c double pointer giving error that doesn't make sense

Question

int p_entity(char **data){
  char *pch;

  pch = strtok(*data, " \n");
  // printf("%s ", pch);
  pch = strtok(NULL, " \n");
  // (*data) = (*data) + 1;   // 1. this is okay
  //  (*data) = pch;          // 2. but doing this will cause an error
  printf("%c %d \n", *pch, pch); 
  printf("%c %d \n", **data, *data);
}

From the above code I will get the following if I uncomment 1:

g 4927479
e 4927456

I will get the following if I uncomment 2:

g 4927479
g 4927479
      3 [main] main 8172 exception::handle: Exception: STATUS_ACCESS_VIOLATION
    470 [main] main 8172 open_stackdumpfile: Dumping stack trace to main.exe.stackdump

Can anyone explain why I am getting that error? I would think both assignments would be legal and incrementing (*data) would be equivalent to straight assignning the address I want it to be at.

what sort of evil code is that? :) why are you even passing as char**? why are you trying to reassign (*data) ? — Mitch Wheat
– Mitch Wheat, Commented Sep 28, 2011 at 0:08
Identifiers like entity, data, pch for "pointer to char" come straight from mindprod.com/jgloss/unmainnaming.html — Dour High Arch
– Dour High Arch, Commented Sep 28, 2011 at 0:28
I am writing a code beautifier, so I load the file into memory and begin priting it out in a beautified format. So *data is the location of the current position in the memory. **data is the actual current char. So I pass this function the current location on **data, and I process it, then I move foward in the data by incrementing *data. The think I don't understand is that I can increment *data but I can't directly assign it an address. — user968102
– user968102, Commented Sep 30, 2011 at 0:11

Matthieu · Accepted Answer · 2011-09-28 00:11:45Z

2

Seems like even when you get the error (and uncomment 2), you still get the two printfs... seems like it is crashing much later in your code.

answered Sep 28, 2011 at 0:11

Matthieu

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1 Comment

user968102 Over a year ago

I think error happens when I try to leave the function, something gets popped out of the stack that shouldn't or something by assigning the location ch to *data

Santosh · Accepted Answer · 2011-09-30 07:04:21Z

0

Answer really depends on how you are handling data before and after calling this function.

if you are allocating memory to *data before calling this function and tying to free it after this function returns, it could crash, as you have changed its value.

answered Sep 30, 2011 at 7:04

Santosh

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