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

NAME
       std::ftell - std::ftell

Synopsis
	  Defined in header <cstdio>
	  long ftell( std::FILE* stream	);

	  Returns  the	current	 value	of the file position indicator for the
       file stream stream.

	  If the stream	is open	in binary mode,	the  value  obtained  by  this
       function	is the
	  number of bytes from the beginning of	the file.

	  If the stream	is open	in text	mode, the value	returned by this func-
       tion is
	  unspecified and is only meaningful as	the input to std::fseek.

Parameters
	  stream - file	stream to examine

Return value
	  File	position  indicator  on	success	or -1L if failure occurs. Also
       sets errno on
	  failure.

Example
	  Demonstrates std::ftell() with error checking. Writes	then  reads  a
       few
	  floating-point (FP) values to/from a file.

       // Run this code

	#include <cstdio>
	#include <cstdlib>
	#include <iostream>

	//  If	the condition is not met then exit the program with error mes-
       sage.
	void check(bool	condition, const char* func, int line)
	{
	    if (condition)
		return;
	    std::perror(func);
	    std::cerr << func << " failed in file " << __FILE__	<< " at	line #
       " << line - 1
		      << '\n';
	    std::exit(EXIT_FAILURE);
	}

	int main()
	{
	    // Prepare an array	of FP values.
	    constexpr int SIZE {5};
	    double A[SIZE] = {1.1, 2.2,	3.3, 4.4, 5.5};

	    // Write array to a	file.
	    const char*	fname =	"/tmp/test.bin";
	    FILE* file = std::fopen(fname, "wb");
	    check(file != NULL,	"fopen()", __LINE__);

	    const  int	write_count  =	std::fwrite(A,	sizeof(double),	 SIZE,
       file);
	    check(write_count == SIZE, "fwrite()", __LINE__);

	    std::fclose(file);

	    // Read the	FP values into array B.
	    double B[SIZE];
	    file = std::fopen(fname, "rb");
	    check(file != NULL,	"fopen()", __LINE__);

	    long  pos  =  std::ftell(file);  //	position indicator at start of
       file
	    check(pos != -1L, "ftell()", __LINE__);
	    std::cout << "pos: " << pos	<< '\n';

	    const int read_count = std::fread(B, sizeof(double), 1, file);  //
       read one	FP value
	    check(read_count ==	1, "fread()", __LINE__);

	    pos	= std::ftell(file); // position	indicator after	reading	one FP
       value
	    check(pos != -1L, "ftell()", __LINE__);
	    std::cout << "pos: " << pos	<< '\n';
	    std::cout << "B[0]:	" << B[0] << '\n'; // print one	FP value

	    return EXIT_SUCCESS;
	}

Possible output:
	pos: 0
	pos: 8
	B[0]: 1.1

See also
	  fgetpos gets the file	position indicator
		  (function)
	  fseek	   moves the file position indicator to	a specific location in
       a file
		  (function)
	  fsetpos moves	the file position indicator to a specific location  in
       a file
		  (function)
	  tellg	  returns the input position indicator
		  (public member function of std::basic_istream<CharT,Traits>)
	  tellp	  returns the output position indicator
		  (public member function of std::basic_ostream<CharT,Traits>)

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

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