c - How do I set a buffer in a possibly recursive procedure? -

i realized major flaw in program writing. it's program symbolically differentiates math functions, e.g. "x^2+5" --> "2*x", , master function involved starts off like

char * derivefromtree ( node * rt ) {     char * dfdx = malloc(100*sizeof(char)); // buffer     if (rt->op) // if rt of form rt = gx op hx     {         char * dgdx = derivefromtree(rt->gx); // g'(x)         char * dhdx = derivefromtree(rt->hx); // h'(x)         char thisop = *rt->op;         if (thisop == '+' || thisop == '-')         {             //   addition/subtraction rule:             //   dfdx = dgdx + thisop + dhdx             dfdx = strcat(dfdx, dgdx);             dfdx = strcat(dfdx, chartostring(thisop));             dfdx = strcat(dfdx, dhdx);          } 

the problem, might see, result of derivefromtree fits in buffer of length 100, result composed of several other results, won't work. alternative solution getting rid of buffer , setting dfdx exact length needs be:

char * derivefromtree ( node * rt ) {     char * dfdx;     if (rt->op) // if rt of form rt = gx op hx     {         char * dgdx = derivefromtree(rt->gx); // g'(x)         char * dhdx = derivefromtree(rt->hx); // h'(x)         char thisop = *rt->op;         if (thisop == '+' || thisop == '-')         {             //   addition/subtraction rule:             //   dfdx = dgdx + thisop + dhdx             dfdx = malloc(strlen(dgdx) + strlen(dhdx) + 2);             dfdx = strcat(dfdx, dgdx);             dfdx = strcat(dfdx, chartostring(thisop));             dfdx = strcat(dfdx, dhdx);          } 

but inefficient because of calls strlen(dgdx) , strlen(dhdx) iterate through strings dgdx , dhdx , iterated through again in strcat calls.

what best solution problem?

since have tagged question c++, suggest chuck char * , use std::string. code this:

string derivefromtree ( node * rt ) {     string dfdx // buffer     if (rt->op) // if rt of form rt = gx op hx     {         string dgdx = derivefromtree(rt->gx); // g'(x)         string dhdx = derivefromtree(rt->hx); // h'(x)         char thisop = *rt->op;         if (thisop == '+' || thisop == '-')         {             //   addition/subtraction rule:             //   dfdx = dgdx + thisop + dhdx             dfdx =  dfdx + dgdx + thisop + dhdx; //strcat(dfdx, dgdx);         }