在keil中使用printf()函数的要点

　　在keil中printf默认是向串口中发送数据的，所以，如果应用该函数，必须先初始化串口，否则可能引起死机的情况，并且在printf之前应该先将TI置位，摘抄原因如下:
　　1、printf函数是调用putchar函数输入的，而putchar应该是先判断ti是否为1，不为1则等待为1。如果为1则清0，然后送出一个字符。因此你如果直接使用printf函数，你的程序就会在putchar函数中等待ti为1。这时你的程序就相当于直接死掉了。你可以通过改写putchar函数实现自己的目的。TI相当于是初始化~不给赋初值就不干活
　　2、Keil的串口处理比较巧妙的，我的分析如下：
　　putchar.c里面，是先检测TI再发送。这样做的目的是把尽可能多的时间留给2次串口操作之间的程序，而不是把等待字节发送的时间白白空等待浪费掉。所以，在系统初始化的时候，一定要令TI=1; 就可以顺畅的使用printf函数了。搂主sbuf=" "的办法，其实就是令TI=1. 另外要特别注意，printf函数执行完毕后，最后一个字节并未发送完毕，例如在485通讯中，此时如果切换为收模式，会丢失最后一字节.
　　3、一般串口发送都是等TI(字节发送完标志)为1就马上发送下一字节，由于不管是中断还是查询TI标志的方法，都会检测TI，因此首次发送必须置位TI标志，使串口开始发送你的“在程序的初始化部分往串口数据寄存器SBUF里放一个字符来起用终端显示;”方法最终作用也就是把TI置1，改成TI＝1;来启动发送也是一样的（当然，不会发出那个' '字符了）。
　　4、<stdio.h>中定义，调用底层的putchar()来实现.底层发送数据到串口时，先查TI=1是否成立，死等直到TI=1时将新数据写入SBUF，函数返回，所以要先将TI置1，启动第一次传输操作.可查看反汇编相关代码理解其工作机理!
　　5、自己理解:在多机通讯中，应该也要形成像putchar()函数的机理，要有串口中断服务程序，并且是在该程序中判断TI，根据TI是不是该发送下一组数据~同样对于接收的一方..

　　下面举一个简单的例子：

//===========================
#include <reg51.h>
#include <stdio.h>
//-------------------------------
int main()
{
    Uart_init();   //初始化串口，这里就不写具体代码了。
    TI = ;                        //keil 调用stdio.h中printf函数前要置位。

    )
    {
         printf("Hello world!\n");
         delay_ms();   //延时程序，这里也不写具体代码了。
    }
    ;
}

printf

http://www.keil.com/support/man/docs/c51/c51_printf.htm

Home »Library Reference »Reference » printf

Summary

#include <stdio.h>

int printf (
  const char *fmtstr       /* format string */
  <[>, arguments ... <]>);   /* additional arguments */

Description

The printf function formats a series of strings and numeric values and builds a string to write to the output stream using theputchar function. The fmtstrargument is a format string that may be composed of characters, escape sequences, and format specifications.

Ordinary characters and escape sequences are copied to the stream in the order in which they are interpreted. Format specifications always begin with a percent sign ('%') and require that additionalarguments are included in the printf function call.

The format string is read from left to right. The first format specification encountered references the firstargument after fmtstr and converts and outputs it using the format specification. The second format specification accesses the secondargument after fmtstr, and so on. If there are more arguments than format specifications, extra arguments are ignored. Results are unpredictable if there are not enough arguments for the format specifications or if the argument types do not match those specified byfmtstr.

Format specifications have the following general format:

% <[>flags<]> <[>width<]> <[>.precision<]> <[>{b|B|l|L}<]> type

Each field in the format specification may be a single character or a number which specifies a particular format option.

The type field is a single character that specifies whether the argument is interpreted as a character, string, number, or pointer, as shown in the following table.

Type	Argument Type	Input Format
d	int	Signed decimal number.
u	unsigned int	Unsigned decimal number.
o	unsigned int	Unsigned octal number.
x	unsigned int	Unsigned hexadecimal number using "0123456789abcedf".
X	unsigned int	Unsigned hexadecimal number using "0123456789ABCDEF".
f	float	Floating-point number formatted as <[>-<]>dddd.dddd.
e	float	Floating-point number formatted as <[>-<]>d.dddde<[>-<]>dd.
E	float	Floating-point number formatted as <[>-<]>d.ddddE<[>-<]>dd.
g	float	Floating-point number using either the e or f format, whichever is more compact for the specified value and precision.
G	float	Floating-point number using either the E or f format, whichever is more compact for the specified value and precision.
c	char	A single character.
s	*	A string of characters terminated by a null character ('\0').
p	*	A generic pointer formatted as t:aaaa where t is the memory type andaaaa is the hexadecimal address.

 Note

• The optional characters l or L may immediately precede the type character to respectively specify long types for d, i, u, o, x, and X.
• The optional characters b or B may immediately precede the type character to respectively specify char types for d, i, u, o, x, and X.

Characters following a percent sign that are not recognized as a format specification are treated as ordinary characters. For example, "%%" writes a single percent sign to the output stream.

