ptg10805159158 StandardI/O Library Chapter 5
#include <stdio.h>
long ftell(FILE *fp);
Returns: current file position indicator if OK,−1L on error
int fseek(FILE *fp,long offset,intwhence);
Returns: 0 if OK,−1 on error
void rewind(FILE *fp);For a binary file, a file’s position indicator is measured in bytes from the beginning of
the file. The value returned byftellfor a binary file is this byte position. To position
abinary file usingfseek, we must specify a byteoffsetand indicate how that offset is
interpreted. The values for whencearethe same as for the lseekfunction from
Section 3.6:SEEK_SETmeans from the beginning of the file,SEEK_CURmeans from the
current file position, andSEEK_ENDmeans from the end of file. ISO C doesn’t require
an implementation to support theSEEK_ENDspecification for a binary file, as some
systems requireabinary file to be padded at the end with zeros to make the file size a
multiple of some magic number.Under the UNIX System, however,SEEK_ENDis
supported for binary files.
For text files, the file’s current position may not be measurable as a simple byte
offset. Again, this is mainly under non-UNIX systems that might storetext files in a
different format. To position a text file,whencehas to beSEEK_SET,and only two
values foroffsetareallowed: 0—meaning rewind the file to its beginning—oravalue
that was returned byftellfor that file. Astream can also be set to the beginning of
the file with therewindfunction.
Theftellofunction is the same asftell,and thefseekofunction is the same as
fseek,except that the type of the offset isoff_tinstead oflong.#include <stdio.h>
off_t ftello(FILE *fp);
Returns: current file position indicator if OK,(off_t)− 1 on error
int fseeko(FILE *fp,off_toffset,intwhence);
Returns: 0 if OK,−1 on errorRecall the discussion of theoff_tdata type in Section 3.6. Implementations can define
theoff_ttype to be larger than 32 bits.
As we mentioned earlier,thefgetposandfsetposfunctions wereintroduced by
the ISO C standard.#include <stdio.h>
int fgetpos(FILE *restrictfp,fpos_t *restrictpos);
int fsetpos(FILE *fp,const fpos_t *pos);
Both return: 0 if OK, nonzero on error