13 Low-Level Input/Output

This chapter describes functions for performing low-level input/output operations on file descriptors. These functions include the primitives for the higher-level I/O functions described in I/O on Streams, as well as functions for performing low-level control operations for which there are no equivalents on streams.

Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons:

• For reading binary files in large chunks.
• For reading an entire file into core before parsing it.
• To perform operations other than data transfer, which can only be done with a descriptor. (You can use fileno to get the descriptor corresponding to a stream.)
• To pass descriptors to a child process. (The child can create its own stream to use a descriptor that it inherits, but cannot inherit a stream directly.)

• Opening and Closing Files: How to open and close file descriptors.
• I/O Primitives: Reading and writing data.
• File Position Primitive: Setting a descriptor's file position.
• Descriptors and Streams: Converting descriptor to stream or vice-versa.
• Stream/Descriptor Precautions: Precautions needed if you use both descriptors and streams.
• Scatter-Gather: Fast I/O to discontinuous buffers.
• Memory-mapped I/O: Using files like memory.
• Waiting for I/O: How to check for input or output on multiple file descriptors.
• Synchronizing I/O: Making sure all I/O actions completed.
• Asynchronous I/O: Perform I/O in parallel.
• Control Operations: Various other operations on file descriptors.
• Duplicating Descriptors: Fcntl commands for duplicating file descriptors.
• Descriptor Flags: Fcntl commands for manipulating flags associated with file descriptors.
• File Status Flags: Fcntl commands for manipulating flags associated with open files.
• File Locks: Fcntl commands for implementing file locking.
• Interrupt Input: Getting an asynchronous signal when input arrives.
• IOCTLs: Generic I/O Control operations.