OsReadFile

OsReadFile is a low-level operating system service that reads data from an open file into memory. It is part of the I/O subsystem and is typically exposed to applications through a library interface or directly by the kernel, depending on the system design. The function reads up to a specified number of bytes from the file and stores them in a user-supplied buffer.

Typical form: OsReadFile takes a file handle or descriptor, a buffer pointer, and a byte count. Some variants also accept an offset parameter to perform a positional read without changing the file's current position. On success, the function returns the number of bytes actually read, which may be less than requested near end-of-file. A return value of 0 indicates end-of-file. On error, a negative value or an error code is returned, and no data is written to the buffer.

Behavior may be blocking or non-blocking depending on the file and flags set by the caller. In buffered I/O environments the OS may return data from internal caches, while unbuffered I/O passes data directly between the file and the buffer. OsReadFile is generally thread-safe and designed to interact with the system’s file system and virtual I/O layer.

Usage notes: The function is typically used by applications that need to read data sequentially or at a known offset from a file. It may require proper error handling and resource management, such as ensuring the underlying file is opened before the call and closed afterward.

See also: OsWriteFile, Open, Close, Seek, ReadFile.