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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 Input/Output 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
• Input and Output Primitives
• Setting the File Position of a Descriptor
• Descriptors and Streams
• Dangers of Mixing Streams and Descriptors
• Fast Scatter-Gather I/O
• Memory-mapped I/O
• Waiting for Input or Output
• Synchronizing I/O operations
• Perform I/O Operations in Parallel
• Control Operations on Files
• Duplicating Descriptors
• File Descriptor Flags
• File Status Flags
• File Locks
• Open File Description Locks
• Open File Description Locks Example
• Interrupt-Driven Input
• Generic I/O Control operations