FILE header and stream differentiation

atlantis43 · 08-18-2015, 10:04 AM

Hope my following dilemma is clear!
I'm used to addressing an overall file (eg: text file)in C with a pointer.

Code:

FILE* fp;

I hope someone can explain (in text form) how, in a file which is supposed to contain a "header" for metadata as well as a "stream" (eg Huffman encoded file or jpeg file), one addresses these separate file sections for reading or writing.
Thanks for any help.

sundialsvcs · 08-18-2015, 11:13 AM

In your program, fp is a pointer to a FILE structure, which is simply the runtime library's data structure for managing access to a file.

Most files contain a fixed "header" portion, usually at the beginning of the file but occasionally at the end, followed by variable data. Simply fseek() to whatever you want.

Guttorm · 08-29-2015, 08:37 AM

If you are looking for a library:

Code:

apt-get source libmagic-dev

atlantis43 · 08-31-2015, 04:59 PM

Quote:

Originally Posted by sundialsvcs

In your program, fp is a pointer to a FILE structure, which is simply the runtime library's data structure for managing access to a file.

Most files contain a fixed "header" portion, usually at the beginning of the file but occasionally at the end, followed by variable data. Simply fseek() to whatever you want.

With

Code:

int fseek(FILE *stream, long offset, int whence);

I think that I understand that this can be used to go to a chosen position in "stream". My question is more one of how do I initially write a "header" or a "stream" into a FILE denoted by

Code:

FILE* fp

i.e., how do I address these particular portions of the file?

atlantis43 · 08-31-2015, 05:01 PM

Quote:

Originally Posted by Guttorm

If you are looking for a library:

Code:

apt-get source libmagic-dev

Sorry, but my knowledge isn't adequate to know what this means, or how it applies to my original question.

ntubski · 08-31-2015, 05:55 PM

Quote:

Originally Posted by atlantis43

My question is more one of how do I initially write a "header" or a "stream" into a FILE denoted by

Code:

FILE* fp

i.e., how do I address these particular portions of the file?

Have you done IO in C before? FILE* doesn't point to part of a file, or even a file at all. It points to an opaque (meaning the implementation details are inaccessible) structure representing a (metaphorical) tape head which advances through the file as you read or write to it.

To write a header and data to a file, first write the header, then write the data. Maintaining the distinction between header and data is completely up to you. A file is just a series of bytes.

sundialsvcs · 09-01-2015, 09:08 AM

The thing-in-memory called FILE is an internal data structure used by the C runtime-library to manage access by this program to a particular file. It is not a pointer to data within the file.

A common strategy is to (having opened the file for exclusive access) write a dummy header record to the start of the file, then write the stream of data. Finally, fseek() back to the beginning of the file and rewrite the header with the final information.