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It looks like you are trying to compile the program (make hello). To run the program the command is usually something like ./hello.


The spec is perfectly correct. More than likely, you don't have #define _XOPEN_SOURCE #include <unistd.h> At the very top of your program (ie, before any other #includes). If you move that to the top even above the other includes in your header, it will compile fine.

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