Incorrect result returned after reading binary file (int value)

Question

I try to read value from binary file. I know offset (3201) and use it.

Example code:

FILE *bin_file;
int *job_id_buffer; 

bin_file = fopen("sample.sgy", "rb");

if (bin_file == NULL)
{
// ... skipped ... 
}

fseek(bin_file, 3201, SEEK_SET);

job_id_buffer = (int*)malloc(sizeof(int));

fread(job_id_buffer, sizeof(int), 1, bin_file);

printf("%d\n", (int)job_id_buffer[0]);

fclose(bin_file);

Looks like I don't know how to read value correctly.

But problem is that when I get result, the value is 993024, while I 100% know that correct value is 9999.

Could you, please, help me to understand what I do incorrectly?

Thank you in advance!

You seem to be off by one byte, and have your byte order wrong. — JimmyB
– JimmyB, Commented May 11, 2016 at 17:13
Are you sure the file endianness is the same as the program's? — stark
– stark, Commented May 11, 2016 at 17:18
firstly @JimmyB you are right! The problem was in 1 byte, I used 3201 offset instead of 3200. Thank you! Now I've got a value - 254214144 (Little endian) - if convert it to Big Endian is is 9999. P.S. I would mark you answer as correct but I don't know how to it with your comment ^( — Sergey Svetlikov
– Sergey Svetlikov, Commented May 11, 2016 at 17:26

JimmyB · Accepted Answer · 2016-05-12 14:24:44Z

A quick look at the values you got:

Expected: 9999 = 0x0000270F

Received: 993024 = 0x000F2700

From this we can see two things:

0F27 is 270F with swapped bytes. Apparently, the byte order (endiannes) inside the file is not the same as that in memory;
If only the byte order was swapped, you would be getting 0x0F270000 from the file, but the value seems to be shifted by one byte.

This can mean one of two things, depending on the endianness of the file:

If the file is little-endian (and the CPU is big-endian), you should be seeking to offset 3202 instead of 3201.

If the file is big-endian (and the CPU is little-endian), you should be seeking to offset 3200 instead of 3201.

So, check the endianness of the file and correct your offset; then, convert the data read from the endianness of the file to the endianness of your CPU (reverse the bytes in this case) to get the correct value.

To create code that is correct on any CPU, you may be able to use be32toh(), or le32toh(), to convert big-endian, or little-endian respectively, file data to the right CPU endianness.

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