06. Memory

Command line

• input-output redirection:

  • program > file: redirect output

  • program < file: redirect input

  • program1 | program 2: redirect output of program1 to input of program2

E. g. (try i):

• Calendar: cal

• Calendar to file: cal > file

• Words counter: wc < file (also lines and characters counter)

• What this does: cal | wc

• Print some: echo some    words        with  spaces

  • note spaces are gone. Why?
• Print exact string"

  [@sugon ~]$ echo 'dafg sdfg sdfg
  sdfgdfger qgrdqwfgwerg erg  ergfer
  xcv  werg    wergwe wer'

• Task: print some strings and count words/characters/lines

less:

• used to browse text page by page

• e. g. less /usr/share/dict/words

• q to quit

maan:

• browse documentation to any system component (if any)

• uses less for browsing

• e. g. man man or man cal

seq:

• generate and display number sequences

• seq 10

• seq 5 -1 0

• man seq

Static Memory allocation

What this code does?

   1 #include <stdio.h>
   2 
   3 int A[5];
   4 int B[5];
   5 
   6 int main(int argc, char *argv[]) {
   7   int i=0;
   8   do
   9         scanf("%d", &A[i++]);
  10   while(A[i-1]);
  11 
  12   for(i=0; i<5; i++)
  13         printf("%d ", A[i]);
  14   for(i=0; i<5; i++)
  15         printf("%d ", B[i]);
  16   putchar('\n');
  17 
  18   return 0;
  19 }

Compile it statically:

• with compile

• or with cc -static

Try it:

• on 3 2 1 0

• on 7 6 5 4 3 2 1 0

  • Describe what you see (Hint: use previous class)

• on 100 99 98 … 0 (user seq and | redirection)

  • Notice «segmentation fault»: trying to access (virtual) memory address, that is not mapped in TLB to any physical address

Dynamic memory allocation

• malloc(nbytes): allocate nbytes of memory

     1 #include <stdio.h>
     2 #include <stdlib.h>
     3 
     4 int main(int argc, char *argv[]) {
     5   long int n;
     6   int *p;
     7 
     8   scanf("%ld", &n);
     9   p = malloc(n);
    10   printf("%p\n", p);
    11   return 0;
    12 }
  

• try to detect maximum possible size of malloc-ed memory
• notice the address changes randomly (?)

• IRL you need to free() all the memory you've malloc-ed:

     1 #include <stdio.h>
     2 #include <stdlib.h>
     3 
     4 int main(int argc, char *argv[]) {
     5   long int n;
     6   int *p;
     7 
     8   scanf("%ld", &n);
     9   p = malloc(n);
    10   /* useful code here */
    11   free(p);
    12   printf("%p\n", p);
    13   return 0;
    14 }
  

  • Note: it's safe to print p after it was freed

  • It is unsafe to free that already free-d (try it)

  • It is highly unsafe to read/write into already free-d memory, because of possible memory corruption without actual address violation (try it for various values of n)

Important: while memory/stack corruption is deadly for a process, it is completely safe for other processes, because of memory isolation

H/W

• write a program that detects maximum possible size of malloc-ed memory

  • read man malloc to learn how malloc() reports a failure

  • spoiler: it returns NULL

  • do not forget to free()