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 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: