek pointer to 5+n byte location
Step3: Read the next 2 bytes - if these match a prespecified pattern ( alternative is to ignore/skip these 2 bytes)



Step 3: read next 2 byte and 3 bytes again. Repeat step1 and step 2. If condition is met then append bytes to output file from step2.

The binary file is:

0000000: 00570101080100ac8b905751485801bb .W........WQHX..
0000010: 8d5c0afdf44304b80000011480a5041d .\...C..........
0000020: 0000092cdd1ddd000f08000006013372 ...,..........3r
0000030: 6240808edea900a000000e7bbead0180 b@.........{....
0000040: 1f720301801f7100f7018029e2017001 .r....q....)..p.
0000050: 801f720100faae009a0101060100ac8b ..r.............
0000060: 90576284b502f8053c0afdf443027800 .Wb.....<...C.x.
0000070: 0001148105050d02100d02422e41010f ...........B.A..
0000080: 08000006013375837080b2ec1d000000 .....3u.p.......
0000090: 0000030d01802e9100016e01801ff200 ..........n.....
00000a0: 003f018022e20001f201802cf1000018 .?.."......,....
00000b0: 01802e1300007e01802f910001350180 ......~../...5..
00000c0: 2cf300002401801f720000c0018022e1 ,...$...r.....".
00000d0: 0001ec01801ff1000039018022e30001 .........9.."...
00000e0: f801802d6300001201802d61000006fa ...-c.....-a....
00000f0: ae01560101020100ac8b905781321202 ..V........W.2..
0000100: f8053d0afdf443027800000114810505 ..=...C.x.......
0000110: 0d02100d02422e41010f080000060133 .....B.A.......3
0000120: 75837080b2ec1d00000006030300ff05 u.p.............
0000130: 000000060000013d0000010e0000013d .......=.......=
0000140: 0a0000000000000301802e9300010180 ................
0000150: 2e930025000001017a01802e93010180 ...%....z.......
0000160: 2e93017a00007000322c6c000000017a ...z..p.2,l....z
0000170: 01802e930201802e93017a1d00670032 ..........z..g.2
0000180: 2c6c01802e92017415006800322c6c06 ,l.....t..h.2,l.
0000190: 06000000000000198f1a0000012d3400 .............-4.
00001a0: 0000004e000000006800000002820000 ...N....h.......
00001b0: 000a9c00000026b600000000d0000000 ......&.........
00001c0: 00ea00000000ff000000000000000000 ................
00001d0: 00ea231a0000000034000000004e0000 ..#.....4....N..
00001e0: 0000680000000082000000009c000000 ..h.............
00001f0: 00b60000000dd00000012dea00000000 ..........-.....
0000200: ff00000000000d000700070201000000 ................
0000210: 0000087db00000014304000000000004 ...}....C.......
0000220: a65b0000001c0201000000000000d79e .[..............
0000230: 00000120040000000000001285000000 ... ............
0000240: 1a00010501faae00500101090100ac8b ........P.......
0000250: 9057950ff902f8053e0afdf443027800 .W......>...C.x.
0000260: 0001148105050d02100e78022e41240f ..........x..A$.
0000270: 0800000228217304704c85681201802f ....(!s.pL.h.../
0000280: 3301802f3300000000000000b5000000 3../3...........
0000290: 0e000000b5faae00a10101060100ac8b ................
00002a0: 905795164502f8053f0afdf443027800 .W..E...?...C.x.
00002b0: 0001148105050d02100e78022e41240f

So I want to read 0057 convert to decimal. This gives me number of bytes 'n' to read.
Then I read next 3 bytes- 010108. If this is equal to say 010102 then I read the next 'n' bytes where 'n' is from hex to decimal conversion of 0057. Write these 'n' bytes to output file.

This will take (seek) to position in file where next 2 bytes are 'faae'. I will ignore these two bytes.

Read the next 2 (after faae) and subsequent 3 bytes again. Test the condition (equal to 010102) and if yes append 'm' bytes to file generated in above step

Repeat this till the end of file.

In short 'faae' indicats end of one chunk and I need to output chunk to file only if it meets condition The size of chunk is variable specified by 2 bytes after faae.

If someone can help me I will really appreciate.

Thanks

Last edited by Kbenipel; 05-22-2009 at 05:29 PM..
# 2  
Old 05-22-2009
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