C:string getting corrupted between malloc and strcpy

Question

I am very confused why this is happening though I've used such statements well before. I am copying one string into other simply and my original string is getting corrupted, even before strtok gets called. This is the relevant code snippet, after this I am using strtok. But before that I'm seeing reference got corrupted.

int j, sizeref;
char *str1, *token;

printf("%s :", reference);
sizeref = strlen(reference);
printf("%d\n", sizeref);
track = malloc(sizeref);
printf("%s :", reference);
strcpy(track, reference);
printf("%d\n", strlen(track));

This is the output below. The first line prints whole reference and correct size as 234. After that I see only 106 characters got copied and original string reference is also truncated to 106 characters. track is declared as char *

+918956549122,9527529747,09847778399,08596774833,9867859469,+919999866778,6985888696,5968939898,6959869856,9898495895,6986596865,09847765399,88596774833,9967859469,+917899866778,6985889696,9527567747,09848778399,08596756733,9867999469 :234
+918956549122,9527529747,09847778399,08596774833,9867859469,+919999866778,6985888696,5968939898,69598698 :106

MORE INFO: reference is getting build up incrementally in chunks of 50 bytes. Once completely built , then only being used as mentioned above. char * reference is global , and getting built up inside main. Above mentioned code is inside a separate function

realloc(reference,strlen(reference) + CHUNK);

track = malloc(sizeref + 1)... you don't allocate space for the NUL terminator. Apart from that, your program has another UB. strlen() returns size_t and not int, so you must print that using %zu and not %d. — user529758
– user529758, Commented Nov 1, 2013 at 6:47
@H2CO3 you save me so much damn typing sometimes. I need to repay that favor someday =P — WhozCraig
– WhozCraig, Commented Nov 1, 2013 at 6:48

Michael · Accepted Answer · 2013-11-01 08:38:10Z

3

According to manual on strlen():

DESCRIPTION
       The  strlen()  function calculates the length of the string s,
       excluding the terminating null byte ('\0').

So, as mentioned above, you should allocate buffer of size strlen() + 1 for correct null termination.

answered Nov 1, 2013 at 8:38

Michael

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3 Comments

Diwakar Sharma Over a year ago

Yea that is true , but how will that solve this problem. I tried strlen()+1 , still same results. If I'm not using +1 then it should at least allocate 234 bytes , but it's stopping at 106 .. that is weird.

Michael Over a year ago

@DiwakarSharma I just checked your small part of the code from question with strlen(.)+1 and it works just fine. I got output like that: %all the input numbers% : 236\n%all the input numbers% : 235. And this is right (second strlen gives us 236-1). Guess, if you have some error than it in the other parts of the code. So, if you still working on this issue - post some test code (wrapped with standalone main() {}), which is as small as possible and which contains error. Then be sure, people will help you.

Diwakar Sharma Over a year ago

Well, I have changed my approach and logic to a better one I feel and have posted a different question for problem there. stackoverflow.com/questions/19761453/…

Lundin · Accepted Answer · 2013-11-01 07:32:51Z

1

C strings are null terminated. Change the code to track=malloc(sizeref + 1);, otherwise strcpy will attempt to write the null termination outside valid memory.

answered Nov 1, 2013 at 7:32

Lundin

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Comments

John · Accepted Answer · 2013-11-01 06:47:13Z

0

You are probably trying to get the size of the array by using sizeof in a function where the array decays to a pointer and sizeof returns only the size of pointer.

answered Nov 1, 2013 at 6:47

John

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