C Quiz Part II -- Pointers into Arrays

Q. What is the output of this program?

int f(int b[][2]){ return ++b[0][0]; }
int g(int b[][2]){ return ++b,b[0][0]; }
int h(int b[][2]){ return (++b,b[0][0]); }
int i(int b[][2]){ return (1+b,b)[0][0]; }
main(){
    int a[][2]={101,201,301,401};
    printf("f=%d\n",f(a)); 
    printf("g=%d\n",g(a)); 
    printf("h=%d\n",h(a)); 
    printf("i=%d\n",i(a)); 
}    

Further reading.

C Quiz Part I

Q1. What is the output of this program:

main(){
  unsigned  int   nnn = -3;
  signed    int   ppp =  3;
  if( nnn >= ppp )
     printf("%d\n", nnn, ppp); // -3 >= 3
}  

Hint: Look up promotions.

Q2. What is the output of this program? and why

main(){
  int x=1,y=1;
  printf("s=%d\n",sizeof(printf("x=%d, y=%d\n",++x,++y)));
}  

Hint: Lookup Evaluation time of sizeof operator.

Q3. What about this?

main(){
  int x=1,y=1;
  printf("s=%d\n",printf("x=%d, y=%d\n",++x,++y));
}                                                                        

Q4. And this one

void main(){
  int x=1, y=100;
  x,y = y,x; // swap numbers, python style.
  printf("x=%d, y=%d\n",x,y);
}        

Q5. And this one:

void main(){
  int x=1, y=100;
  (x,y) = (y,x);
  printf("x=%d, y=%d\n",x,y); // swap number, perl style
}        

Put your answers as comments to this blog. Next check your answers with gcc, by compiling and running each of the programs. Windows users can download cygwin for gcc (the cygwin setup on windows is a breeze, and you get bash too). For explanations, see these two books: C Programming Language by Kernighan and Ritchie (aka KNR, K&R) C Reference Manual by Harbison and Steele Now try this excellent quiz by Pathak.

Dictor.py program to annotate meanings.

> cat dictor.py # text2html by vim63

 # GPL(C)
# Annontate text with meanings of words in tooltips, or links to lookup the word.
# usage: python $0 infile.txt > outfile.html

import os
import sys
import string
import commands
import re

if len(sys.argv)>1 and os.path.isfile(sys.argv[1]):
    pass
else:
    print "Usage: dictor.py infile > outfile.html"
    sys.exit()

# Read ascii dictionary
dict = os.path.expandvars('$DOC/dict/dict.txt')
meanings = {}
if os.path.isfile(dict):
    f = open(dict, 'r')
    while 1:
        line = f.readline()
        if not line:
            break
        if not line.find('--'):
            continue
        line = line.rstrip("\n\r")
        try:
            word,meaning = line.split('--')
        except ValueError:
            continue
        for worda in word.split(','):
            meanings[worda.lower()] = meaning.strip()
    f.close()

# Read the text file and annotate it from the dictionary or link to web.
splitter = re.compile('([^a-zA-Z]+)')
for infile in sys.argv[1:]:
    if not os.path.isfile(infile):
        print 'not a file', infile
        continue
    else:
        print 'reading', infile
    i = open(infile,'r')
    while 1:
        line = i.readline()
        if not line:
            break
        line = line.rstrip("\n\r")
        for word in splitter.split(line):
            if meanings.has_key(word.lower()):
                print '<a href="%s">%s</a>' % ( meanings[word.lower()], word )
            elif splitter.search(word):
                word = word.replace('&','&amp;')
                word = word.replace('<','&lt;')
                word = word.replace('>','&gt;')
                word = word.replace('~','&ntilde;')
                word = word.expandtabs()
                word = word.replace(' ',' ')
                print '%s' % word,
            else:
                print '<a href="http://www.google.com/search?q=%s">%s</a>' % ( word, word )
        print '<br>'
    i.close()
pass

Output of ana.py is a list of anagrams

Only anagrams of words longer than 17 characters are shown to avoid blogbloat.

