strcmp segmentation fault - c++

Question

Here is a problem from spoj. nothing related to algorithms, but just c
Sample Input
2
a aa bb cc def ghi
a a a a a bb bb bb bb c c
Sample Output
3
5
it counts the longest sequence of same words
http://www.spoj.pl/problems/WORDCNT/
The word is less than 20 characters
But when i run it, it's giving segmentation fault. I debugged it using eclipse. Here's where it crashes
if (strcmp(previous, current) == 0)
currentLength++;
with the following message
No source available for "strcmp() at 0x2d0100"
What's the problem?
#include <iostream>
#include <cstring>
#include <string>
#include <cstdio>
using namespace std;
int main(int argc, const char *argv[])
{
int t;
cin >> t;
while (t--) {
char line[20000], previous[21], current[21], *p;
int currentLength = 1, maxLength = 1;
if (cin.peek() == '\n') cin.get();
cin.getline(line, 20000);
p = strtok(line, " '\t''\r'");
strcpy(previous, p);
while (p != NULL) {
p = strtok(NULL, " '\t''\r'");
strcpy(current, p);
if (strcmp(previous, current) == 0)
currentLength++;
else
currentLength = 1;
if (currentLength > maxLength)
maxLength = currentLength;
}
cout << maxLength << endl;
}
return 0;
}

Answer 1

The problem is probably here:
while (p != NULL) {
p = strtok(NULL, " '\t''\r'");
strcpy(current, p);
While p may not be NULL when the loop is entered.
It may be NULL when strcpy is used on it.
A more correct form of the loop would be:
while ((p != NULL) && ((p = strtok(NULL, " \t\r")) != NULL))
{
strcpy(current, p);
Note. Tokenizing a stream in C++ is a lot easier.
std::string token;
std::cin >> token; // Reads 1 white space seoporated word
If you want to tokenize a line
// Step 1: read a single line in a safe way.
std::string line;
std::getline(std::cin, line);
// Turn that line into a stream.
std::stringstream linestream(line);
// Get 1 word at a time from the stream.
std::string token;
while(linestream >> token)
{
// Do STUFF
}

Answer 2

Forgot to check for NULL on strtok, it will return NULL when done and you cannot use that NULL on strcpy, strcmp, etc.
Note that you do a strcpy right after the strtok, you should check for null before doing that using p as a source.

Answer 3

The strtok man page says:
Each call to strtok() returns a pointer to a null-terminated string containing the next
token. This string does not include the delimiting character. If no more tokens are found,
strtok() returns NULL.
And in your code,
while (p != NULL) {
p = strtok(NULL, " '\t''\r'");
strcpy(current, p);
you are not checking for NULL (for p) once the whole string has been parsed. After that you are trying to copy p (which is NULL now) in current and so getting the crash.

Answer 4

You will find that one of previous or current does not point to a null-terminated string at that point, so strcmp doesn't know when to stop.
Use proper C++ strings and string functions instead, rather than mixing C and C++. The Boost libraries can provide a much safer tokeniser than strtok.

Answer 5

You probably undersized current and previous. You should really use std::string for this kind of thing- that's what it's for.

Answer 6

You are doing nothing to check your string lengths before copying them into buffers of size 21. I bet that you are somehow overwriting the end of the buffer.

Answer 7

If you insist on using C strings, I'd suggest using strncmp instead of strcmp. That way, in case you are ending up with a non-null terminated string (which is what I suspect is the case), you can restrict your compare to the max length of the string (in this case 21).

Answer 8

Try this one...
#include <cstdio>
#include <cstring>
#define T(x) strtok(x, " \n\r\t")
char line[44444];
int main( )
{
int t; scanf("%d\n", &t);
while(t--)
{
fgets(line, 44444, stdin);
int cnt = 1, len, maxcnt = 0, plen = -1;
for(char *p = T(line); p != NULL; p = T(NULL))
{
len = strlen(p);
if(len == plen) ++cnt;
else cnt = 1;
if(cnt > maxcnt)
maxcnt = cnt;
plen = len;
}
printf("%d\n", maxcnt);
}
return 0;
}