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The GNU C Library

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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 12. 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.)

13.1 Opening and Closing Files    How to open and close file descriptors.

13.2 Input and Output Primitives    Reading and writing data.

13.3 Setting the File Position of a Descriptor    Setting a descriptor's file position.

13.4 Descriptors and Streams    Converting descriptor to stream or vice-versa.

13.5 Dangers of Mixing Streams and Descriptors    Precautions needed if you use both descriptors and streams.

13.6 Fast Scatter-Gather I/O    Fast I/O to discontinuous buffers.

13.7 Memory-mapped I/O    Using files like memory.

13.8 Waiting for Input or Output    How to check for input or output on multiple file descriptors.

13.9 Synchronizing I/O operations    Making sure all I/O actions completed.

13.10 Perform I/O Operations in Parallel    Perform I/O in parallel.

13.11 Control Operations on Files    Various other operations on file descriptors.

13.12 Duplicating Descriptors    Fcntl commands for duplicating file descriptors.

13.13 File Descriptor Flags    Fcntl commands for manipulating flags associated with file descriptors.

13.14 File Status Flags    Fcntl commands for manipulating flags associated with open files.

13.15 File Locks    Fcntl commands for implementing file locking.

13.16 Interrupt-Driven Input    Getting an asynchronous signal when input arrives.

13.17 Generic I/O Control operations