std::basic_filebuf::seekoff in C++

In file-based I/O operations, we often need to manipulate the position where the data is read or written. This means that you change the "file pointer" inside the file so that it points to a specific place. std::basic_filebuf::seekoff offers a solution for changing the position of a file pointer in an instance of std::basic_filebuf.

Introduction

Stream buffer for I/O files is what std::basic_filebuf really does; it serves as a basic class in C++ standard library. It mediates between the standard streams (std::istream and std::ostream) and files. A protected member function of std::basic_filebuf, seekoff allows you to control its associated file's position of its internal file pointer.

Implementation

Here's a breakdown of std::basic_filebuf::seekoff's behavior and parameters:

1. Prototype:

2. Parameters:

• off: It is an offset value of type off_type (usually streamoff), which indicates the relative position from where to move the file pointer. Positive values move forward, negative values move backward.
• dir: An object of type ios_base::seekdir, specifying the reference point for the offset (off). It can be:
• ios_base::beg: Beginning of the file (default)
• ios_base::cur: Current position of the file pointer
• ios_base::end: End of the file
• which (optional): An object of type ios_base::openmode (usually ignored), indicating which stream (input or output) to affect. This parameter often has no effect because the std::basic_filebuf maintains a single position.

3. Return Value:

The seekoff function returns an object of type pos_type (usually streampos), which represents the new position of the file pointer after the seek operation. If the seek fails, a value representing an error position is returned.

Key Points and Considerations

• Positioning: The seekoff function repositions the file pointer within the associated file based on the specified offset and reference point.
• Seek Modes: The dir parameter allows you to seek from the beginning, current position, or end of the file.
• Fixed-Width vs Variable-Width Encoding: For fixed-width character encodings (where each character occupies a constant number of bytes), seekoff can calculate the exact byte offset in the file based on the character offset (off). For variable-width encodings (e.g., UTF-8), seekoff might need to perform additional processing to determine the precise byte position, potentially leading to less precise seeking.
• Error Handling: It is essential to check the return value of seekoff to ensure successful positioning. An error position on return indicates a failure.

Program 1:

Let us take an example to illustrate the std::basic_filebuf seekoff function in C++.

Output:

f1's locale's encoding() returns 1
pubseekoff(3, beg) returns 3
pubseekoff(0, end) returns 10
f2's locale's encoding() returns 0
pubseekoff(3, beg) returns -1
pubseekoff(0, end) returns 10

Explanation:

In this example, we open a file in binary mode and then use seekoff to position the file pointer at the 10th byte from the beginning. If successful, we read the next 100 bytes into a buffer.

1. Header Inclusions:

• #include <fstream>: This header file provides classes for file I/O, including std::ifstream for reading files.
• #include <iostream>: This header file provides standard input/output streams like std::cerr for error messages.

2. Main Function:

• int main(): It is the entry point of the program.

3. Opening the File:

• std::ifstream file("data.txt", std::ios::binary);:
• std::ifstream: It creates an input filestream object named file.
• "data.txt": It specifies the filename to open.
• std::ios::binary: It opens the file in binary mode. It ensures that the file contents are read/written byte-by-byte without interpreting characters based on encoding.
• if (!file.is_open()): It checks if the file was opened successfully.
• !file.is_open(): If file.is_open() returns false (indicating an error), the if block executes.
• std::cerr << "Error opening file!" << std::endl;: It prints an error message to the standard error stream (std::cerr) if the file couldn't be opened.
• return 1;: It exits the program with a non-zero exit code, signifying an error.

4. Seeking to a Specific Position:

• if (file.seekoff(10, std::ios_base::beg) == std::streampos::eof()):
• seekoff(10, std::ios_base::beg): This line attempts to move the file pointer 10 bytes forward from the beginning of the file (std::ios_base::beg).
• std::streampos::eof(): It is a special value representing the end-of-file position.
• The if statement checks if the return value of seekoff is equal to std::streampos::eof(). If so, it means the seek operation failed.
• std::cerr << "Error seeking in file!" << std::endl;: Prints an error message if seeking failed.
• return 1;: Exits the program with a non-zero exit code.

