Question

I’m writing a C code for a class. This class requires that our code compile and run on the school server, which is a sparc solaris machine. I’m running Linux x64.

I have this line to parse (THIS IS NOT ACTUAL CODE BUT IS INPUT TO MY PROGRAM):

while ( cond1 ){ 

I need to capture the “while” and the “cond1” into separate strings. I’ve been using strtok() to do this. In Linux, the following lines:

char *cond = NULL;
cond = (char *)malloc(sizeof(char));
memset(cond, 0, sizeof(char));
strcpy(cond, strtok(NULL, ": \t\(){")); //already got the "while" out of the line

will correctly capture the string “cond1”.Running this on the solaris machine, however, gives me the string “cone1”.

Note that in plenty of other cases within my program, strings are being copied correctly. (For instance, the “while”) was captured correctly.

Does anyone know what is going on here?

Answer 1

The line:

cond = (char *)malloc(sizeof(char));

allocates exactly one char for storage, into which you are then copying more than one – strcpy needs to put, at a bare minimum, the null terminator but, in your case, also the results of your strtok as well.

The reason it may work on a different system is that some implementations of malloc will allocate at a certain resolution (e.g., a multiple of 16 bytes) no matter what actual value you ask for, so you may have some free space there at the end of your buffer. But what you’re attempting is still very much undefined behaviour.

The fact that the undefined behaviour may be to work sometimes in no way abrogates your responsibility to avoid such behaviour.

Allocate enough space for storing the results of your strtok and you should be okay.

The safest way to do this is to dynamically allocate the space so that it’s at least as big as the string you’re passing to strtok. That way there can be no possibility of overflow (other than weird edge cases where other threads may modify the data behind your back but, if that were the case, strtok would be a very bad choice anyway).

Something like (if instr is your original input string):

cond = (char*)malloc(strlen(instr)+1);

This guarantees that any token extracted from instr will fit within cond.

As an aside, sizeof(char) is always 1 by definition, so you don’t need to multiply by it.