libstorj | Asynchronous multi-platform C library | Cryptography library

No, neither C nor POSIX (since you mention dup()) has a function for opening a new, independent file handle based on an existing file handle. As you observed, you can dup() a file descriptor, but the result refers to the same underlying open file description.

To get an independent handle, you need to open() or fopen() the same path (which is possible only if the FILE refers to an object accessible through the file system). If you don't know what path that is, or if there isn't any in the first place, then you'll need a different approach.

Some alternatives to consider:

• buffer some or all of the file contents in memory, and read as needed from the buffer to serve your needs for independent file offsets;
• build an internal equivalent of the tee command; this will probably require a second thread, and you'll probably not be able to read one file too far ahead of the other, or to seek in either one;
• copy the file contents to a temp file with a known name, and open that as many times as you want;
• if the FILE corresponds to a regular file, map it into memory and access its contents there. The POSIX function fmemopen() could be useful in this case to adapt the memory mapping to your existing stream-based usage.