user-0 提问: 在一个返回值为int型的函数中，在printf语句之后接return；返回的结果是printf输出的长度，有什么原理吗？

回复:
user: user-1
content: printf平日你头发 拼音输入法

user: user-2
content: 汇编可以解释……

user: user-1
content: printf返回平日你头发输出的长度，保存到eax中，返回函数后，认为函数的返回值在eax中。如此而已。（Win32）  理解和讨论之前请先学会如何观察！  计算机组成原理→DOS命令→汇编语言→C语言（不包括C++）、代码书写规范→数据结构、编译原理、操作系统→计算机网络、数据库原理、正则表达式→其它语言（包括C++）、架构……  对学习编程者的忠告： 多用小脑和手，少用大脑、眼睛和嘴，会更快地学会编程！ 眼过千遍不如手过一遍！ 书看千行不如手敲一行！ 手敲千行不如单步一行！ 单步源代码千行不如单步Debug版对应汇编一行！ 单步Debug版对应汇编千行不如单步Release版对应汇编一行！  VC调试时按Alt+8、Alt+7、Alt+6和Alt+5,打开汇编窗口、堆栈窗口、内存窗口和寄存器窗口看每句C对应的汇编、单步执行并观察相应堆栈、内存和寄存器变化，这样过一遍不就啥都明白了吗。 对VC来说，所谓‘调试时’就是编译连接通过以后，按F10或F11键单步执行一步以后的时候，或者在某行按F9设了断点后按F5执行停在该断点处的时候。 （Turbo C或Borland C用Turbo Debugger调试,Linux或Unix下用GDB调试时,看每句C对应的汇编并单步执行观察相应内存和寄存器变化。）  printf, wprintf Print formatted output to the standard output stream.  int printf( const char *format [, argument]... );  int wprintf( const wchar_t *format [, argument]... );  Routine Required Header Compatibility  printf <stdio.h> ANSI, Win 95, Win NT  wprintf <stdio.h> or <wchar.h> ANSI, Win 95, Win NT    For additional compatibility information, see Compatibility in the Introduction.  Libraries  LIBC.LIB Single thread static library, retail version  LIBCMT.LIB Multithread static library, retail version  MSVCRT.LIB Import library for MSVCRT.DLL, retail version    Return Value  Each of these functions returns the number of characters printed, or a negative value if an error occurs.  Parameters  format  Format control  argument  Optional arguments  Remarks  The printf function formats and prints a series of characters and values to the standard output stream, stdout. If arguments follow the format string, the format string must contain specifications that determine the output format for the arguments. printf and fprintf behave identically except that printf writes output to stdout rather than to a destination of type FILE.  wprintf is a wide-character version of printf; format is a wide-character string. wprintf and printf behave identically otherwise.   Generic-Text Routine Mappings  TCHAR.H Routine  _UNICODE & _MBCS Not Defined _MBCS Defined _UNICODE Defined  _tprintf printf printf wprintf    The format argument consists of ordinary characters, escape sequences, and (if arguments follow format) format specifications. The ordinary characters and escape sequences are copied to stdout in order of their appearance. For example, the line  printf("Line one\n\t\tLine two\n");   produces the output  Line one         Line two  Format specifications always begin with a percent sign (%) and are read left to right. When printf encounters the first format specification (if any), it converts the value of the first argument after format and outputs it accordingly. The second format specification causes the second argument to be converted and output, and so on. If there are more arguments than there are format specifications, the extra arguments are ignored. The results are undefined if there are not enough arguments for all the format specifications.  Example  /* PRINTF.C: This program uses the printf and wprintf functions  * to produce formatted output.  */  #include <stdio.h>  void main( void ) {    char   ch = 'h', *string = "computer";    int    count = -9234;    double fp = 251.7366;    wchar_t wch = L'w', *wstring = L"Unicode";     /* Display integers. */    printf( "Integer formats:\n"            "\tDecimal: %d  Justified: %.6d  Unsigned: %u\n",            count, count, count, count );     printf( "Decimal %d as:\n\tHex: %Xh  C hex: 0x%x  Octal: %o\n",             count, count, count, count );     /* Display in different radixes. */    printf( "Digits 10 equal:\n\tHex: %i  Octal: %i  Decimal: %i\n",             0x10, 010, 10 );     /* Display characters. */     printf("Characters in field (1):\n%10c%5hc%5C%5lc\n", ch, ch, wch, wch);    wprintf(L"Characters in field (2):\n%10C%5hc%5c%5lc\n", ch, ch, wch, wch);     /* Display strings. */     printf("Strings in field (1):\n%25s\n%25.4hs\n\t%S%25.3ls\n",    string, string, wstring, wstring);    wprintf(L"Strings in field (2):\n%25S\n%25.4hs\n\t%s%25.3ls\n",        string, string, wstring, wstring);     /* Display real numbers. */    printf( "Real numbers:\n\t%f %.2f %e %E\n", fp, fp, fp, fp );     /* Display pointer. */    printf( "\nAddress as:\t%p\n", &count);     /* Count characters printed. */    printf( "\nDisplay to here:\n" );    printf( "1234567890123456%n78901234567890\n", &count );    printf( "\tNumber displayed: %d\n\n", count ); }   Output  Integer formats:    Decimal: -9234  Justified: -009234  Unsigned: 4294958062 Decimal -9234 as:    Hex: FFFFDBEEh  C hex: 0xffffdbee  Octal: 37777755756 Digits 10 equal:    Hex: 16  Octal: 8  Decimal: 10 Characters in field (1):          h    h    w    w Characters in field (2):          h    h    w    w Strings in field (1):                  computer                      comp    Unicode                      Uni Strings in field (2):                  computer                      comp    Unicode                      Uni Real numbers:    251.736600 251.74 2.517366e+002 2.517366E+002  Address as:   0012FFAC  Display to here: 123456789012345678901234567890    Number displayed: 16   Floating-Point Support Routines |  Stream I/O Routines |  Locale Routines  See Also   fopen, fprintf, scanf, sprintf, vprintf Functions

user: user-3
content: 这个应该是函数栈表的问题，跟函数栈表压栈顺序有关

user: user-4
content: 这个是未定义行为, 返回值未知，只是这里恰好是printf的返回值. printf返回值是int, 直接存放在寄存器里面, 调用printf后没有进行其他操作, 这个值没变就又直接返回了。

user: user-5
content: http://www.cplusplus.com/reference/cstdio/printf/Return Value On success, the total number of characters written is returned.  If a writing error occurs, the error indicator (ferror) is set and a negative number is returned.  If a multibyte character encoding error occurs while writing wide characters, errno is set to EILSEQ and a negative number is returned.