Plab Tirgul 4 structs

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Plab Tirgul 4structs arrays, file
I/O,debugging memory errors
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Assignment struct vs. pointer to struct
struct Complex double x,y
Complex cp1, cp2cp1 (Complex)malloc
...cp2 (Complex)malloc ...cp1-gtx7
cp1-gty3 cp2 cp1
Complex c1, c2c1.x7 c1.y3 c2 c1
x7
garbage
x7
x7
y3
garbage
y3
y3
c1
c2
cp1
cp2
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Arrays structs as arguments

• When an array is passed as an argument to a
  function, the address of the 1st element is
  passed.
• Structs are passed by value, exactly as the basic
  types.

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Arrays structs as arguments
struct MyStr int a10 void f(int a)
a7 89 void g(MyStr s)
s.a7 84
main() MyStr x x.a7 0 f(x.a)
printf("d\n", x.a7) g(x)
printf("d\n", x.a7)
Output 89 89
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argv argc

• To pass command line arguments to our program we
  should use the following main declaration main(i
  nt argc, char argv) ...
• Compare to main(String args) in java.
• Unlike java the first argument is the name of
  the program itself.

• char argv
• char argv

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argv argc example

• prog1 u danny p 1234argc 5 argv0
  prog1 argv1 -u ... argv4
  1234Always argvargc 0

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Default function arguments

• We can specify default value for trailing
  arguments of a function.
• An alternative to overloading.
• Exampleprint(int value, int base10) . .
  .print(31, 16)print(31)// equivalent to
  print(31,10) h(int x0, int y) // error!

Only in C
Only in C
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File I/O

• File I/O is mostly similar to stdin stdout I/O.
• Most I/O functions we encountered have a file
  counterpart which receives a FILE pointer
  (handle).
• Examplesgetchar(void)
  fgetc(FILE) scanf(const char ,...)
  fscanf(FILE, const char,...)printf(const char
  ,...) fprintf(FILE, const char,...)
• The standard streams (stdin, stdout, stderr) are
  also of FILE type.

See related man pages fprintf, fscanf, etc.
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File I/O example mywc

• include ltstdio.hgtinclude lterrno.hgtinclude
  ltctype.hgtmain(int argc, char argv)
  FILE fp int wc 0, ch if (argc !
  2) printf("Usage mywc
  ltfilenamegt\n") exit(1)
  errno 0 fp fopen(argv1, "r")
• if (fp NULL) perror()
  exit(1)

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File I/O example

• while (1) while ((ch fgetc(fp)) ! EOF
  isspace(ch)) if (ch
  EOF) break wc
  while ((ch fgetc(fp)) ! EOF !isspace(ch))
  if (ch EOF)
  break fclose(fp)
  printf("There are d words in s\n", wc,
  argv1) return 0

Related man pages fopen, fclose
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Memory related bugs

• Memory leaks.
• Accessing random/freed memory addresses (e.g.
  off-by-one errors).

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malloc_stats()

• By including malloc.h you can use the
  malloc_stats() function which prints to the
  stderr information about the amount of used
  memory.
• Example

. . . malloc_stats() destroyDictionary(dict) mal
loc_stats() . . .
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malloc_stats() cntd.
With memory leak Arena 0 system bytes 8140
in use bytes 6860 . . . Arena 0 system
bytes 8140 in use bytes 4084 . . .
Without memory leak Arena 0system bytes
8124 in use bytes 6860. . . Arena 0system
bytes 8124 in use bytes 4 . . .
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mtrace

• Log all memory allocations to a file.
• The file name is contained in the MALLOC_TRACE
  environment variable.
• For example setenv MALLOC_TRACE
  /plab/ex1/trace
• Analyze the file to find memory leaks using the
  mtrace utility.
• The program must
• be compiled with g flag
• include ltmcheck.hgt

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mtrace example
The program include ltmcheck.hgt int main()
mtrace() // later we can call muntrace() . . .
The trace file looks like this Start _at_
0x80486fd 0x804a0e0 0x8 _at_ 0x804887d
0x804a0f0 0x8 _at_ 0x8048c7d 0x804a100 0x14 _at_
/lib/libc.so.6(__strdup0x29)0x400d7a29
0x804a118 0x5 _at_ 0x8048c7d 0x804a128 0x14 _at_
/lib/libc.so.6(__strdup0x29)0x400d7a29
0x804a140 0x3 _at_ 0x8048c7d 0x804a150 0x14. .
.
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mtrace example cntd.
The result of analysis ( mtrace ex1
trace) Memory not freed -----------------
Address Size Caller 0x0804a100 0x14 at
/home/mush/plab/ex1/strBinTree.c65 0x0804a128
0x14 at /home/mush/plab/ex1/strBinTree.c65
0x0804a150 0x14 at /home/mush/plab/ex1/strBinTree.
c65
Another example
Memory not freed ----------------- Address
Size Caller 0x0804a118 0x5 at /lib/libc.so.6(__s
trdup0x29)0x400d7a29 0x0804a140 0x3 at
/lib/libc.so.6(__strdup0x29)0x400d7a29
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MALLOC_CHECK_

• By setting this environment variable to 0, 1, 2
  we can handle some bugs, most notably freeing
  twice the same memory.
• Usually double free causes segmentation fault.
• When MALLOC_CHECK_ is 0 freeing twice works.
• When MALLOC_CHECK_ is 1 an error message is
  printed.
• Example free() invalid pointer 0x80497b8!
• When MALLOC_CHECK_ is 2 the program (gracefully)
  aborts.

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ElectricFence

• ElectricFence is a library which allows to catch
  accesses to memory that was already freed, as
  well as off-by-one errors.
• It will cause the program to segfault in the
  above cases, which is usually better than
  continue running and have unpredictable errors
  later.

Example char a (char)malloc(100sizeof(char))
. . . a100 'c' // ElectricFence will
cause segfault
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ElectricFence cntd.
Example Node n1 (Node)malloc(sizeof(Node)).
. . free(n1) . . . n1-gtx 7 //
ElectricFence will cause segfault

• To use ElectricFence you should link your program
  with the efence library.
• For exampleg prog1.o list.o read.o -lefence

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Commercial products

• Purify
• BoundsChecker
• MSs VisualStudio