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):
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- #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):
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- 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:
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- /* 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):
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- _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):
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- 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
- };
下面我们看看_IO_2_1_stdout_的定义(glibc-2.3.6/libio/stdfiles.c),顺便给出_IO_2_1_stdin_和_IO_2_1_stderr_的定义:
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- # define DEF_STDFILE(NAME, FD, CHAIN, FLAGS) \
- struct _IO_FILE_plus NAME \
- = {FILEBUF_LITERAL(CHAIN, FLAGS, FD, NULL), \
- &_IO_file_jumps};
-
-
- DEF_STDFILE(_IO_2_1_stdin_, 0, 0, _IO_NO_WRITES);
- DEF_STDFILE(_IO_2_1_stdout_, 1, &_IO_2_1_stdin_, _IO_NO_READS);
- DEF_STDFILE(_IO_2_1_stderr_, 2, &_IO_2_1_stdout_, _IO_NO_READS+_IO_UNBUFFERED);
从这里我们可以看到,_IO_2_1_stdout_的FD = 0、_IO_2_1_stdin_的FD = 1、_IO_2_1_stderr_的FD = 2。FILEBUF_LITERAL用于初始化_IO_FILE,定义如下(glibc-2.3.6/libio/libioP.h):
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- # define FILEBUF_LITERAL(CHAIN, FLAGS, FD, WDP) \
- { _IO_MAGIC+_IO_LINKED+_IO_IS_FILEBUF+FLAGS, \
- 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, (_IO_FILE *) CHAIN, FD, \
- 0, _IO_pos_BAD, 0, 0, { 0 }, 0, _IO_pos_BAD, \
- 0 }
其中,FD赋值给了_fileno。我们回到vfprintf的分析,vfprintf的具体实现本文就不详细讲解,主要原理是格式化字符串,最后将字符串输出到文件中,也就是stdout中。至于如何输出,则和_IO_file_jumps关系密切,_IO_file_jumps的定义如下:
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- const struct _IO_jump_t _IO_file_jumps =
- {
- JUMP_INIT_DUMMY,
- JUMP_INIT(finish, INTUSE(_IO_file_finish)),
- JUMP_INIT(overflow, INTUSE(_IO_file_overflow)),
- JUMP_INIT(underflow, INTUSE(_IO_file_underflow)),
- JUMP_INIT(uflow, INTUSE(_IO_default_uflow)),
- JUMP_INIT(pbackfail, INTUSE(_IO_default_pbackfail)),
- JUMP_INIT(xsputn, INTUSE(_IO_file_xsputn)),
- JUMP_INIT(xsgetn, INTUSE(_IO_file_xsgetn)),
- JUMP_INIT(seekoff, _IO_new_file_seekoff),
- JUMP_INIT(seekpos, _IO_default_seekpos),
- JUMP_INIT(setbuf, _IO_new_file_setbuf),
- JUMP_INIT(sync, _IO_new_file_sync),
- JUMP_INIT(doallocate, INTUSE(_IO_file_doallocate)),
- JUMP_INIT(read, INTUSE(_IO_file_read)),
- JUMP_INIT(write, _IO_new_file_write),
- JUMP_INIT(seek, INTUSE(_IO_file_seek)),
- JUMP_INIT(close, INTUSE(_IO_file_close)),
- JUMP_INIT(stat, INTUSE(_IO_file_stat)),
- JUMP_INIT(showmanyc, _IO_default_showmanyc),
- JUMP_INIT(imbue, _IO_default_imbue)
- };
至于怎么跳转到这些函数,以及如何跳转到linux内核,由于涉及到glibc的一些细节,这里简单介绍一下进入内核后的情况:进入linux内核后,调用write(),在write之前所有的代码都是C库的代码,可以说是和平台无关的。而涉及到具体输出,就要调用操作系统提供给的接口。调用write()后,通过系统调用进入内核空间,首先是sys_write(),这个函数代码位于fs/read_write.c中。一进入sys_write(),就要根据传进来的fd描述符找到相应的file结构。对于标准输出,fd= 1,每个进程的进程控制块都有一个打开文件的数组files。file结构就是根据fd在这个数组中查找到相应的结构。找到结构后,就会调用file->write()来向外输出。具体输出到哪里,就要看file结构对应的设备驱动是什么。
通过本文可以理解:文件描述符0、1和2和stdout、stdin和stderr对应,如果要修改linux内核中文件描述符相关代码,一定要注意文件描述符0、1和2的分配和回收,否则会导致终端没有输出信息,也无法和内核输入信息。