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std::fsetpos(3)		      C++ Standard Libary	       std::fsetpos(3)

NAME
       std::fsetpos - std::fsetpos

Synopsis
	  Defined in header <cstdio>
	  int fsetpos( std::FILE* stream, const	std::fpos_t* pos );

	  Sets the file	position indicator and the multibyte parsing state (if
       any) for	the C
	  file stream stream according to the value pointed to by pos.

	  Besides  establishing	 new  parse state and position,	a call to this
       function	undoes
	  the effects of std::ungetc and clears	the end-of-file	state,	if  it
       is set.

	  If  a	 read or write error occurs, the error indicator (std::ferror)
       for the stream is
	  set.

Parameters
	  stream - file	stream to modify
	  pos	 - pointer to  a  fpos_t  object  obtained  from  std::fgetpos
       called on a stream
		   associated with the same file

Return value
	  0  upon  success, nonzero value otherwise. Also, sets	errno on fail-
       ure.

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.

Example
       // Run this code

	#include <cstdio>
	#include <cstdlib>

	int main()
	{
	    // Prepare an array	of floating-point values.
	    const int SIZE = 5;
	    double A[SIZE] = {1., 2., 3., 4., 5.};
	    // Write array to a	file.
	    std::FILE *	fp = std::fopen("test.bin", "wb");
	    std::fwrite(A, sizeof(double), SIZE, fp);
	    std::fclose(fp);

	    // Read the	values into array B.
	    double B[SIZE];
	    fp = std::fopen("test.bin",	"rb");
	    std::fpos_t	pos;
	    if (std::fgetpos(fp, &pos) != 0)	  // current  position:	 start
       of file
	    {
	       std::perror("fgetpos()");
	       std::fprintf(stderr,  "fgetpos()	 failed	 in  file %s at	line #
       %d\n",
			    __FILE__, __LINE__-3);
	       std::exit(EXIT_FAILURE);
	    }

	    int	ret_code = std::fread(B, sizeof(double), 1, fp);      //  read
       one value
	    // current position: after reading one value
	    std::printf("%.1f; read count = %d\n", B[0], ret_code);   // print
       one value and ret_code

	    if	(std::fsetpos(fp,  &pos)  != 0)	  // reset current position to
       start of	file
	    {
	       if (std::ferror(fp))
	       {
		  std::perror("fsetpos()");
		  std::fprintf(stderr, "fsetpos() failed in file %s at line  #
       %d\n",
			       __FILE__, __LINE__-5);
		  std::exit(EXIT_FAILURE);
	       }
	    }

	    ret_code  =	 std::fread(B,	sizeof(double),	1, fp);		// re-
       read first value
	    std::printf("%.1f; read count = %d\n", B[0], ret_code);  //	 print
       one value and ret_code
	    std::fclose(fp);

	    return EXIT_SUCCESS;
	}

Output:
	1.0; read count	= 1
	1.0; read count	= 1

See also
	  fgetpos gets the file	position indicator
		  (function)
	  ftell	  returns the current file position indicator
		  (function)
	  fseek	   moves the file position indicator to	a specific location in
       a file
		  (function)

http://cppreference.com		  2022.07.31		       std::fsetpos(3)

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