Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>The C FAQ has some examples of historical machines with non-0 NULL representations.</p> <p>From <a href="http://c-faq.com/" rel="noreferrer">The C FAQ List</a>, <a href="http://c-faq.com/null/machexamp.html" rel="noreferrer">question 5.17</a>:</p> <blockquote> <p>Q: Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?</p> <p>A: The Prime 50 series used segment 07777, offset 0 for the null pointer, at least for PL/I. Later models used segment 0, offset 0 for null pointers in C, necessitating new instructions such as TCNP (Test C Null Pointer), evidently as a sop to [footnote] all the extant poorly-written C code which made incorrect assumptions. Older, word-addressed Prime machines were also notorious for requiring larger byte pointers (char *'s) than word pointers (int *'s).</p> <p>The Eclipse MV series from Data General has three architecturally supported pointer formats (word, byte, and bit pointers), two of which are used by C compilers: byte pointers for char * and void *, and word pointers for everything else. For historical reasons during the evolution of the 32-bit MV line from the 16-bit Nova line, word pointers and byte pointers had the offset, indirection, and ring protection bits in different places in the word. Passing a mismatched pointer format to a function resulted in protection faults. Eventually, the MV C compiler added many compatibility options to try to deal with code that had pointer type mismatch errors.</p> <p>Some Honeywell-Bull mainframes use the bit pattern 06000 for (internal) null pointers.</p> <p>The CDC Cyber 180 Series has 48-bit pointers consisting of a ring, segment, and offset. Most users (in ring 11) have null pointers of 0xB00000000000. It was common on old CDC ones-complement machines to use an all-one-bits word as a special flag for all kinds of data, including invalid addresses.</p> <p>The old HP 3000 series uses a different addressing scheme for byte addresses than for word addresses; like several of the machines above it therefore uses different representations for char * and void * pointers than for other pointers.</p> <p>The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric pointers; it uses the pair (basically a nonexistent handle) as a C null pointer.</p> <p>Depending on the ``memory model'' in use, 8086-family processors (PC compatibles) may use 16-bit data pointers and 32-bit function pointers, or vice versa.</p> <p>Some 64-bit Cray machines represent int * in the lower 48 bits of a word; char * additionally uses some of the upper 16 bits to indicate a byte address within a word.</p> </blockquote>
    singulars
    1. This table or related slice is empty.
    1. POWhen was the NULL macro not 0?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USBrendan Long
    1. USjanks
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POWhen was the NULL macro not 0?
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    1. CO+1 for finding the solid examples. FWIW, I think ISO C now mandates that 0 in the source code means null pointer no matter the underlying representation used. But that's from memory and I don't have a copy of C1x floating around at the moment.
      singulars
      1. PO
      1. USpaxdiablo
    2. COExactly what I was looking for. Thank you.
      singulars
      1. PO
      1. USGyörgy Andrasek
    3. COIn C source code the NULL macro would still be either an integer constant expression that evaluates to 0 or a (void *) cast of the same. The resulting pointer value might have a representation that isn't 0 but that doesn't mean the source code can assume it will be 0xffff or anything like that.
      singulars
      1. PO
      1. USWindows programmer
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload