c - what is difference between fgetpos/fsetpos and ftell/fseek

Question

What's the difference between using the functions fgetpos() and fsetpos() and using the functions ftell() and fseek() to get and set a position in a file?

What are fgetpos() and fsetpos() good for? Why would they be used instead of ftell() and fseek()?

Answer 1

None of the above answers are correct - in fact if you use fsetpos interchangeably with fseek you could introduce a security flaw (https://www.securecoding.cert.org/confluence/pages/viewpage.action?pageId=20087255).

The reason is that the fpos_t *pos argument to fsetpos isn't actually an integer so it can't be used to seek to arbitrary locations in a file. The only valid values are therefore ones obtained from fgetpos. As the docs say,

The internal file position indicator associated with stream is set to the position represented by pos, which is a pointer to an fpos_t object whose value shall have been previously obtained by a call to fgetpos.

(http://www.cplusplus.com/reference/cstdio/fsetpos/)

If all you want is the ability to seek to arbitrary locations beyond 32-bit boundary, then use ftello / fseeko and compile with #define _FILE_OFFSET_BITS 64.