Software 1 - Bitwise operations, fork, & pipes(9)

administrivia

Web Page - http://www.cs.tau.ac.il/~efif/courses/Software1_Summer_03

Riddle

• Find three ways to change one character in the code bellow so that the resulting code will print 20 times the minus sign ('-').
  • Remember, you can change one character only for each solution.

  int i, n=20; for (i = 0; i < n; i--) printf("-");

Student Accounts

• bit-wise operations usage for flags handling
  • accounts.c

The Dining Philosophers - the setup

• Five philosophers are seated around a round table
• Each philosopher has a bowl of rice in front
• Between each philosopher and his neighbor is a chopstick
• Each philosopher thinks for a while, then gets hungry and wants to eat
• Eating rice requires 2 chopsticks
  • however there are only five chopsticks
  • at most 2 philosophers can be eating at the same time
• A philosopher may acquire only the chopsticks to his immediate left or right
• Once a philosopher possesses two chopsticks, he will eat for a while, then put both chopsticks back down on each side

The Dining Philosophers - the problem

• Find a solution so that
  • deadlock does not occur
  • each philosopher gets to eat fairly often

The Dining Philosophers - the code

• diners.c

A program that prints out it's own code?

• wio2.c

  char s[] = {0x23, 0x69, 0x6e, 0x63, 0x6c, 0x75, 0x64, 0x65, 0x20, 0x3c, 0x73, 0x74, 0x64, 0x69, 0x6f, 0x2e, 0x68, 0x3e, 0xa, 0x69, 0x6e, 0x74, 0x20, 0x6d, 0x61, 0x69, 0x6e, 0x28, 0x29, 0xa, 0x7b, 0xa, 0x20, 0x20, 0x69, 0x6e, 0x74, 0x20, 0x69, 0x3b, 0xa, 0xa, 0x20, 0x20, 0x70, 0x72, 0x69, 0x6e, 0x74, 0x66, 0x28, 0x22, 0x63, 0x68, 0x61, 0x72, 0x20, 0x73, 0x5b, 0x5d, 0x20, 0x3d, 0x20, 0x7b, 0x22, 0x29, 0x3b, 0xa, 0x20, 0x20, 0x66, 0x6f, 0x72, 0x28, 0x69, 0x3d, 0x30, 0x3b, 0x69, 0x3c, 0x31, 0x36, 0x37, 0x3b, 0x69, 0x2b, 0x2b, 0x29, 0x20, 0x70, 0x72, 0x69, 0x6e, 0x74, 0x66, 0x28, 0x22, 0x30, 0x78, 0x25, 0x78, 0x2c, 0x20, 0x22, 0x2c, 0x20, 0x73, 0x5b, 0x69, 0x5d, 0x29, 0x3b, 0xa, 0x20, 0x20, 0x70, 0x72, 0x69, 0x6e, 0x74, 0x66, 0x28, 0x22, 0x30, 0x78, 0x30, 0x7d, 0x3b, 0x5c, 0x6e, 0x22, 0x29, 0x3b, 0xa, 0x20, 0x20, 0x70, 0x72, 0x69, 0x6e, 0x74, 0x66, 0x28, 0x22, 0x25, 0x73, 0x22, 0x2c, 0x20, 0x73, 0x29, 0x3b, 0xa, 0x20, 0x20, 0x72, 0x65, 0x74, 0x75, 0x72, 0x6e, 0x20, 0x30, 0x3b, 0xa, 0x7d, 0xa, 0x0}; #include <stdio.h> int main() { int i; printf("char s[] = {"); for(i=0;i<167;i++) printf("0x%x, ", s[i]); printf("0x0};\n"); printf("%s", s); return 0; }

• wio1.c

  int printf(const char *, ...);char p[]="int printf(const char *, ...);char p[]=%c%s%c;int main(){printf(p,34,p,34,10);return 0;}%c";int main(){printf(p,34,p,34,10);return 0;}

• ASCII - American Standard Code for Information Interchange

A shell script that prints out it's own code?

• wio2

  #!/bin/sh s="\x070 \x072 \x069 \x06e \x074 \x066 \x020 \x022 \x023 \x021 \x02f \x062 \x069 \x06e \x02f \x073 \x068 \x05c \x06e \x073 \x03d \x05c \x022 \x022 \x03b \x00a \x065 \x063 \x068 \x06f \x020 \x022 \x024 \x073 \x05c \x022 \x03b \x022 \x03b \x00a \x070 \x072 \x069 \x06e \x074 \x066 \x020 \x022 \x025 \x062 \x022 \x020 \x024 \x073 \x03b \x00a"; printf "#!/bin/sh\ns=\""; echo "$s\";"; printf "%b" $s;