Info Catalog web2c: Determining the memory dump to use web2c: Memory dumps

web2c: Hardware and memory dumps

 
 4.3.3 Hardware and memory dumps
 -------------------------------
 
 By default, memory dump files are sharable between architectures of
 different types; specifically, on machines of different endianness
 (⇒(libc)Byte order) and with different word sizes (4-byte 'long'
 vs. 8-byte 'long').  This is a feature of the Web2c implementation, and
 is not true of all TeX implementations.
 
    The script 'tl-check-fmtshare' in the TeX Live source tree
 ('Master/tlpkg/bin') provides a relatively easy way to test that a
 '.fmt' built on the local host can be loaded by a TeX engine built on
 some remote host.
 
    If you specify '--disable-dump-share' to 'configure', however, memory
 dumps will be endian-dependent.  The reason to do this is speed.  To
 achieve endian-independence, the reading of memory dumps on LittleEndian
 architectures, such as PC's and DEC architectures, is somewhat slowed
 (all the multibyte values have to be swapped).  Usually, this is not
 noticeable, and the advantage of being able to share memory dumps across
 all platforms at a site far outweighs the speed loss.  But if you're
 trying to squeeze out every possible bit of performance, you may wish to
 do this.
 
    TeXnically, even without '--disable-dump-share', sharing of '.fmt'
 files cannot be guaranteed to work.  Floating-point values are always
 written in native format, and hence will generally not be readable
 across platforms.  Fortunately, TeX uses floating point only to
 represent glue ratios, and none of the common formats (plain, LaTeX,
 AMSTeX, ...) do any glue setting at '.fmt'-creation time.  Metafont does
 not use floating point in any dumped value at all.
 
    Incidentally, different memory dump files will never compare equal
 byte-for-byte, because the programs dump the current date and time.  So
 don't be alarmed by a few bytes difference.
 
    If you don't know what endianness your machine is, and you're
 curious, here is a little C program to tell you.  (The 'configure'
 script contains a similar program.)  This is from the book 'C: A
 Reference Manual', by Samuel P. Harbison and Guy L. Steele Jr.  (⇒
 References).
 
      main ()
      {
        /* Are we little or big endian?  From Harbison&Steele.  */
        union
        {
          long l;
          char c[sizeof (long)];
        } u;
        u.l = 1;
        if (u.c[0] == 1)
          printf ("LittleEndian\n");
        else if (u.c[sizeof (long) - 1] == 1)
          printf ("BigEndian\n");
        else
          printf ("unknownEndian");
 
        exit (u.c[sizeof (long) - 1] == 1);
      }
 
    You can add 'printf("long %d\n", sizeof(long));' to see the size of
 the 'long' data type.
 

Info Catalog web2c: Determining the memory dump to use web2c: Memory dumps