Question

I’m updating a fairly old C extension bound to a ruby gem that I didn’t write. Currently, when I execute the script that will use the C extension, I get this error:

symbol lookup error: /path/to/lib.so: undefined symbol: rb_str2cstr

There was also problems with STR2CSTR in the same library, which was fixed by replacing it with StringValuePtr. My initial thought was to replace rb_str2cstr with rb_string_value_ptr, but when I try that, the C extension will not build. I’m by no means a C programmer, and am not sure where to go with this. I know I have to replace rb_str2cstr with its replacement, but I can’t seem to figure out what that is.

Answer 1

In the source of ruby.h for Ruby 1.8.7-p357 it has:

/* obsolete API - use StringValue() */
char *rb_str2cstr _((VALUE,long*));

StringValue returns a VALUE that you can use RSTRING on to get the length and char * pointer:

RSTRING(s)->len /* length */
RSTRING(s)->ptr  /* char pointer */

So if the existing code looked something like this:

VALUE str = ...; /* The ruby String */

long  len;
char* words;
words = rb_str2cstr(str, &len);

Then change it to:

VALUE str = ...;
str = StringValue(str);

char* words = RSTRING(str)->ptr;
long  len   = RSTRING(str)->len;

If you’re sure that str is already a Ruby String then you could omit the StringValue call, but it’d be safer to keep it.