c++ - sebuf设置缓冲区大小，但并不是按照设定的大小刷新

Question

 #define BUFSIZE 3

int main(){

char c;
static char buf[BUFSIZE];
setbuf(stdout, buf);
while ((c = getchar()) != EOF){
    putchar(c);
   }
}

但是结果并不是像我想象的按照输入三个字符会自动刷新？请问这是什么问题？

Answer 1

Read the document clearly for yourself:

void setbuf(FILE *stream, char *buf);
...
The setbuf() function is exactly equivalent to the call

   setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

To specify the size, use setbuffer.

Answer 2

$ man setbuf

The setbuf() function is exactly equivalent to the call

setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);

Note the BUFSIZ at the end.