Why Does tellg() Report Inconsistent File Sizes, and How Can I Accurately Determine File Size?

tellg() Function Reporting Inconsistent File Size

In an attempt to read a file into a buffer, users may encounter incorrect results when utilizing the tellg() function. Unlike expectations, tellg() reports a token value instead of the actual file size or byte offset. This discrepancy can be attributed to the function's intended purpose.

Technically, tellg() does not directly indicate the file size or its precise position in bytes. Rather, it provides a token that enables the program to revisit the same location within the file. The token's value is not guaranteed to be convertible to an integral data type.

In Unix-based systems, tellg() generally represents the byte offset from the file's beginning. On Windows systems, however, its behavior differs based on the file's open mode. For binary files, the offset resembles Unix behavior. However, in text mode, there is no direct correlation between tellg()'s output and the actual number of bytes required to reach a given position.

To accurately determine the number of readable bytes in a file, the most reliable approach is to attempt the reading process. This can be achieved using the following steps:

file.ignore( std::numeric_limits<std::streamsize>::max() );
std::streamsize length = file.gcount();
file.clear();   //  Since ignore will have set eof.
file.seekg( 0, std::ios_base::beg );

Here, ignore is employed to read until the end of the file. The value of gcount() then provides the actual number of bytes.

Additional Notes:

1. The line *buffer = new char[length]; should be avoided as it attempts to assign a pointer to a character variable. Instead, consider declaring buffer as a char** or using a vector or string.
2. The loop condition at the end of the code for (int i = 0; i < size; i ) should be revised to account for potential issues while reading.
3. Failing to handle file opening errors (e.g., by setting size to 0) is a potential oversight.

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