Having issue with copy string values im between arrays

Question

I have been working on some code that tokenizes a string from a line and then creates a temp array to copy the string into it (called copy[]) and it is filled with 0's initially (The end game is to split this copy array into temp arrays of length 4 and store them in a struct with a field char* Value). For some reason my temp arrays of size 4 end up having a size of 6.

char* string = strtok(NULL, "\"");
printf("%s", string);                         
int len = (int)strlen(string);

while(len%4 != 0) {
   len++;
}
char copy[len];
for(int i = 0; i < len; i++){
   copy[i] = '0';
}
printf("%s\n", copy);

int copyCount = 0;
int tmpCount = 0;
char temp[4];
while (copyCount < len) {
   if(tmpCount == 4) {
       tmpCount = 0;
   }
   while(tmpCount < 4) {
       temp[tmpCount] = copy[copyCount];
       tmpCount++;
       copyCount++;
   }
   printf("%s %d\n", temp, (int)strlen(temp));  
}

This yields:

This is the end
0000000000000000
This is the end0
This� 6
 is � 6
the � 6
end0� 6

And should yield:

This is the end
0000000000000000
This is the end0
This 4
 is  4
the  4
end0 4

I've been messing around with this for awhile and can't seem to figure out why its making temp have a length of 6 when I set it to 4. Also I'm not sure where the random values are coming from. Thanks!

Your snippet of code shows the first call to strtok as char* string = strtok(NULL, "\""); Have you called it once with the string to tokenize passed as the first argument? — ffhaddad
– ffhaddad, Commented Dec 3, 2013 at 22:21
@AdrianKrupa I thought so too, but after looking around on SO for a minute it looks like char copy[len]; is valid C code nowadays, even in the middle of a function like this. — Andrey Mishchenko
– Andrey Mishchenko, Commented Dec 3, 2013 at 22:30

Andrey Mishchenko · Accepted Answer · 2013-12-03 22:20:25Z

2

The reason is that your string temp is not null-terminated. C-style strings should be terminated with a \0 character. For some (lucky) reason there is a \0 three bytes in memory after wherever the end of temp lives, so when strlen tries to compute its length, it gets 6. This is also why printf is printing garbage: it will print temp until it finds the null terminator, and there are garbage characters in memory before printf reaches the null terminator.

answered Dec 3, 2013 at 22:20

Andrey Mishchenko

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2 Comments

CSDUDE Over a year ago

So would making temp size 5 and adding a \0 after copying the 4 chars work?

Andrey Mishchenko Over a year ago

Yes, it should. You don't need to add the \0 repeatedly. For example: char temp[5] = "xxxx"; will add the \0 for you automatically. You could do this explicitly with char temp[5]; temp[4] = '\0';.

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