Linux内核版本：2.6.14
glibc版本：2.3.6
CPU平台：arm
printf的输出不一定是串口，也可以是LCD，甚至是文件等，这里仅以输出到串口为例。本文分析了printf和文件描述符0、1和2以及stdout、stdin和stderr的关系，通过这篇文章可以知道文件描述符0、1和2为什么对应着stdout、stdin和stderr，因为glibc就是这么定义的！！！
首先看glibc中printf函数的定义(glibc-2.3.6/stdio-common/printf.c)：
#undef printf
/* Write formatted output to stdout from the format string FORMAT. */
/* VARARGS1 */
int
printf (const char *format, ...)
{
va_list arg;
int done;
va_start (arg, format);
done = vfprintf (stdout, format, arg);//主要是这个函数
va_end (arg);
return done;
}
#undef _IO_printf
/* This is for libg++. */
strong_alias (printf, _IO_printf);
strong_alias，即取别名。网上有人提及这个strong alias好像是为了防止c库符号被其他库符号覆盖掉而使用的，如果printf被覆盖了，还有_IO_printf可以用。跟踪vfprintf函数()，我们先给出该函数的声明，如下(glibc-2.3.6/libio/stdio.h)：
extern int vfprintf (FILE *__restrict __s, __const char *__restrict __format,
_G_va_list __arg);
printf函数是通过vfprintf将format输出到stdout文件中，stdout是(FILE *)类型。stdout的定义如下(glibc-2.3.6/libio/stdio.h)，顺被也给出stdin和stderr的定义：
/* Standard streams. */
extern struct _IO_FILE *stdin; /* Standard input stream. */
extern struct _IO_FILE *stdout; /* Standard output stream. */
extern struct _IO_FILE *stderr; /* Standard error output stream. */
/* C89/C99 say they're macros. Make them happy. */
#define stdin stdin
#define stdout stdout
#define stderr stderr
继续跟踪stdout(glibc-2.3.6/libio/stdio.c)：
_IO_FILE *stdin = (FILE *) &_IO_2_1_stdin_;
_IO_FILE *stdout = (FILE *) &_IO_2_1_stdout_;
_IO_FILE *stderr = (FILE *) &_IO_2_1_stderr_;
在继续分析_IO_2_1_stdout_之前，我们先看一下_IO_FILE(FILE和_IO_FILE是一回事，#define FILE _IO_FILE)的定义(glibc-2.3.6/libio/libio.h)：
struct _IO_FILE {
int _flags; /* High-order word is _IO_MAGIC; rest is flags. */
#define _IO_file_flags _flags
/* The following pointers correspond to the C++ streambuf protocol. */
/* Note: Tk uses the _IO_read_ptr and _IO_read_end fields directly. */
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;//这个就是linux内核中文件描述符fd
#if 0
int _blksize;
#else
int _flags2;
#endif
_IO_off_t _old_offset; /* This used to be _offset but it's too small. */
#define __HAVE_COLUMN /* temporary */
/* 1+column number of pbase(); 0 is unknown. */
unsigned short _cur_column;
signed char _vtable_offset;
char _shortbuf[1];
/* char* _save_gptr; char* _save_egptr; */
_IO_lock_t *_lock;
#ifdef _IO_USE_OLD_IO_FILE
};
struct _IO_FILE_plus
{
_IO_FILE file;
const struct _IO_jump_t *vtable;//IO函数跳转表
};
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