segmentation fault checking user arguments c

Question

I am trying to validate user input from the command line with argc and argv but am running into a segmentation fault and am not sure why.

I want to validate the user has

Answer 1

This is incorrect, since in C string is not a built-in type (and is not a standard type defined in some standard header - notice that C++ has std::string defined in its standard header).

The signature of main has to be

int main(int argc, char**argv);

The convention is that argv[argc] is the null pointer, and the argc pointers from argv[0] to argv[argc-1] are non-null pointers to null-terminated strings.

^{Notice that (at least on Linux and Posix systems) program arguments are not user input to the program. The shell has been given a command line as input and has expanded that command line into program arguments. Your program could later read some input, e.g. with scanf.}

You cannot convert a NULL pointer with atoi, so you should check argc before doing the conversion.

int main(int argc, char**argv) {
   if (argc!=2) { 
      fprintf(stderr, "%s needs two arguments\n", argv[0]);
      exit(EXIT_FAILURE);
   };
   int k = atoi(argv[1]);
   if (k<=0) {
      fprintf(stderr,
              "%s wants a positive first argument, but got %s\n",
              argv[1]);
      exit(EXIT_FAILURE);
   }
   // now do the real thing
 }

BTW, I believe that (at least on Linux or Posix system) a program should accept the --help argument per the GNU coding standard. I suggest you to use getopt (notably getopt_long) or argp to parse program arguments.

At last, compile with all warnings and debug information (e.g. with gcc -Wall -g) and learn how to use the debugger (e.g. gdb).