MLReleaseString()

#include "mathlink.h"

/* read a C string from a link and then release that memory to MathLink for deallocation */

void f(MLINK lp)
{
    const char *s;

    if(! MLGetString(lp, &s))
        { /* unable to read the string from lp */ }

    /* ... */

    MLReleaseString(lp, s);
}