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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 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.)