Question

I am parsing a macro definition from a Makefile into two strings, the name of the macro and the body. For example here is a macro definition line from my Makefile:

macro-1 = body-1

My code produces a bus error/segmentation fault.

static void parse_macro_def(const char* line)
{
   char* m_name;
   int name_pos = 0;

   int i = 0;
   while(line[i++] != '=')                    //iterate until an equal sign is found
   {
      if(!isspace(line[i]))          //copy characters to m_name  unless the character is whitespace
      {
         m_name[name_pos++] = line[i];
      }
   }
}
m_name[name_pos] = '\0';

m_name should be set to macro-1
Thanks for all the help!

Answer 1

You don’t initialize m_name so it points anywhere, so you are writing at random, and crashing.

You need to allocate and return the space, or pass the space in (along with the length of the space), ensuring you don’t overwrite in every case.

---

You should note that white space characters in a macro name are a bug in the macro definition. There can be leading white space; there can be trailing white space; but there can’t be white space in the middle of a name. Now, if you are assuming that you have been given a valid, working Makefile to parse, you may be able to get away with ignoring this subtlety. If you’re writing a replacement for make, you can’t.

Unless you’ve previously validated that there is an equals sign in the string, you should also check that you don’t run off the end of the string (you don’t scan past a NUL '\0'). In fact, in robust code, you’d probably ensure that out of paranoia.

while (line[i] != '\0' && line[i] != '=')
{
    ...
}

And, while writing that, I realized that you increment i in the while condition, and then check whether the next character is a space in the body of the loop. That’s a little aconventional, shall we say. If you come across a macro:

MACRO=value

you will copy the = into m_name when the loop condition checks the O. And, AFAICS, you won’t copy the M.

---

Note that your line:

m_name[name_pos] = '\0';

is outside any function and therefore a syntax error.