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   I checked out the MSDN page  which offered some insight  but I was hoping to hear more from people with experience  I ve seen it in code before  though I can t seem to find where anymore

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I ve used it in code before  though only to interface with legacy code  This was a Mac OS X Cocoa application that needed to load preference files from an earlier  Carbon version  which was itself backwards-compatible with the original M68k System 6 5 version   you get the idea   The preference files in the original version were a binary dump of a configuration structure  that used the  pragma pack 1  to avoid taking up extra space and saving junk  i e  the padding bytes that would otherwise be in the structure    The original authors of the code had also used  pragma pack 1  to store structures that were used as messages in inter-process communication  I think the reason here was to avoid the possibility of unknown or changed padding sizes  as the code sometimes looked at a specific portion of the message struct by counting a number of bytes in from the start  ewww

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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

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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

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A compiler may place structure members on particular byte boundaries for reasons of performance on a particular architecture   This may leave unused padding between members   Structure packing forces members to be contiguous     This may be important for example if you require a structure to conform to a particular file or communications format where the data you need the data to be at specific positions within a sequence   However such usage does not deal with endian-ness issues  so although used  it may not be portable   It may also to exactly overlay the internal register structure of some I O device such as a UART or USB controller for example  in order that register access be through a structure rather than direct addresses

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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

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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  lt stdio h gt   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

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Data elements  e g  members of classes and structs  are typically aligned on WORD or DWORD boundaries for current generation processors in order to improve access times  Retrieving a DWORD at an address which isn t divisible by 4 requires at least one extra CPU cycle on a 32 bit processor  So  if you have e g  three char members char a  b  c   they actually tend to take 6 or 12 bytes of storage     pragma allows you to override this to achieve more efficient space usage  at the expense of  access speed  or for consistency of stored data between different compiler targets  I had a lot of fun with this transitioning from 16 bit to 32 bit code  I expect porting to 64 bit code   will cause the same kinds of headaches for some code

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Why one want to use it    To reduce the memory of the structure  Why one should not use it     This may lead to performance penalty  because some system works better on aligned data Some machine will fail to read unaligned data Code is not portable

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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  times  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  times  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  times  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

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It tells the compiler the boundary to align objects in a structure to  For example  if I have something like   struct foo        char a      int b       With a typical 32-bit machine  you d normally  want  to have 3 bytes of padding between a and b so that b will land at a 4-byte boundary to maximize its access speed  and that s what will typically happen by default     If  however  you have to match an externally defined structure you want to ensure the compiler lays out your structure exactly according to that external definition  In this case  you can give the compiler a  pragma pack 1  to tell it not to insert any padding between members -- if the definition of the structure includes padding between members  you insert it explicitly  e g   typically with members named unusedN or ignoreN  or something on that order

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Compiler could align members in structures to achieve maximum performance on the certain platform   pragma pack directive allows you to control that alignment  Usually you should leave it by default for optimum performance  If you need to pass a structure to the remote machine you generally will use  pragma pack 1 to exclude any unwanted alignment

[c] #pragma pack effect

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