std::fseek(3)

NAME
std::fseek - std::fseek

Synopsis
   Defined in header <cstdio>
   int fseek( std::FILE* stream, long offset, int origin );
   Sets the file position indicator for the file stream stream.
   If the stream is open in binary mode, the new position is exactly offset bytes
   measured from the beginning of the file if origin is SEEK_SET, from the current file
   position if origin is SEEK_CUR, and from the end of the file if origin is SEEK_END.
   Binary streams are not required to support SEEK_END, in particular if additional
   null bytes are output.
   If the stream is open in text mode, the only supported values for offset are zero
   (which works with any origin) and a value returned by an earlier call to std::ftell
   on a stream associated with the same file (which only works with origin of
   SEEK_SET).
   If the stream is wide-oriented, the restrictions of both text and binary streams
   apply (result of std::ftell is allowed with SEEK_SET and zero offset is allowed from
   SEEK_SET and SEEK_CUR, but not SEEK_END).
   In addition to changing the file position indicator, fseek undoes the effects of
   std::ungetc and clears the end-of-file status, if applicable.
   If a read or write error occurs, the error indicator for the stream (std::ferror) is
   set and the file position is unaffected.

Parameters
   stream - file stream to modify
   offset - number of characters to shift the position relative to origin
   origin - position to which offset is added. It can have one of the following values:
            SEEK_SET, SEEK_CUR, SEEK_END

Return value
   0 upon success, nonzero value otherwise.

Notes
   After seeking to a non-end position in a wide stream, the next call to any output
   function may render the remainder of the file undefined, e.g. by outputting a
   multibyte sequence of a different length.
   POSIX allows seeking beyond the existing end of file. If an output is performed
   after this seek, any read from the gap will return zero bytes. Where supported by
   the filesystem, this creates a sparse file.
   POSIX also requires that fseek first performs fflush if there are any unwritten data
   (but whether the shift state is restored is implementation-defined). The standard
   C++ file streams guarantee both flushing and unshifting:
   std::basic_filebuf::seekoff.
   POSIX specifies, that fseek should return -1 on error, and set errno to indicate the
   error.

Example
// Run this code
 #include <cassert>
 #include <cstdio>
 #include <cstdint>
 #include <fstream>
 #include <vector>
 int main()
 {
     std::ofstream("dummy.nfo") << "8 bytes\n"; // create the file
     std::FILE* fp = std::fopen("dummy.nfo", "rb");
     assert(fp);
     std::fseek(fp, 0, SEEK_END); // seek to end
     const std::size_t filesize = std::ftell(fp);
     std::vector<std::uint8_t> buffer(filesize);
     std::fseek(fp, 0, SEEK_SET); // seek to start
     std::fread(buffer.data(), sizeof(std::uint8_t), buffer.size(), fp);
     std::fclose(fp);
     std::printf("I've read %zi bytes\n", filesize);
 }

Possible output:
 I've read 8 bytes

See also
   fsetpos moves the file position indicator to a specific location in a file
           (function)
   fgetpos gets the file position indicator
           (function)
   ftell returns the current file position indicator
           (function)
   rewind moves the file position indicator to the beginning in a file
           (function)
   C documentation for
   fseek