basiparachromatin	marsipobranchiata
anatomicopathological	pathologicoanatomical
glossolabiopharyngeal	labioglossopharyngeal
glossolabiolaryngeal	labioglossolaryngeal
anatomicopathologic	pathologicoanatomic
laryngopharyngeal	pharyngolaryngeal
mechanicochemical	chemicomechanical
phrenicopericardiac	pericardiacophrenic
siliceocalcareous	calcareosiliceous
clinicopathological	pathologicoclinical
duodenopancreatectomy	pancreatoduodenectomy
myxochondrosarcoma	chondromyxosarcoma
magnetoelectrical	electromagnetical
anatomicophysiologic	physiologicoanatomic
acetmethylanilide	methylacetanilide
octakishexahedron	hexakisoctahedron
pericardiopleural	pleuropericardial
tracheolaryngotomy	laryngotracheotomy
tetrakishexahedron	hexakistetrahedron
galvanothermometer	thermogalvanometer
esophagogastrostomy	gastroesophagostomy
hysterolaparotomy	laparohysterotomy
laryngopharyngitis	pharyngolaryngitis
meningoencephalocele	encephalomeningocele
psychoneurological	neuropsychological
chronophotographic	photochronographic
physiopsychological	psychophysiological
microphotographic	photomicrographic
pneumohydropericardium	hydropneumopericardium
polygamodioecious	dioeciopolygamous
meningoencephalitis	encephalomeningitis
encephalomyelitis	myeloencephalitis
cephalomeningitis	meningocephalitis
chromophotolithograph	photochromolithograph
lithochromography	chromolithography
photochromography	chromophotography
photochronography	chronophotography
misconstitutional	constitutionalism
epididymovasostomy	vasoepididymostomy
hydropneumothorax	pneumohydrothorax
representationism	misrepresentation
cerebromeningitis	meningocerebritis
incontrovertibility	introconvertibility
spectromicroscope	microspectroscope
cholecystoduodenostomy	duodenocholecystostomy
chorioidoretinitis	retinochorioiditis
thermoelectrometer	electrothermometer
myomohysterectomy	hysteromyomectomy

Finding anagrams with ana.py

# WHAT: generate anagrams from a list of words.
# GPL(C)
import os
import sys
import commands
import string
import re
wf = os.path.expandvars('$DOC/dict/words')
if not os.path.isfile(wf) :
    print "Cannot read wf=",wf
    sys.exit()
f = open(wf,'r')
anagrams = {}
count = 0
while 1:
    word = f.readline()
    if not word:
        break
    word = word.rstrip("\n\r").lower()
    if len(word) <= 2: # skip trivial anagrams.
        continue
    charlist = list(word)
    # charlist.sort(lambda x, y : cmp( string.lower(x), string.lower(y)) )
    charlist.sort()
    sword = string.join( charlist, '' )
    if not anagrams.has_key(sword):
        anagrams[sword] = set()
    anagrams[sword].add(word)
    count += 1

f.close()

key = anagrams.keys()
key.sort()
acount = 0
for sword in key:
    anas = anagrams[sword]
    if len(anas) <= 1:
        continue
    acount = acount + 1
    for word in anas:
        print word,
    print

print 'Found %d anagrams in %d words from %s' % (acount,count,wf)

sys.exit()

Turbo C Source code for Instant Insanity Game

/* 
  What: Instant Insanity Game source in Turbo C. GPL(C)
   =======================
   Col1        Col4*4
    3   3   3   3     Row1
   405 405 405 405    Row2
    1   1   1   1     Row3
    2   2   2   2     Row4
   =======================
    c:\mosh\games>tcc -If:\tcc30\include -Lf:\tcc30\lib instanti.cpp
*/

