Set buffer array.
  Calls protected virtual member setbuf.
  The standard behavior of function setbuf on derived stream buffer classes is to set the array pointed by parameter s as the character buffer to be used.
  The character buffer is the array of characters in memory used by the stream buffer.

Parameters.

s

Pointer to an array of n characters already allocated in memory. If this parameter is NULL the stream is unbuffered.

n

Length in characters of the character buffer pointed by s. 0 means unmbuffered.
This is an interger value of type streamsize.

Return Value.
  In case of success the member function should return this, otherwise a null pointer.

Example.

// set character buffer (pubsetbuf)
#include <fstream>
using namespace std;

int main () {

  char mybuffer [512];
  fstream filestr;
  filestr.rdbuf()->pubsetbuf(mybuffer,512);

  // operations with file stream here.

  return 0;
}

Basic template member declaration ( basic_streambuf<charT,traits> ):

basic_streambuf<charT,traits>* pubsetbuf (charT* s, streamsize n);

See also.
  setbuf
  streambuf class