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by markps
3808 days ago
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> If the fork() call succeeds, this means the OS has guaranteed that all the memory in the child process is available to be written over. i.e. it has had to allocate 'lots of memory' x 2 in total, even if only 'lots of memory' x 1 will actually be used. That isn't the case anymore, at least on Linux and BSD. fork() uses a copy-on-write scheme so the OS only allocates/copies the parent memory space if the child attempts to write to it. |
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The point was that after you fork() in a copy-on-write scheme, the OS now has promised that more memory is available to write on than may actually exist. If the OS avoided overallocating, it would have to right there and then reserve lots of memory (without necessarily writing to it) just to be sure that you wouldn't run out at a later date.