Structure Padding in C
1. Is alignment applied for stack?
Yes. The stack is also memory. The system programmer should load the stack pointer with a memory address that is properly aligned. Generally, the processor won’t check stack alignment, it is the programmer’s responsibility to ensure proper alignment of stack memory. Any misalignment will cause run-time surprises.
For example, if the processor word length is 32-bit, the stack pointer also should be aligned to be a multiple of 4 bytes.
2. If char data is placed in a bank other than bank 0, it will be placed on the wrong data lines during memory reading. How the processor handles char type?
Usually, the processor will recognize the data type based on instruction (e.g. LDRB on an ARM processor). Depending on the bank it is stored, the processor shifts the byte onto the least significant data lines.
3. When arguments are passed on the stack, are they subjected to alignment?
Yes. The compiler helps the programmer in making proper alignment. For example, if a 16-bit value is pushed onto a 32-bit wide stack, the value is automatically padded with zeros out to 32 bits. Consider the following program.
c
void argument_alignment_check( char c1, char c2 ) { // Considering downward stack // (on upward stack the output will be negative) printf("Displacement %d\n", (int)&c2 - (int)&c1); } |
The output will be 4 on a 32-bit machine. It is because each character occupies 4 bytes due to alignment requirements.
4. What will happen if we try to access misaligned data?
It depends on the processor architecture. If the access is misaligned, the processor automatically issues sufficient memory read cycles and packs the data properly onto the data bus. The penalty is on performance. Whereas few processors will not have the last two address lines, which means there is no way to access the odd byte boundary. Every data access must be aligned (4 bytes) properly. Misaligned access is a critical exception on such processors. If the exception is ignored, read data will be incorrect and hence the results.
5. Is there any way to query the alignment requirements of a data type?
Yes. Compilers provide non-standard extensions for such needs. For example, __alignof() in Visual Studio helps in getting the alignment requirements of data type. Read MSDN for details.
6. When memory reading is efficient in reading 4 bytes at a time on a 32-bit machine, why should a double type be aligned on an 8-byte boundary?
It is important to note that most of the processors will have a math co-processor, called Floating Point Unit (FPU). Any floating point operation in the code will be translated into FPU instructions. The main processor is nothing to do with floating-point execution. All this will be done behind the scenes.
As per standard, the double type will occupy 8 bytes. And, every floating point operation performed in FPU will be of 64-bit length. Even float types will be promoted to 64 bits prior to execution.
The 64-bit length of FPU registers forces double type to be allocated on an 8-byte boundary. I am assuming (I don’t have concrete information) in the case of FPU operations, data fetch might be different, I mean the data bus since it goes to FPU. Hence, the address decoding will be different for double types (which are expected to be on an 8-byte boundary). It means the address decoding circuits of the floating point unit will not have the last 3 pins.
This article is contributed by Venki.
Structure Member Alignment, Padding and Data Packing
In C, the structures are used as data packs. They don’t provide any data encapsulation or data hiding features.
In this article, we will discuss the property of structure padding in C along with data alignment and structure packing.