一、Newlib
arm-none-eabi-gcc編譯器組件中包含了一些庫文件，這些庫文件的具體信息可以查閱官網(https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads)中相應版本的Release Note得到。
以gcc-arm-none-eabi-9-2019-q4-major為例，它的C庫使用的是 newlib and newlib-nano : newlib-3.1.0，如果想要查看C庫函數的具體實現，可以通過該信息找到對應源代碼。
Newlib是一個面向嵌入式系統的C運行庫。最初是由Cygnus Solutions收集組裝的一個源代碼集合，取名為newlib，現在由Red Hat維護，官網地址https://sourceware.org/newlib/。
通過Git下載好對應版本的源代碼之后，可以看到C庫只是newlib的一部分，C庫具體路徑位於newlib-cygwin-cygwin-3_1_0-release/newlib-cygwin/newlib/libc 。
二、printf實現
printf函數的具體實現位於newlib/libc/stdio/printf.c:
int printf (const char *__restrict fmt, ...) { int ret; va_list ap; struct _reent *ptr = _REENT; _REENT_SMALL_CHECK_INIT (ptr); va_start (ap, fmt); ret = _vfprintf_r (ptr, _stdout_r (ptr), fmt, ap); va_end (ap); return ret; }
其中va_list, va_start, va_end幾個宏用於實現可變參數的解析，具體原理這里不再贅述。
struct _reent *ptr = _REENT;
這行代碼用於獲取printf的關鍵參數ptr, 該參數是一個指向_reent結構體的指針，_reent結構體具體為：
struct _reent { int _errno; /* local copy of errno */ /* FILE is a big struct and may change over time. To try to achieve binary compatibility with future versions, put stdin,stdout,stderr here. These are pointers into member __sf defined below. */ __FILE *_stdin, *_stdout, *_stderr; int _inc; /* used by tmpnam */ char _emergency[_REENT_EMERGENCY_SIZE]; /* TODO */ int _unspecified_locale_info; /* unused, reserved for locale stuff */ struct __locale_t *_locale;/* per-thread locale */ int __sdidinit; /* 1 means stdio has been init'd */ void (*__cleanup) (struct _reent *); /* used by mprec routines */ struct _Bigint *_result; int _result_k; struct _Bigint *_p5s; struct _Bigint **_freelist; /* used by some fp conversion routines */ int _cvtlen; /* should be size_t */ char *_cvtbuf; union { struct { unsigned int _unused_rand; char * _strtok_last; char _asctime_buf[_REENT_ASCTIME_SIZE]; struct __tm _localtime_buf; int _gamma_signgam; __extension__ unsigned long long _rand_next; struct _rand48 _r48; _mbstate_t _mblen_state; _mbstate_t _mbtowc_state; _mbstate_t _wctomb_state; char _l64a_buf[8]; char _signal_buf[_REENT_SIGNAL_SIZE]; int _getdate_err; _mbstate_t _mbrlen_state; _mbstate_t _mbrtowc_state; _mbstate_t _mbsrtowcs_state; _mbstate_t _wcrtomb_state; _mbstate_t _wcsrtombs_state; int _h_errno; } _reent; /* Two next two fields were once used by malloc. They are no longer used. They are used to preserve the space used before so as to allow addition of new reent fields and keep binary compatibility. */ struct { #define _N_LISTS 30 unsigned char * _nextf[_N_LISTS]; unsigned int _nmalloc[_N_LISTS]; } _unused; } _new; # ifndef _REENT_GLOBAL_ATEXIT /* atexit stuff */ struct _atexit *_atexit; /* points to head of LIFO stack */ struct _atexit _atexit0; /* one guaranteed table, required by ANSI */ # endif /* signal info */ void (**(_sig_func))(int); /* These are here last so that __FILE can grow without changing the offsets of the above members (on the off chance that future binary compatibility would be broken otherwise). */ struct _glue __sglue; /* root of glue chain */ # ifndef _REENT_GLOBAL_STDIO_STREAMS __FILE __sf[3]; /* first three file descriptors */ # endif };
_REENT是一個宏，將其展開后就指向了全局變量_impure_ptr，該變量就是一個（struct _reent *）類型的指針，並具體指向了impure_data結構體：
static struct _reent __ATTRIBUTE_IMPURE_DATA__ impure_data = _REENT_INIT (impure_data); struct _reent *__ATTRIBUTE_IMPURE_PTR__ _impure_ptr = &impure_data;
_REENT_INIT 宏用於impure_data結構體初始化，例如三個文件指針_stdin, _stdout, _stderr分別指向了impure_data結構體中最后的三個文件結構體__FILE __sf[3]。