5. Reading Data:

• char buffer[100];: It declares a character array named buffer with a size of 100. It will be used to store the data read from the file.
• read(buffer, sizeof(buffer));: It reads the data from the file into the buffer.
• read(): It is a member function of std::ifstream that reads data from the file.
• buffer: The destination buffer where the read data will be stored.
• sizeof(buffer): This expression calculates the size of the buffer array in bytes. It ensures that the entire buffer is filled or all available data is read (up to the buffer size) until the end of the file is reached.

6. Closing the File:

• close();: It closes the file stream object file, releasing the resources associated with the file.

7. Exiting the Program:

• return 0;: It exits the program with a zero exit code, indicating successful execution.

Time Complexity:

The time complexity of this code is linear i.e. O(n) because it uses one single read operation which reads data from the file into a buffer array, and hence is dominant. The size of the buffer array used here is fixed (100 bytes) but the number of times this call to file.read happens depends on the size of the whole file. In case there exists much large size files, more calls will be needed when all data is being read by use of file.read().

Space Complexity:

Also linearly, it is the space complexity of the code that is O(n). This is because the major space cost will come from the buffer array. Although the size of this buffer array is known, how many loops get to be carried out depends on the file size. If a larger file has more data that needs to be read into the buffer, its actual size does not change.

Program 2:

Let us take another example to illustrate the std::basic_filebuf seekoff function in C++.

Output:

f1's locale's encoding() returns 1
pubseekoff(3, beg) returns 3
pubseekoff(0, end) returns 10
f2's locale's encoding() returns 0
pubseekoff(3, beg) returns -1
pubseekoff(0, end) returns 10

Explanation:

1. get_encoding Function Template:

• This function template takes a std::basic_istream (input stream) as its parameter.
• It extracts the codecvt facet from the stream's locale and retrieves the encoding using std::use_facet.
• The encoding() function of the codecvt facet returns an integer representing the encoding.

2. main Function:

• The program first creates a file named "text.txt" with UTF-8 encoded characters ("zß水?").
• After that, it opens the file using two different input stream objects: std::ifstream and std::wifstream.
• For the std::ifstream (f1), the program retrieves the encoding of the stream's locale and seeks to specific positions in the file using pubseekoff.
• For the std::wifstream (f2), the program sets the locale to "en_US.UTF-8" to specify UTF-8 encoding. After that, it performs similar operations as in the case of f1.

3. Output:

• The program prints the encoding returned by get_encoding for each stream (f1 and f2).
• It also demonstrates seeking positions within the streams using pubseekoff.

Conclusion:

In conclusion, the std::basic_filebuf::seekoff function is a valuable tool for controlling the file pointer's position. The std::basic_filebuf::seekoff provides a way to reposition the file pointer within a file stream associated with a std::basic_filebuf object. It allows you to jump to specific locations within the file for reading or writing data.

Key points to remember:

Takes three arguments:

• off: Offset value relative to the starting position (std::ios_base::beg), current position (std::ios_base::cur), or end of the file (std::ios_base::end).
• dir: It specifies the reference point for the offset (std::ios_base::seekdir).
• which (often ignored): It indicates which sequence (input or output) to modify (usually for streams supporting both).
• It returns a std::streampos object representing the new position if successful, or std::streampos::eof() if there's an error.
• It works best with files opened in binary mode (std::ios::binary) for accurate byte-level positioning.
• Limited impact on space complexity (usually constant) as it mainly manipulates pointers.
• Time complexity can be constant (seeking within a buffer) or linear (seeking within a large file) depending on usage.

Effective for:

• Random access within a file for reading or writing specific data chunks.
• Implementing custom file processing algorithms that require precise positioning.