How to remove a header from file without creating additional temporary files?

Question

I'm developing an application for embedded system running Linux.

In my case, I have a quite big file (compared to the capability of the system) as input. The file has a small header whose size is just a few hundred bytes. In my application, I need to remove that header from the file so the file will have no header and contain only relevant data. Normally, I'd implement like following (pseudo code):

char *input_file  = "big_input.bin";
char *tmp_file1 = "header.bin";
char *tmp_file2 = "data.bin";
/* Copy the content of header from input file to tmp_file1 */ 
_copy_header(tmp_file1, input_file); 
/* Copy the data from input file to tmp_file2 */ 
_copy_data(tmp_file2, input_file);
/* Rename temp file to input file */
unlink(input_file);
rename(tmp_file2, input_file);

The problem with this approach is that it creates a temporary file tmp_file2 whose size is almost as big as input file (because the header is very small). In my system, everything is stored on RAM, which is very limited. Creating a big temporary file causes an out-of-memory error.

So how can I avoid creating a big temporary file?

Answer 1

Open the same file twice, once for reading, once for writing.

Seek the read pointer behind the header.

Read from the read-pointer and write to the write-pointer.

Make sure the size you read and write at once is not larger than the header.

Cut off the header's size at the end of the file.