_REENT_SMALL_CHECK_INIT (ptr);
這行代碼用於ptr指針的初始化，根據不同的宏定義進行展開。
ret = _vfprintf_r (ptr, _stdout_r (ptr), fmt, ap);
最后調用_vfprintf_r 進行下一步操作，_stdout_r (ptr)指向_stdout文件指針，即_impure_ptr->_stdout 。
三、_vfprintf_r實現
_vfprintf_r函數實現位於newlib/libc/stdio/vfprintf.c:
int _VFPRINTF_R (struct _reent *data, FILE * fp, const char *fmt0, va_list ap) { register char *fmt; /* format string */ register int ch; /* character from fmt */ register int n, m; /* handy integers (short term usage) */ register char *cp; /* handy char pointer (short term usage) */ register int flags; /* flags as above */ char *fmt_anchor; /* current format spec being processed */ #ifndef _NO_POS_ARGS int N; /* arg number */ int arg_index; /* index into args processed directly */ int numargs; /* number of varargs read */ char *saved_fmt; /* saved fmt pointer */ union arg_val args[MAX_POS_ARGS]; int arg_type[MAX_POS_ARGS]; int is_pos_arg; /* is current format positional? */ int old_is_pos_arg; /* is current format positional? */ #endif int ret; /* return value accumulator */ int width; /* width from format (%8d), or 0 */ int prec; /* precision from format (%.3d), or -1 */ char sign; /* sign prefix (' ', '+', '-', or \0) */
... ...
該函數實現了將可變參數進行格式化並輸出到stdout的過程，代碼比較龐大復雜，上面只貼了開頭的一部分。_vfprintf_r主要調用了以下幾個接口：
__sinit
__swsetup_r (cantwrite )
__sbprintf
__sprint_r
__sinit:
用於ptr指針的初始化，根據不同的宏定義進行展開。
__swsetup_r :
__swsetup_r的作用是配置stdout輸出文件，具體實現如下：
/* * Various output routines call wsetup to be sure it is safe to write, * because either _flags does not include __SWR, or _buf is NULL. * _wsetup returns 0 if OK to write, nonzero and set errno otherwise. */ int __swsetup_r (struct _reent *ptr, register FILE * fp) { /* Make sure stdio is set up. */ CHECK_INIT (_REENT, fp); /* * If we are not writing, we had better be reading and writing. */ if ((fp->_flags & __SWR) == 0) { if ((fp->_flags & __SRW) == 0) { ptr->_errno = EBADF; fp->_flags |= __SERR; return EOF; } if (fp->_flags & __SRD) { /* clobber any ungetc data */ if (HASUB (fp)) FREEUB (ptr, fp); fp->_flags &= ~(__SRD | __SEOF); fp->_r = 0; fp->_p = fp->_bf._base; } fp->_flags |= __SWR; } /* * Make a buffer if necessary, then set _w. * A string I/O file should not explicitly allocate a buffer * unless asprintf is being used. */ if (fp->_bf._base == NULL && (!(fp->_flags & __SSTR) || (fp->_flags & __SMBF))) __smakebuf_r (ptr, fp); if (fp->_flags & __SLBF) { /* * It is line buffered, so make _lbfsize be -_bufsize * for the putc() macro. We will change _lbfsize back * to 0 whenever we turn off __SWR. */ fp->_w = 0; fp->_lbfsize = -fp->_bf._size; } else fp->_w = fp->_flags & __SNBF ? 0 : fp->_bf._size; if (!fp->_bf._base && (fp->_flags & __SMBF)) { /* __smakebuf_r set errno, but not flag */ fp->_flags |= __SERR; return EOF; } return 0; }
注意在調用該函數之前，stdout->_p指針（指向stdout文件的輸出Buffer）還是空的，通過該函數會給這個Buffer分配內存，分配內存的調用路徑為：
__smakebuf_r ----> _malloc_r(nano_malloc) ---> sbrk_aligned ---> _sbrk_r ---> _sbrk
_sbrk為malloc的底層實現,用於開辟新的大塊內存空間，該函數實現與體系結構相關，ARM體系的實現位於newlib/libc/sys/arm/syscall.c,函數具體實現如下：
void * __attribute__((weak)) _sbrk (ptrdiff_t incr) { extern char end asm ("end"); /* Defined by the linker. */ static char * heap_end; char * prev_heap_end; if (heap_end == NULL) heap_end = & end; prev_heap_end = heap_end; if ((heap_end + incr > stack_ptr) /* Honour heap limit if it's valid. */ || (__heap_limit != 0xcafedead && heap_end + incr > (char *)__heap_limit)) { /* Some of the libstdc++-v3 tests rely upon detecting out of memory errors, so do not abort here. */ #if 0 extern void abort (void); _write (1, "_sbrk: Heap and stack collision\n", 32); abort (); #else errno = ENOMEM; return (void *) -1; #endif } heap_end += incr; return (void *) prev_heap_end; }