#include "cubeinit.cpp"

int cube[4][6];  /* four cubes with six faces */

enum { W,R,G,B } virtual_colors;
static int screen_colors[5] = {WHITE, RED,GREEN,BLUE,YELLOW};

void setpuzzle(int p){
    int i,k;
    static int puzzle[1][4][6] = {
        {{W,W,W,G,R,B},{G,W,G,R,B,R},{W,W,B,R,G,B},{B,W,G,G,B,R}},
    };
    for(i=0;i<6;i++)
        for(k=0;k<4;k++)
            cube[k][i]=puzzle[p%(sizeof(puzzle)/sizeof(cube))][k][i];
}

int sameface(int cube[4][6]){ /* return 0, if all sides have different colors */
    int i,j,k,dup=0;
    for(i=0;i<4;i++)
      for(j=i+1;j<4;j++)
        for(k=0;k<4;k++)
            dup += (cube[i][k]==cube[j][k]);
    return dup;
}

void putcharc(int row, int kol, int c){
    int t;
    textcolor(screen_colors[c]);
    mvaddch(row,kol,'0'+c);
}

void drawcubes(int cc){
    int i;
    clrscr();
    for(i=0;i<4;i++){
        putcharc(4+i*4   ,1,cube[i][3]);
        putcharc(4+i*4   ,2,cube[i][0]);
        putcharc(4+i*4-1 ,3,cube[i][4]);
        putcharc(4+i*4+1 ,3,cube[i][5]);
        putcharc(4+i*4   ,3,cube[i][1]);
        putcharc(4+i*4   ,4,cube[i][2]);
        if(i==cc)
            putcharc(4+i*4,5,4);
    }
}

void upcube(int cc){
    int t=cube[cc][3];
    cube[cc][3]=cube[cc][0]; cube[cc][0]=cube[cc][1];
    cube[cc][1]=cube[cc][2]; cube[cc][2]=t;
}
void rocube(int cc){
    int t=cube[cc][3];
    cube[cc][3]=cube[cc][4]; cube[cc][4]=cube[cc][1];
    cube[cc][1]=cube[cc][5]; cube[cc][5]=t;
}
void main(){
    chtype ch=' ';
    int cc=0; /* current cube to operate on */
    initscr(); cbreak(); noecho(); refresh();
    setpuzzle(0);
    drawcubes(cc);

    do{
        textcolor(FGCOL); mvaddch(24,0,':');
        clrtoeol(); mvaddch(24,1,ch); ch = mvgetch(24,2); clrtoeol();
        amessage("ok.");
        switch(ch){
        case 'L': if(--cc<0)cc=3; break;
        case 'R': if(++cc>3)cc=0; break;
        case 'l': upcube(cc); break;
        case 'r': upcube(cc); upcube(cc); upcube(cc); break;
        case 'u': rocube(cc); break;
        case 'd': rocube(cc);  rocube(cc);  rocube(cc); break;
        case '1': cc=0; break;
        case '2': cc=1; break;
        case '3': cc=2; break;
        case '4': cc=3; break;
        }
        if( sameface(cube) == 0 ){
            printf("Done\n");
        }
        drawcubes(cc);
    }while(ch!='q');
    clear(); refresh();
    endwin();
}

Set of Sets in STL

 // WHAT: print set of set in C++ STL with generate_allsubets example.
 // file setofsets.cpp
 // Testing VC6.

 #include <set>
 #include <iostream>
 #include <algorithm>
 using namespace std;

 #pragma warning(disable:4786) // warning: debug identifier trunc to 255.

 typedef set< set<int> > tsos;
 typedef set<int> ts;

 void generate_allsubets( tsos& sos, ts& choices ){
     if( choices.size() < 1 ) return; // end of recursion.
     if( sos.size() < 1 )
         sos.insert(*new set<int>); // insert emptyset first time.
     int c1 = *(choices.begin());
     // make a copy of sos, in each subset insert c1.
     tsos sos_c1;
     for( tsos::iterator p = sos.begin(); p != sos.end(); ++p ){
         ts s_c1 = *p; // make a copy of the subset, then insert c1.
         s_c1.insert(c1);
         sos_c1.insert(s_c1);
     }
     // we have two sets of subset now: sos_c1 with c1, original sos without c1.
     // merge sos_c1 into sos to double the size of sos.

