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(libc.info)Dynamic String Input

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Dynamically Allocating String Conversions
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   A GNU extension to formatted input lets you safely read a string
with no maximum size.  Using this feature, you don't supply a buffer;
instead, `scanf' allocates a buffer big enough to hold the data and
gives you its address.  To use this feature, write `a' as a flag
character, as in `%as' or `%a[0-9a-z]'.

   The pointer argument you supply for where to store the input should
have type `char **'.  The `scanf' function allocates a buffer and
stores its address in the word that the argument points to.  You should
free the buffer with `free' when you no longer need it.

   Here is an example of using the `a' flag with the `%[...]'
conversion specification to read a "variable assignment" of the form
`VARIABLE = VALUE'.

     {
       char *variable, *value;
     
       if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
                      &variable, &value))
         {
           invalid_input_error ();
           return 0;
         }
     
       ...
     }