How to capture output of printf?

Question

I am calling a function funcB from funcA. funcB uses several printf statements to output data. Is there a way for me to c

Answer 1

If you're willing to play a dirty game interposing on printf you can 'steal' its output doing something like:

#include 
#include 

static char buffer[1024];
static char *next = buffer;

static void funcB(){
     printf( "%s", "My Name is" );
     printf( "%s", "I like ice cream" );
}

static void funcA(){
    funcB();
    // Do stuff iwth buffer here
    fprintf(stderr, "stole: %s\n", buffer);
    next=buffer; // reset for later.
}

int main() {
  funcA();
}


int printf(const char *fmt, ...) {
   va_list argp;
   va_start(argp, fmt);
   const int ret = vsnprintf(next, sizeof buffer-(next-buffer), fmt, argp);
   next += ret;
   va_end(argp);
   return ret;
}

You could use a flag to indicate how to handle the instances where you want printf to work as normal. (E.g. map it onto fprintf or use dlsym()/similar to find the real call).

You could also use realloc to manage the size of the buffer more sensibly.