#include <stdio.h>

void setbuf(FILE * stream, char * buf);

#include <stdio.h> 
char outbuf[BUFSIZ]; 
int main(void) 
{ 
  setbuf(stdout, outbuf); // 把缓冲区与流相连
  puts("This is a test of buffered output.\n");
  puts(outbuf);
  fflush(stdout); // 刷新
  puts(outbuf); // 输出
  return 0; 
}

This is a test of buffered output..
This is a test of buffered output..
This is a test of buffered output..
This is a test of buffered output..

 #include <stdio.h>

  int setvbuf(FILE * stream, char * buf, int type, unsigned size);

#include <stdio.h> 
int main(void) 
{ 
  FILE *input, *output; 
  char bufr[512]; 
  input = fopen("file.in", "w+");   /*打开文件*/
  output = fopen("file.out", "w");
  if (setvbuf(input, bufr, _IOFBF, 512) != 0) /*失败*/
  {
   printf("failed to set up buffer for input file\n");
  }
  else 
  {
   printf("buffer set up for input file\n"); 
}
  if (setvbuf(output, NULL, _IOLBF, 132) != 0) /*为流指定特殊的缓冲区*/
  {
   printf("failed to set up buffer for output file\n"); 
}
  else 
  {
   printf("buffer set up for output file\n"); 
  }
  fclose(input); 
  fclose(output); 
  return 0; 
}

Buffer set up for input file
Buffer set up for output file