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Byte Swaping a structure

Posted on 2003-12-02
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Last Modified: 2008-02-01
I would like to set data elements to a struction and then byte swap the entire structure.
The part that makes it difficult is the struction contains elements of diffrent sizes.  Below is an example of a data structure that uses shorts, ints, and chars all in the same structure.
I do not have to perform any word swapping just byte swapping.

Is there some inherent function in C++ like the _swab function that performs this.  If not does someone have an example of how to do this. Once the structure is swapped it will be written to a file.


#pragma pack(1)//eliminate padding in structs
typedef struct {
      short            IDMSAReportingMode : 2;
      short            IDMSAReportingSpare : 1;
      short            IDMSATimeFilter : 13;
      short            IDMSAMotionSpare : 4;
      short            IDMSAMotionFilter : 12;
      int            IDMStaleFilterDurationSpare : 13;
      int            IDMStaleFilterDuration : 19;
      int            IDMOldFilterDurationSpare : 13;
      int            IDMOldFilterDuration : 19;
      int            IDMPurgeFilterDurationSpare : 13;
      int            IDMPurgeFilterDuration : 19;
      short            IDMAutoResponseModeSpare : 6;
      short            IDMAutoResponseMode : 2;
      short            IDMChannelTransFlagSpare : 6;
      short            IDMChannelTransFlag : 2;
      char            IDMFSConfigFile[6];
      short            IDMTIInterfaceSpare : 6;
      short            IDMTIInterfaceNum : 2;
      short            IDMTICMSAPortNumSpare : 6;
      short            IDMTICMSAPortNum : 2;
      short            IDMFSInterfaceNumSpare : 6;
      short            IDMFSInterfaceNum : 2;
      short            IDMFSCMSAPortSpare : 6;
      short            IDMFSCMSAPort : 2;
      int            IDMEBC_OWN_URN_Spare : 9;
      int            IDMEBC_OWN_URN : 23;
      int            IDMMulticast_URN_Spare : 9;
      int            IDMMulticast_URN : 23;
      char            IDMTIPassword[8];
      short            IDMFIPRRegNetworkMonitoring : 1;
      short            IDMFIPRRegSystemCoord : 1;
      short            IDMFIPRRegSecurityMsg : 1;
      short            IDMFIPRRegFreetext : 1;
      short            IDMFIPRRegUnitRefQuery : 1;
      short            IDMFIPRRegInfoRequest : 1;
      short            IDMFIPRRegObserverMsnUpdate : 1;
      short            IDMFIPRRegMsgToObserver : 1;
      short            IDMFIPRRegAirborneFireMsn : 1;
      short            IDMFIPRRegBasicWeatherRprt : 1;
      short            IDMFIPRRegMaydayMsg : 1;
      short            IDMFIPRRegSpot_SaluteRprt : 1;
      short            IDMFIPRRegObstacleRprt : 1;
      short            IDMFIPRRegPosRprt : 1;
      short            IDMFIPRRegNBC3Rprt : 1;
      short            IDMFIPRRegREDCONRprt : 1;
      short            IDMFIPRRegThreatWarning : 1;
      short            IDMFIPRRegSitrep : 1;
      short            IDMFIPRRegMOPP : 1;
      short            IDMFIPRRegSupplyPointStatus : 1;
      short            IDMFIPRRegTaskMgmt : 1;
      short            IDMFIPRSpare : 11;
      char            IDMEBC_UTO[64];

} IDM_OPERATOR_CONFIG_TYPE;
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Question by:LBoorn
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3 Comments
 
LVL 86

Expert Comment

by:jkr
ID: 9861229
>>Is there some inherent function in C++ like the _swab function that performs this

'_swab()' works like

/* SWAB.C illustrates:
 *      _swab
 */

#include <stdlib.h>
#include <stdio.h>

char from[] = "BADCFEHGJILKNMPORQTSVUXWZY";
char to[] =   "..........................";

void main()
{
    printf( "Before:\t%s\n\t%s\n\n", from, to );
    _swab( from, to, sizeof( from ) );
    printf( "After:\t%s\n\t%s\n\n", from, to );
}
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LVL 86

Expert Comment

by:jkr
ID: 9861263
BTW, as I can see, you don *NOT* want to use '_swab()' at all - if your goal is to put the above in non-intel byte order, just swapping the byte is not enough, you need to use the appropriate functions on the bitfield members.
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LVL 86

Accepted Solution

by:
jkr earned 2000 total points
ID: 9861291
OK, here we go

#include <winsock.h> // endianity support
#pragma comment ( lib, "ws2_32.lib")

typedef struct {
    short          IDMSA;
    int                     IDMFilter;
    short          IDMShort1;
    char           IDMFSConfigFile[6];
    short          IDMShort2;
    int            IDMInt;
    char           IDMTIPassword[8];
    short          IDMShort3;
    char          IDMEBC_UTO[64];

} IDM_OPERATOR_CONFIG_TYPE_HELPER;

IDM_OPERATOR_CONFIG_TYPE idm; // original struct
IDM_OPERATOR_CONFIG_TYPE_HELPER* p = &idm; // helper needed fow swapping

p->IDMSA= htons ( p->IDMSA);
p->IDMFilter = htonl ( p->IDMFilter);
p->IDMShort1 = htons ( p->IDMShort1);
p->IDMShort2 = htons ( p->IDMShort2);
p->IDMInt = htonl ( p->IDMInt);
p->IDMShort3 = htons ( p->IDMShort2);

This will avoid the byte swapping for the character members, which certainly is somethig you wouldn't want to happen :o)
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