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### 18.1 Host Common

Some things are just not portable, even between similar operating systems, and are too difficult for autoconf to detect. They get implemented using hook functions in the file specified by the host_hook_obj variable in `config.gcc`.

Host Hook: void HOST_HOOKS_EXTRA_SIGNALS (void)

  This host hook is used to set up handling for extra signals. The most common thing to do in this hook is to detect stack overflow.

Host Hook: void * HOST_HOOKS_GT_PCH_GET_ADDRESS (size_t size, int fd)

  This host hook returns the address of some space that is likely to be free in some subsequent invocation of the compiler. We intend to load the PCH data at this address such that the data need not be relocated. The area should be able to hold size bytes. If the host uses `mmap`, fd is an open file descriptor that can be used for probing.

Host Hook: int HOST_HOOKS_GT_PCH_USE_ADDRESS (void * address, size_t size, int fd, size_t offset)

  This host hook is called when a PCH file is about to be loaded. We want to load size bytes from fd at offset into memory at address. The given address will be the result of a previous invocation of `HOST_HOOKS_GT_PCH_GET_ADDRESS`. Return -1 if we couldn’t allocate size bytes at address. Return 0 if the memory is allocated but the data is not loaded. Return 1 if the hook has performed everything.

  If the implementation uses reserved address space, free any reserved space beyond size, regardless of the return value. If no PCH will be loaded, this hook may be called with size zero, in which case all reserved address space should be freed.

  Do not try to handle values of address that could not have been returned by this executable; just return -1. Such values usually indicate an out-of-date PCH file (built by some other GCC executable), and such a PCH file won’t work.

Host Hook: size_t HOST_HOOKS_GT_PCH_ALLOC_GRANULARITY (void);

  This host hook returns the alignment required for allocating virtual memory. Usually this is the same as getpagesize, but on some hosts the alignment for reserving memory differs from the pagesize for committing memory.

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