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Using the Inline and Embedded Assemblers of the AR
Unaligned pointers in C and C++ code
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| Home > Compiler Coding Practices > Unaligned pointers in C and C++ code | |||
By default, the compiler expects conventional C and C++ pointers to point to naturally aligned words in memory because this enables the compiler to generate more efficient code.
If you want to define a pointer that can point to a word at
any address, you must specify the __packed qualifier
when defining the pointer. For example:
__packed int *pi; // pointer to unaligned int
When a pointer is declared as __packed,
the compiler generates code that correctly accesses the dereferenced
value of the pointer, regardless of its alignment. The generated
code consists of a sequence of byte accesses, or variable alignment-dependent
shifting and masking instructions, rather than a simple LDR instruction.
Consequently, declaring a pointer as __packed incurs
a performance and code size penalty.
See also- Reference
Compiler Reference:
| Copyright © 2007-2008, 2011-2012 ARM. All rights reserved. | ARM DUI 0375D |
| Non-Confidential | ID062912 |
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