     #if 0 // USE set_union
     tsos both;
     set_union( sos.begin(), sos.end(), sos_c1.begin(), sos_c1.end(), 
                         insert_iterator<tsos>(both,both.begin()) );
     sos = both;
     #else
     for( tsos::iterator pc1 = sos_c1.begin(); pc1 != sos_c1.end(); ++pc1 )
         sos.insert( *pc1 );
     #endif

     // Recursively insert remaining choices.
     if( choices.erase(c1) > 0 ) // should always be able to erase c1.
         generate_allsubets( sos, choices );
 }

 void print_allsubsets( tsos& sos ){
     int i=0;
     for( tsos::iterator p = sos.begin(); p != sos.end(); ++p ){
         cout << "subset " << ++i << " = { ";
         for( ts::iterator q = p->begin(); q != p->end(); ++q )
              cout << *q << ", ";
         cout << "}\n";
     }
 }

 int main( int argc, char *argv[] ){
     set< set<int> > sos;
     set< int > choices;
     for(int i=0;i<3;i++)
         choices.insert(i);
     generate_allsubets( sos, choices );
     cout << "The "<< sos.size() << " subsets of {0.."<<i-1<<"} are\n";
     print_allsubsets( sos );
     return 0;
 }
 

64Bit C Programmer's Tips, Tricks and Traps

 P64 Gotcha List
 ----------------

 1. sizeof long array can overflow on lp64.

   uchar n = sizeof( 50*sizeof(long)); // 200 bytes in ilp32, 400 bytes in lp64.

 2. Use u U L l suffix on large const.

   long l = 1 <<32; // 0 on ilp32,lp64
   long l = 1L<<32; // 2^32 on lp64, 0 on ilp32.

   Solaris: see /usr/include/sys/inttypes.h

   INT64_C(5)  => 5ll for ILP32, and 5l for LP64.

   0x80000000L  ulong in lp32,  long in lp64.

 3. Bit fields are converted to int and sign-extended (or uint if too large) in expr.
   struct { uint b:19, r:13 }x;
   uint y = x.b<<13 // x.b first converted to int and sign extended!
   uint y = (uint)x.b << 13; // ok.

 4. Integer Arithmetic on ptr, ptr is trucated to int32.

   char *p="x";
   (int)p+1=='\0' // p is trunc to int32.
   (uintptr_t)p+1=='\0' // ok.

 5. Unprototyped func returning ptr is trucated to int32.

   // int login(...); -- implicit C decl.
   char *p = getenv("HOME");  // p is trucated to int32, crash.

 6. Saving ptr in int, losses upper 4 bytes of ptr.

   void *q;
   int p = (int)q; // truncated to int32

 7. Unions with long.

   union {
     long x;
     int  y; // on lp64, y gets lsb of x.
   }

 8. Print format specifier

   char *p="x";
   printf("p=0x%08x",p); // losses msb.
   printf("p=%p",   p); // ok.

   long x=LONG_MAX;

   printf("x=%d ",x); // truncated, wrong.
   printf("x=%ld",x); // ok.

 9. Casting *int into *long.

   int  *i;
   long *p;
   p+1; // p+=4 on ilp32
   p+1; // p+=8 on  lp64
   p = (long*) i; // p misaligned pointing to *i and *(i+1).
   i = ( int*) p; // i pointing to lsb of p.

 10. Unsigned bit fields (default sign and alignment).

   struct { b:12, int:0, d:5 } x;  // b is unsigned in lp64, signed
 in ilp32.
   // :0 affects alignment in ilp32, no effect in lp64.

 11. LP64 Perf Problems

   Larger exe, need more TLB, cache, mem (wrt same src compiled as ILP32).
   Long div slow in lp64, eg. 1L/9.
   Int Array indexes are sign-ext on every use, need more instruction in LP64, eg. int i; a[i];

 12. Expression sizing:

  Example.


   int x,y;
   long z = x + y;  // x+y done in 32bit, then assignment will resize result for z.

  Example.

   ulong y = (1 <<32); // overflows in lp32 and lp64.
   ulong y = (1L<<32); // overflows in lp32, ok 2^32 in lp64.

 13. Promotions:

   result = a op b;

   // 1. type of tmp is of max of a and b.
   // 2. tmp = a op b
   // 3. promote tmp to type of result, ptmp.
   // 4. result = ptmp;

  Example.

   long l=-1; uint i=1;
   if( l > i ) printf("ilp32"); // -1 is promoted to uint32.
   else     printf("lp64");  // both promoted to signed 64.

   if( l > (long)i ) // fix
     ...;

  Example.

   main(){  // k is -1 in LP32, 4294967295 in LP64
     int i = -2;
     unsigned int j = 1;
     long k = i+j;
     printf("int %d+ uint %ld= long k is %ld\n",i,j,k);
   }

 15. Prototype:

   // #include <stdlib.h> missing.
   int *i = malloc(sizeof(*i));
   // Ok in lp32, Crash on lp64 because
   // malloc will return 32bit int value because of missing prototype.

