根据unix advanced programming：

We can see that the default for this system is to have standard input and standard
output line buffered when they’re connected to a terminal.

从这个例子：

#include <stdio.h>
#include <stdlib.h>

long buffer_size(FILE *fp)
{
    return (fp->_IO_buf_end - fp->_IO_buf_base);
}

int is_linebf(FILE *fp)
{
    return (fp->_flags & _IOLBF);
}

int is_unbf(FILE *fp)
{
    return (fp->_flags & _IONBF);
}

void stream(const char *name, FILE *fp)
{
    printf("stream = %s, ", name);
    if (is_unbf(fp))
    {
        printf("unbuffered");
    }
    else if (is_linebf(fp))
    {
        printf("line buffered");
    }
    else
    {
        printf("fully buffered");
    }
    printf(", buffer size = %ld\n", buffer_size(fp));
}

int main()
{
    FILE *fp;

    fputs("enter any character: ", stdout);
    if (getchar() == EOF)
    {
        perror("getchar error");
    }
    fputs("one line to standard error\n", stderr);

    stream("stdin", stdin);
    stream("stdout", stdout);
    stream("stderr", stderr);

    if ((fp = fopen("/etc/passwd", "r")) == NULL)
    {
        perror("fopen error");
    }
    if (fgetc(fp) == EOF)
    {
        perror("fgetc error");
    }
    stream("/etc/passwd", fp);
    exit(0);
}

如果我运行它：

$ ./a.out
enter any character: a
one line to standard error
stream = stdin, fully buffered, buffer size = 1024
stream = stdout, fully buffered, buffer size = 1024
stream = stderr, unbuffered, buffer size = 1
stream = /etc/passwd, fully buffered, buffer size = 4096

即使将std[in，out]连接到tty，它们也是 * 完全缓冲 * 而不是 * 行缓冲 *。到底是书写错了，还是我错过了什么？
uname -a：

Linux 5.8.0-44-generic #50-Ubuntu SMP Tue Feb 9 06:29:41 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

书中的例子使用了4个操作系统（solaris，openBSD，Linux和macos）的特定于操作系统的宏，所以我的操作系统也在其中。
编辑：在 my 机器中，宏和FILE结构的实现：

• <stdio.h>-

/* The possibilities for the third argument to `setvbuf'.  */
#define _IOFBF 0        /* Fully buffered.  */
#define _IOLBF 1        /* Line buffered.  */
#define _IONBF 2        /* No buffering.  */

• <struct_FILE.h>-

struct _IO_FILE
{
  int _flags;       /* High-order word is _IO_MAGIC; rest is flags. */

  /* The following pointers correspond to the C++ streambuf protocol. */
  char *_IO_read_ptr;   /* Current read pointer */
  char *_IO_read_end;   /* End of get area. */
  char *_IO_read_base;  /* Start of putback+get area. */
  char *_IO_write_base; /* Start of put area. */
  char *_IO_write_ptr;  /* Current put pointer. */
  char *_IO_write_end;  /* End of put area. */
  char *_IO_buf_base;   /* Start of reserve area. */
  char *_IO_buf_end;    /* End of reserve area. */

  /* The following fields are used to support backing up and undo. */
  char *_IO_save_base; /* Pointer to start of non-current get area. */
  char *_IO_backup_base;  /* Pointer to first valid character of backup area */
  char *_IO_save_end; /* Pointer to end of non-current get area. */

  struct _IO_marker *_markers;

  struct _IO_FILE *_chain;

  int _fileno;
  int _flags2;
  __off_t _old_offset; /* This used to be _offset but it's too small.  */

  /* 1+column number of pbase(); 0 is unknown. */
  unsigned short _cur_column;
  signed char _vtable_offset;
  char _shortbuf[1];

  _IO_lock_t *_lock;
#ifdef _IO_USE_OLD_IO_FILE
};

struct _IO_FILE_complete
{
  struct _IO_FILE _file;
#endif
  __off64_t _offset;
  /* Wide character stream stuff.  */
  struct _IO_codecvt *_codecvt;
  struct _IO_wide_data *_wide_data;
  struct _IO_FILE *_freeres_list;
  void *_freeres_buf;
  size_t __pad5;
  int _mode;
  /* Make sure we don't get into trouble again.  */
  char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
};

FILE就是typedef struct _IO_FILE FILE