...pack C structures [ARM]?

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Paul Curtis

• July 13, 2009 12:11

I have a structure containing a number of other structures. When I look at how the structure has been laid out in memory it seems to be using too much memory. For example in the following code, type c_t is 8 bytes in length and there are 3 bytes of wasted space between a and b:

typedef struct
{
  unsigned char a[1];
} a_t;

typedef struct
{
  unsigned char b[2];
} b_t;

typedef struct
{
  a_t a;
  b_t b;
} c_t;

How can I make structure c_t occupy as little memory as possible?

You can used the packed variable attribute. For example in the following code, type c_t will have a size of 3 bytes and there will be no wasted space between a and b:

typedef struct
{
  unsigned char a[1];
} __attribute__ ((packed)) a_t;

typedef struct
{
  unsigned char b[2];
} __attribute__ ((packed)) b_t;

typedef struct
{
  a_t a;
  b_t b;
} __attribute__ ((packed)) c_t;

2

• 

  John Russell

  • December 29, 2009 03:44

  I tried the __attribute__ ((packed)) method and got compile errors like

  found '(', expecting ';'

  and ! missing prototype

  when using the MSP430 version, so am I missing an include or similar?

  #pragma pack(1) ............... #pragma pack() is a commonly used pack definition method (Visual C, IAR)

   

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• 

  John Russell

  • December 29, 2009 03:48

  Btw, I'm using the Windows 30 day evaluation version with the pack problem.

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• 

  Paul Curtis

  • December 29, 2009 11:25

  I've updated the title to show this is for ARM only.

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• 

  Andy Heilveil

  • January 05, 2010 23:07

  the packed attribute has to be applied to data allocations, you can't apply it to a typedef.

  IE you have to remember to apply it to all instances of a type if you want them to be byte-identical.

  typedef struct {
    a_t a;
    b_t b;
  }  c_t;
  c_t  anInstanceOfC __attribute__ ((packed));
  
  

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• 

  Paul Curtis

  • January 05, 2010 23:24

  ...you apply the packed attribute to a structure (or enumeration).  I use it for a number of things, e.g.

  typedef struct
  {
    unsigned short tcpSrcPort; // network bo
    unsigned short tcpDstPort; // network bo
    unsigned long tcpSeqNo; // SEND data pointer, network bo
    unsigned long tcpAckNo; // Next expected RECEIVE data pointer, network bo
    unsigned char tcpDataOfst;
    unsigned char tcpFlags;
    unsigned short tcpWindow; // how much data we will accept, network bo
    unsigned short tcpChecksum;
    unsigned short tcpUrgent;
    unsigned char tcpOptions[4];
  } __attribute__((packed)) tcpHeader;
  

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