How to handle structure padding

This is a discussion on How to handle structure padding within the C Programming forums, part of the General Programming Boards category; I don't have a clear example for this. Say I had something like: Code: #include <stdio.h> struct packet { int ...

  1. #1
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    How to handle structure padding

    I don't have a clear example for this. Say I had something like:

    Code:
    #include <stdio.h>
    
    struct packet {
     int head;
     char tail;
    };
    
    int main(void) {
    
    struct packet route;
    
    return 0;
    }
    I know if I compile the code on Suse Linux 9.1 and then on FreeBSD 4.6, the structure would get padded differently. Would using memcpy() be a possible solution to get around the padding problem?

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by cdalten
    Would using memcpy() be a possible solution to get around the padding problem?
    What problem?

    [edit]http://c-faq.com/struct/padding.html
    Last edited by Dave_Sinkula; 03-17-2006 at 09:27 AM.
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  3. #3
    Frequently Quite Prolix dwks's Avatar
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    I think the OP is trying to do something with the structure that depends on the structure having the same padding, such as reading or writing the structure with fread() or fwrite().
    dwk

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    I was seeing if memcpy() would work when reading from the Unix Process Accounting Structure. Ie

    Code:
    typdef u_short comp_t;
    
    struct acct {
     char ac_flag;
     char ac_stat;
    
     uid_t ac_uid;
     gid_t ac_gid;
     dev_t tc_tty:
     time_t ac_btime;
     comp_t ac_utime;
     comp_t ac_stime;
     comp_t ac_etime;
     comp_t ac_mem;
     comp_t ac_io;
    
     comp_t ac_rw;
     char ac_comm[8];
    };
    Instead doing an fread() and risk alignment issues from one platform to another, could I use memcpy() to bypass it?

  5. #5
    ATH0 quzah's Avatar
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    You don't memcpy directly from disk. memcpy is for manipulating stuff loaded into memory.


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    Quote Originally Posted by quzah
    You don't memcpy directly from disk. memcpy is for manipulating stuff loaded into memory.


    Quzah.
    OKay, can you or someone clarify that stateent a bit.

  7. #7
    Just Lurking Dave_Sinkula's Avatar
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    I thought that was pretty straighforward.

    fread (essentially) grabs a bag of bytes from a file and puts it somewhere in memory.
    memcpy grabs a bag of bytes from one place in memory and puts it somewhere else in memory.

    So memcpy doesn't read from a file.


    Perhaps conditionally compile with appropriate pack directives for each compiler? Serialize and use a text file instead?
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    Okay. That makes sense.

  9. #9
    Guest Sebastiani's Avatar
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    most compilers accept:

    Code:
    /* align on 1 byte boundaries (no padding) */
    #pragma pack(push, 1)
    struct X { };
    #pragma pack(pop)
    Code:
    if( numeric_limits< byte >::digits != bits_per_byte )
        error( "program requires bits_per_byte-bit bytes" );
    24bbs.cpp

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