 }

 ////////////////////////////////////////////////////



 Examples

  > cc +DA2.0W +w1 +M2 enquire.c    # +w1 for warnings, +M2 for 64bits

  long double=128 bits

   # Sort LP64 warning by problem:


   > make 2>&1 | grep LP64 | xsort 'warning:\s+\S+:' | align warn > lp64.log
   > vim lp64.log
   :set isk+=46  " Make . part of tag, so file.c is a keyword for tags"

   > emacs lp64.log

   > M-x compilation-minor-mode
   > M-x next-error
   ; if cant find files, set
   ; (setq compilation-search-path '("dir1" "dir2")) C-xC-e

 /714: Function "malloc" called with no prototype or definition in scope
 /724: Initialization converts default int return type to pointer
 /722:.*cast converts 32 bit constant to pointer
 /725:.*cast converts 32 bit integer to pointer
 /727:.*cast truncates pointer into 32 bit integer
 /729:.*converts int* to long*
 /732:.*different types treated as unsigned
 /740:.*casting to a smaller size may truncate
 /530:.*casting from loose to strict alignment
 /720:.*Assignment may overflow

 // Example

 > catt -n x.c

 1  void main(){
 2   int *x, *q, i;
 3   long l=-1;
 4   x = malloc(sizeof(int)*10);
 5   (int*)i;
 6   (int)  &i;
 7   (long*)&i;
 8   i = l;
 9  }

 hp64> cc  +DA2.0W +M2 +w1 -Aa x.c

 /*
 line 4: warning 714: Function "malloc" called with no prototype or definition in scope.
 line 4: warning 724: LP64 migration: Assignment converts default int return type to pointer "x".
 line 5: warning 725: LP64 migration: Cast converts 32 bit integer to pointer.
 line 6: warning 727: LP64 migration: Cast truncates pointer into 32 bit integer.
 line 7: warning 530: LP64 migration: Casting from loose to strict alignment: Resulting pointer may be misaligned.
 line 7: warning 729: LP64 migration: Cast converts int* to long*.
 line 8: warning 720: LP64 migration: Assignment may overflow integer "i".
 */

 /////////////////////////////////////////////////

 A. Sizes
     Ansi sizeof() is always unsigned, try:
     if( 0-sizeof(int) > 0 ) printf("See: 0-sizeof(int) is unsigned!\n");

   ILP32: win32 hp11 solaris linux

   ANSI C Integer Requirements, see C book.  moshtag=ansi_int

     sizeof(int) >= sizeof(short)  >= 2.
     sizeof(long) >=sizeof(int)
     sizeof(long) >= 4.

   LP64:  hp11_64 solaris_64 (assume default for 64bit).

     New: sizeof(long)==sizeof(void*)==8, rest unchanged.

   P64:  win64

     New: sizeof(void*)==8, rest unchanged.

 //////////////////////////////////////////////////////////

 Linux64                       moshtag=linux64

   char = 8 bits, signed
   short=16 int=32 long=64 float=32 double=64 bits
   long double=128 bits
   Type size_t is unsigned long
   ALIGNMENTS
     char=1 short=2 int=4 long=8 float=4 double=8 long double=16
     char*=8 int*=8 func*=8
   short: BA
   int:  DCBA
   long:  HGFEDCBA
   Maximum long = 9223372036854775807 (= 2**63-1)
   Minimum long = -9223372036854775808
   Maximum unsigned short = 65535
   Maximum unsigned int = 4294967295
   Maximum unsigned long = 18446744073709551615
   PROMOTIONS
     unsigned short promotes to signed int
     long+unsigned gives signed long

 HP64 ALIGNMENTS

   char=1 short=2 int=4 long=8 float=4 double=8 long double=16
   char*=8 int*=8 func*=8