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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  func-
     tion is the
	number of bytes from the beginning of the file.

	If  the stream is open in text mode, the value returned by this function
     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 message.
      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>)
	C documentation for
	ftell

http://cppreference.com 	   2024.06.10			   std::ftell(3)
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