Thread: Segmentation fault when appending to strings (char *)

So I'm trying to implement this simple program which essentially parses a simple (untabbed, simple tags) XML file and returns it in a tabbed format, but I've been stuck on trying to append to a string...

Code:

#include <stdio.h>
#include <string.h>
// Echo stdin to stdout with trailing \n newlines
int main(int argc, char**argv) {
  char c;
	char *str;
	int swtch = 0;
  c = getchar();
	int depth = 0;
  while (c != EOF) {
		while (c != '\n')
		{	str = "";
			if (c == '<')
				swtch = 0;
			else if (c == '>')
			{	if (swtch == 0)
					depth++;
			}	else if (c == '/')
				depth-=2;
			strcpy(str,&c);
		}
		int i = 0;
		while (i < depth)
		{	printf("   "); i++;	}
		printf("%s",str);
		putchar(c);
		c = getchar();
		}
  return 1;
}

I've tried strcpy(str,&c), strncpy(str,&c,sizeof(str)), and strlcpy(str,&c,sizeof(str)), but I get a segmentation fault every time..
How do I add one string variable to another?

A string is a char array whose last element is a zero byte.
1) You haven't allocated memory for str. You should probably define it as str[100], or use malloc() to obtain memory of some appropriate number of bytes.
2) You can't concatenate a single char to a string using strcpy, strcat, etc. unless both strings are NULL terminated.
To assign a single character, you need to just str[n] = c, and keep track of the n index, starting at 0 and incrementing each time. Then put in a '\0' as the last element before attempting to print this string using printf.

Thanks for the quick reply.
So how would I go about using malloc(), and how much space should I allocate?

To give you an idea, this is what I am looking to parse (one line at a time):

Code:

<level1>
<level2>
</level2>
<level2>
<deeper>
<evendeeper>
</evendeeper>
</deeper>
</level2>
<notasdeep>
</notasdeep>
</level1>

In addition to nonoob's comments you also can't do this:

Code:

str = "";

C does not assign strings with the = sign. To reset a string use numerical assignment: str[0] = 0;

getchar returns an int, not a char, this is especially important as you are comparing against EOF.

Why does it return an int?
Is there a function that can read a line from standard input (ie: keyboard input) and save it as a string? That's really what I need to make this program function..

Originally Posted by LanguidLegend

Why does it return an int?
Is there a function that can read a line from standard input (ie: keyboard input) and save it as a string? That's really what I need to make this program function..

You could try scanf() or fscanf() whichever is best suited to your application.

Originally Posted by LanguidLegend

Why does it return an int?
Is there a function that can read a line from standard input (ie: keyboard input) and save it as a string? That's really what I need to make this program function..

Because it needs to be able to return all possible char values plus special things like EOF, thus just a char isn't enough. Look into fgets for reading a whole line from stdin.

Thanks everybody, I think I understand the issue much better now.

SO this boils down to the ultimate goal of my program, to parse bracketed tags (opening & closing tags like in XML), and return them in their correct format (tabbed correctly).
This is what I have thus far:

Code:

int main(int argc, char**argv) {
  char c;
  int swtch = 0;
  c = getchar();
  int depth = 0;
  while (c != EOF) {
		if (swtch == 1)
		{	int i = 0;
			while (i < depth)
			{	printf("   "); i++;	}
		}
		if (c == '\n')
			swtch = 1;
		if (c == '<')
			swtch = 0;
		if (c == '>' && swtch == 0)
			depth++;
		if (c == '/')
			depth-=2;
		putchar(c);
		c = getchar();
	}
  return 1;

The issue I'm having is the cycle of reading and writing: by the time my program runs across the '/' character, signalling a closing tag, it has already printed the (incorrect) tab to the screen.
How can I determine if a tag is closing before the line has been written to the screen?

Originally Posted by LanguidLegend

Why does it return an int?
Is there a function that can read a line from standard input (ie: keyboard input) and save it as a string? That's really what I need to make this program function..

Look up fgets().

Originally Posted by quzah

The problem then is the way you are processing things. You should be building an output buffer, and once you are done with all of your reading, then write the output. Not writing while you're reading.


Quzah.

Can you give me an example of how to build an output buffer then write it to the screen? Sorry for my inexperience..
Thanks!