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2507 :: How to change the buffer malloc from c code?

• donydh (orginal) [2016-09-01T06:15:41+02:00] view original

  C code

  char* get_char(int n)
  {
          char* p = (char*)malloc(sizeof(char)*n);
          printf("%p\n", p);
          strcpy(p, "hello,world,from c!");
          return p;
  }
  
  void del_char(char* p)
  {
          free((void*)p);
  }
  Run

  {.compile: "logic.c".}
  proc get_char(n: cint): cstring {.importc.}
  proc del_char(p: cstring){.importc.}
  
  var cs = get_char(100)
  echo cs
  cs = "I change this"
  echo cs
  del_char(cs)
  Run

  E:CODEnim>nim c -r test.nim Hint: used config file 'd:Nimconfignim.cfg' [Conf] Hint: system [Processing] Hint: test [Processing] Hint: [Link] Hint: operation successful (10306 lines compiled; 0.361 sec total; 15.504MiB; De bug Build) [SuccessX]

  0000000000645850

  hello,world,from c!

  I change this

  Error: execution of an external program failed: 'e:\code\nim\test.exe '

---

• wiffel (orginal) [2016-09-01T10:42:54+02:00] view original

  The cs variable that you create is of type cstring. That is a pointer to something in memory. The line

  cs = "I change this" 
  Run

  actually means: change the pointer to the address of the string "I change this".

  That string is located somewhere in memory. It is a literal string, so it's unclear where it actually is pointing at. With

  del_char(cs)
  Run

  you are trying to deallocate (the memory behind) this pointer, which is illegal. Hence the error.

  This little program also prints out the pointer values. I hope this shows what is going on ...

  import strutils
  
  {.compile: "logic.c".}
  proc get_char(n: cint): cstring {.importc.}
  proc del_char(p: cstring){.importc.}
  
  var cs = get_char(100)
  echo "0x", cast[int](cs).toHex(8), " : ", cs
  cs = "I change this"
  echo "0x", cast[int](cs).toHex(8), " : ", cs
  del_char(cs)
  Run

  gives

  
  0x6a6010
  0x006A6010 : hello,world,from c!
  0x00423073 : I change this
  Traceback (most recent call last)
  test.nim(11)             test
  SIGSEGV: Illegal storage access. (Attempt to read from nil?)
  Run

---