C sprintf causing a segmentation fault

Question

I am trying to pass in arguments to the a parent file that is supposed to create a child process for each pair of arguments. The child processes will add up each pair and return their sum to the parent process. If there is an odd number of arguments passed in, I add a 0 to the end of the argv array to make it even. This keeps happening until all the arguments have been added up, and their sum is printed at the end.

Everything is working fine except when I pass in an even amount of arguments. A child process will successfully add the first two arguments and return them, but then when the parent does a sprintf , (line 58) there is always a segmentation fault .

Here is the code for both the parent and child processes (I am using pastebin so it doesn't look very cluttered here. They will expire in one day, so I could re-post them if needed):

• http://pastebin.com/zCB700Et
• http://pastebin.com/VxXKYrdn

The second file has to be called worker when it is compiled in order to be run by the first file. I am using gcc on Ubuntu 9.10

Here are a few examples on how to run the program:

gcc -o parent parent.c
gcc -o worker worker.c
./parent 1 2 3 4

(The above example will end with a segmentation fault like I explained above). The answer should be 10 because (1 + 2) + (3 + 4) = 10 .

This one below will work fine though with an odd number of arguments being passed in:

./parent 1 2 3

The answer to this one should be 6 .

Any help would be greatly appreciated!

Answer 1

Overwriting the argument list is, at best, a serious perversion, and at worst a guaranteed fault. That's what you're doing in your sprintf call - the first argument you pass it is one of the strings passed to main(). Why are you doing that?

[edit] I see you've even got a comment to that effect. Well, I don't know what you think is supposed to happen when you do that; if you explain your thinking then somebody may be able to explain where you've gotten confused. You can't add new stuff to the "argv" array. It makes no sense. That's something allocated by the system when launching your process, and you should basically treat it as being read-only (unless you really know what you're doing).

Answer 2

The C standard allows one to modify the strings pointed-to by argv :

The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.

But it doesn't guarantee you to write past the available number of arguments in argv , which is what you are doing when you store the sum at the end of argv .

You seem to be using argv[i] for char * variables. Why not just declare separate variables for the purpose of storing the sum? You also don't need to store "0" in argv[argc-1] if the number of numbers to sum is odd—you can easily determine that in your loop, and pass "0" to the last worker.

Answer 3

        argc++;
        sprintf(argv[argc-1],"%d",tmp);

you are getting out of the array boundaries here. btw, what is use to write to argv ?