fsetpos

fsetpos is a function in the C standard I/O library that sets the file position indicator for a stream to a position previously obtained by fgetpos. It is declared in stdio.h and has the signature int fsetpos(FILE *stream, const fpos_t *pos).

The pos argument must refer to a position previously obtained from fgetpos for the same stream. The fpos_t type may store a complex representation of the stream position, which can include buffering or multibyte conversion state. Because of this, fsetpos can restore a position more reliably on buffered or wide-character streams than a simple byte offset.

Return value is 0 on success and nonzero on error. Possible failure conditions include an invalid stream, a pos value that does not correspond to the stream, or an internal error during restoration of the stream state.

Relation to fseek and ftell: fsetpos and fgetpos are designed to handle nontrivial stream state beyond a basic numeric offset, making them suitable for complex buffered streams. For simple seeking to byte offsets, fseek and ftell are often more straightforward and portable.

Usage notes and examples: A typical pattern is to save the current position with fgetpos, perform other operations that may move the pointer, and then restore the position with fsetpos. For example, fgetpos(f, &pos); /* perform reads or writes that may advance the pointer */ fsetpos(f, &pos); This approach relies on using the same stream and checking return values for error handling.

Portability: fsetpos is broadly supported in ISO C implementations and in C++ standard libraries that expose stdio functionality. Ensure proper error checking and avoid assuming the saved position remains valid across stream closures or major state changes.