#pragma pack effect

Question

I was wondering if someone could explain to me what the #pragma pack preprocessor statement does, and more importantly, why one would want to use it.

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Answer 1

#pragma is used to send non-portable (as in this compiler only) messages to the compiler. Things like disabling certain warnings and packing structs are common reasons. Disabling specific warnings is particularly useful if you compile with the warnings as errors flag turned on.

#pragma pack specifically is used to indicate that the struct being packed should not have its members aligned. It's useful when you have a memory mapped interface to a piece of hardware and need to be able to control exactly where the different struct members point. It is notably not a good speed optimization, since most machines are much faster at dealing with aligned data.

Answer 2

You'd likely only want to use this if you were coding to some hardware (e.g. a memory mapped device) which had strict requirements for register ordering and alignment.

However, this looks like a pretty blunt tool to achieve that end. A better approach would be to code a mini-driver in assembler and give it a C calling interface rather than fumbling around with this pragma.

Answer 3

#pragma pack instructs the compiler to pack structure members with particular alignment. Most compilers, when you declare a struct, will insert padding between members to ensure that they are aligned to appropriate addresses in memory (usually a multiple of the type's size). This avoids the performance penalty (or outright error) on some architectures associated with accessing variables that are not aligned properly. For example, given 4-byte integers and the following struct:

struct Test
{
   char AA;
   int BB;
   char CC;
};

The compiler could choose to lay the struct out in memory like this:

|   1   |   2   |   3   |   4   |  

| AA(1) | pad.................. |
| BB(1) | BB(2) | BB(3) | BB(4) | 
| CC(1) | pad.................. |

and sizeof(Test) would be 4 × 3 = 12, even though it only contains 6 bytes of data. The most common use case for the #pragma (to my knowledge) is when working with hardware devices where you need to ensure that the compiler does not insert padding into the data and each member follows the previous one. With #pragma pack(1), the struct above would be laid out like this:

|   1   |

| AA(1) |
| BB(1) |
| BB(2) |
| BB(3) |
| BB(4) |
| CC(1) |

And sizeof(Test) would be 1 × 6 = 6.

With #pragma pack(2), the struct above would be laid out like this:

|   1   |   2   | 

| AA(1) | pad.. |
| BB(1) | BB(2) |
| BB(3) | BB(4) |
| CC(1) | pad.. |

And sizeof(Test) would be 2 × 4 = 8.

Order of variables in struct is also important. With variables ordered like following:

struct Test
{
   char AA;
   char CC;
   int BB;
};

and with #pragma pack(2), the struct would be laid out like this:

|   1   |   2   | 

| AA(1) | CC(1) |
| BB(1) | BB(2) |
| BB(3) | BB(4) |

and sizeOf(Test) would be 3 × 2 = 6.

Answer 4

I have seen people use it to make sure that a structure takes a whole cache line to prevent false sharing in a multithreaded context. If you are going to have a large number of objects that are going to be loosely packed by default it could save memory and improve cache performance to pack them tighter, though unaligned memory access will usually slow things down so there might be a downside.

Answer 5

Note that there are other ways of achieving data consistency that #pragma pack offers (for instance some people use #pragma pack(1) for structures that should be sent across the network). For instance, see the following code and its subsequent output:

#include <stdio.h>

struct a {
    char one;
    char two[2];
    char eight[8];
    char four[4];
};

struct b { 
    char one;
    short two;
    long int eight;
    int four;
};

int main(int argc, char** argv) {
    struct a twoa[2] = {}; 
    struct b twob[2] = {}; 
    printf("sizeof(struct a): %i, sizeof(struct b): %i\n", sizeof(struct a), sizeof(struct b));
    printf("sizeof(twoa): %i, sizeof(twob): %i\n", sizeof(twoa), sizeof(twob));
}

The output is as follows: sizeof(struct a): 15, sizeof(struct b): 24 sizeof(twoa): 30, sizeof(twob): 48

Notice how the size of struct a is exactly what the byte count is, but struct b has padding added (see this for details on the padding). By doing this as opposed to the #pragma pack you can have control of converting the "wire format" into the appropriate types. For instance, "char two[2]" into a "short int" et cetera.