another question about strcpy

**kantze** · 11-02-2006

Hello everyone.
I found on a book this function(I suppose it works)
which reads some lines and returns the number of lines readed.

Code:

int readlines(char *lineptr[],int maxlines)
{int len,nlines;
char *p,line[MAXLEN];
nlines=0;
while((len=getline(line,MAXLEN))>0)
  if(nlines>=maxlines || p=alloc(len)==NULL)
    return -1;
else{line[line-1]='\0';
strcpy(p,line);
lineptr(nlines++)=p;
}

return nlines;
}

The problem is why using this

Code:

strcpy(p,line);
lineptr(nlines++)=p;

instead of this?

Code:

lineptr(nlines++)=line;

What's the difference?

**Prelude** · 11-02-2006

>What's the difference?
p refers to dynamic memory that isn't tied to the function. line is a local array that will be destroyed when the function returns. There are two problems with your suggested change (presumably with the following fix):

Code:

lineptr[nlines++]=line;

First, there's only one line. This assignment points every pointer in the array to the same address, so after the loop, even if there are a dozen pointers, they all refer to the last line entered. Second, when the function returns, line is destroyed and all of the pointers will be dangling.

That function looks extremely dicey. What book did you get it out of?

**vart** · 11-02-2006

1.

Code:

line[line-1]='\0'

shoulde be

Code:

line[len-1]='\0'

2.

Code:

lineptr(nlines++)=line;

stores the pointer to the local variable line
this address not changes
so every pointer in the array stores the same address

And after the function exits - the local variable is destroed, and the addres is invalid.

On the other hand
p is allocated for every string
so the allocated address is new for each new line
and this memory is valid till the corresponding free is called (somethere other place)

**kantze** · 11-02-2006

Well my teacher gave as this function and I think it's on K&R2 book. I didn't copy paste it and probably I did a few mistakes such as not declaring

Code:

#define MAXLEN 5000

.
Thanks for both answers

**Prelude** · 11-02-2006

>Well my teacher gave as this function and I think it's on K&R2 book.
I recognize the solution, but it's heavily butchered from the one in K&R.

Thread: another question about strcpy

Thread Tools

Search Thread

Display

another question about strcpy

Similar Threads

another do while question

Strcpy

strcpy question

opengl DC question

strcpy