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__unaligned

When you declare a pointer with the __unaligned modifier, the compiler assumes that the pointer addresses data that is not aligned. Consequently, for an application that targets an Itanium Processor Family (IPF) computer, the compiler generates code that reads the unaligned data one byte at a time.

The __unaligned modifier is valid for the x64 and Itanium compilers but affects only applications that target an IPF computer. This modifier describes the alignment of the addressed data only; the pointer itself is assumed to be aligned.

The Itanium processor generates an alignment fault when it accesses misaligned data, and the time to process the fault weakens performance. Use the __unaligned modifier to cause the processor to read data one byte at a time and avoid the fault. This modifier is not required for x64 applications because the x64 processor handles misaligned data without faulting. 

For more information about alignment, see:

// unaligned_keyword.cpp
// compile with: /c
// processor: x64 IPF
#include <stdio.h>
int main() {
   char buf[100];

   int __unaligned *p1 = (int*)(&buf[37]);
   int *p2 = (int *)p1;

   *p1 = 0;   // ok

   __try {
      *p2 = 0;  // throws an exception
   }
   __except(1) {
      puts("exception");
   }
}

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