注意函數中有個end符號：
extern char end asm ("end"); /* Defined by the linker. */
這個符號需要在Linker腳本中定義，用於指定brk分配內存的起始地址，如果 未定義則會出現鏈接錯誤。
__sbprintf :
這是一個輔助函數，通過一個臨時Buffer, 實現unbufferd文件的輸出。
/* * Helper function for `fprintf to unbuffered unix file': creates a * temporary buffer. We only work on write-only files; this avoids * worries about ungetc buffers and so forth. * * Make sure to avoid inlining. */ _NOINLINE_STATIC int __sbprintf (struct _reent *rptr, register FILE *fp, const char *fmt, va_list ap) { int ret; FILE fake; unsigned char buf[BUFSIZ]; /* copy the important variables */ fake._flags = fp->_flags & ~__SNBF; fake._flags2 = fp->_flags2; fake._file = fp->_file; fake._cookie = fp->_cookie; fake._write = fp->_write; /* set up the buffer */ fake._bf._base = fake._p = buf; fake._bf._size = fake._w = sizeof (buf); fake._lbfsize = 0; /* not actually used, but Just In Case */ #ifndef __SINGLE_THREAD__ __lock_init_recursive (fake._lock); #endif /* do the work, then copy any error status */ ret = _VFPRINTF_R (rptr, &fake, fmt, ap); if (ret >= 0 && _fflush_r (rptr, &fake)) ret = EOF; if (fake._flags & __SERR) fp->_flags |= __SERR; #ifndef __SINGLE_THREAD__ __lock_close_recursive (fake._lock); #endif return (ret); }
__sprint_r：
執行輸出，將格式化完成的字符串輸出到stdout->_p指向的buffer
/* * Flush out all the vectors defined by the given uio, * then reset it so that it can be reused. */ int __sprint_r (struct _reent *ptr, FILE *fp, register struct __suio *uio) { register int err = 0; if (uio->uio_resid == 0) { uio->uio_iovcnt = 0; return (0); } #ifdef _WIDE_ORIENT if (fp->_flags2 & __SWID) { struct __siov *iov; wchar_t *p; int i, len; iov = uio->uio_iov; for (; uio->uio_resid != 0; uio->uio_resid -= len * sizeof (wchar_t), iov++) { p = (wchar_t *) iov->iov_base; len = iov->iov_len / sizeof (wchar_t); for (i = 0; i < len; i++) { if (_fputwc_r (ptr, p[i], fp) == WEOF) { err = -1; goto out; } } } } else #endif err = __sfvwrite_r(ptr, fp, uio); out: uio->uio_resid = 0; uio->uio_iovcnt = 0; return (err); }
__sprint_r隨后調用__sfvwrite_r做進一步輸出.
__sfvwrite_r函數中有兩種路徑進入下一步輸出，分別為：
（1）_fflush_r ------ __sflush_r ------ fp->_write
(2) fp->_write
兩種路徑最終都指向了函數指針 fp->_write，該指針在_fopen_r函數中做了設定：
FILE * _fopen_r (struct _reent *ptr, const char *__restrict file, const char *__restrict mode) { register FILE *fp; register int f; int flags, oflags; if ((flags = __sflags (ptr, mode, &oflags)) == 0) return NULL; if ((fp = __sfp (ptr)) == NULL) return NULL; if ((f = _open_r (ptr, file, oflags, 0666)) < 0) { _newlib_sfp_lock_start (); fp->_flags = 0; /* release */ #ifndef __SINGLE_THREAD__ __lock_close_recursive (fp->_lock); #endif _newlib_sfp_lock_end (); return NULL; } _newlib_flockfile_start (fp); fp->_file = f; fp->_flags = flags; fp->_cookie = (void *) fp; fp->_read = __sread; fp->_write = __swrite; fp->_seek = __sseek; fp->_close = __sclose; if (fp->_flags & __SAPP) _fseek_r (ptr, fp, 0, SEEK_END); #ifdef __SCLE if (__stextmode (fp->_file)) fp->_flags |= __SCLE; #endif _newlib_flockfile_end (fp); return fp; }
fp->_write指向了__swrite 函數，__swrite 函數通過_write_r ---- _write 路徑完成最終輸出，如果想要改變printf輸出方式，可以對_write_r 或者 _write 函數進行重定義，替換成自定義函數，即可實現對應的輸出效果。