The flags field is a single character used to justify the output and to print +/- signs and blanks, decimal points, and octal and hexadecimal prefixes, as shown in the following table.

Flag	Description
-	Left justify the output in the specified field width.
+	Prefix the output value with a + or - sign if the output is a signed type.
blank (' ')	Prefix the output value with a blank if it is a signed positive value. Otherwise, no blank is prefixed.
#	Prefixes a non-zero output value with 0, 0x, or 0X when used with o, x, and X field types, respectively. When used with the e, E, f, g, and G field types, the # flag forces the output value to include a decimal point. The # flag is ignored in all other cases.

The width field is a non-negative number that specifies the minimum number of characters printed. If the number of characters in the output value is less than width, blanks are added on the left (by default) or right (when the - flag is specified) to pad to the minimum width. If width is prefixed with a '0', zeros are padded instead of blanks. The width field never truncates the output. If the length of the output value exceeds the specified width, all characters are output.

The width field may be an asterisk ('*'), in which case an int argument from the argument list provides the width value. Specifying a 'b' in front of the asterisk specifies that the argument is an unsigned char.

The precision field is a non-negative number that specifies the number of characters to print, the number of significant digits, or the number of decimal places. Theprecision field can cause truncation or rounding of the output value in the case of a floating-point number as specified in the following table.

Type	Precision Field Meaning
d,u,o,x,X	The precision field specifies the minimum number of digits that are included in the output value. Digits are not truncated if the number of digits in the argument exceeds that defined in the precision field. If the number of digits in the argument is less than the precision field, the output value is padded on the left with zeros.
f	The precision field specifies the number of digits to the right of the decimal point. The last digit is rounded.
e,E	The precision field specifies the number of digits to the right of the decimal point. The last digit is rounded.
g,G	The precision field specifies the maximum number of significant digits in the output value.
s	The precision field specifies the maximum number of characters in the output value. Excess characters are not output.
c,p	The precision field has no effect on these field types.

The precision field may be an asterisk ('*'), in which case an int argument from the argument list provides the value. Specifying a 'b' in front of the asterisk specifies that the argument is an unsigned char.

 Note

• You must ensure that the argument type matches that of the format specification. You may use type casts to ensure that the proper type is passed to printf.
• This function is implementation-specific and is based on the operation of the_getkey and putchar functions. These functions, as provided in the standard library, read and write characters using the microcontroller's serial port. Custom functions may use other I/O devices.
• The total number of bytes that may be passed to this function is limited due to the memory restrictions imposed by the 8051. A maximum of 15 bytes may be passed in SMALL or COMPACT model. A maximum of 40 bytes may be passed in LARGE model.

Return Value

The printf function returns the number of characters actually written to the output stream.

See Also

gets,printf517,puts,scanf,scanf517,sprintf,sprintf517,sscanf,sscanf517,vprintf,vsprintf

Example

#include <stdio.h>

void tst_printf (void) {
  char a = 1;
  int b  = 12365;
  long c = 0x7FFFFFFF;

  unsigned char x = 'A';
  unsigned int y  = 54321;
  unsigned long z = 0x4A6F6E00;

  float f = 10.0;
  float g = 22.95;

  char buf [] = "Test String";
  char *p = buf;

  printf ("char %bd int %d long %ld\n",a,b,c);
  printf ("Uchar %bu Uint %u Ulong %lu\n",x,y,z);
  printf ("xchar %bx xint %x xlong %lx\n",x,y,z);
  printf ("String %s is at address %p\n",buf,p);
  printf ("%f != %g\n", f, g);
  printf ("%*f != %*g\n", (int)8, f, (int)8, g);
}

--------------------------------

C51: PRINTF GIVES WRONG VALUES

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Information in this article applies to:

• C51 All Versions

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QUESTION

In C51, I am using printf and sprintf with many arguments and it is printing out wrong values. What is the reason for this behavior?

ANSWER

There are any number of reasons why printf can output incorrect values.

Probably the most common reason is that the format specifier and the argument type do not match. To solve this kind of problem, explicitly type cast each argument to the desired type. Be sure that you accurately specify the correct format specifier for that type.

Another reason may be that the number of bytes you may pass to functions with variable-length argument lists is fixed (since arguments are not passed on the stack). In small and compact memory model, printf and sprintf only reserve 15 bytes for the argument list. In the large memory model 40 bytes are reserved. For example, 15 bytes allow you to pass up to five generic (3-byte) pointers, or one generic pointer and three long arguments. This parameter limitation is a compromise to the limited memory available in 8051 devices.

Eight arguments will not fit in the space reserved for small or compact memory models. So, instead of one sprintf call with many arguments you must perform two or three calls and distribute the arguments among the sprintf calls. For example:

The following code exceeds the parameter space limits since seven generic pointers (21 bytes) are passed to sprintf.

char str[] = "Any Text";
sprintf (buff_out, "%s %s %s %s %s", str, str, str, str, str);

The above example may be rewritten as follows:

char str[] = "Any Text";
int len;

len = sprintf (buff_out, "%s %s %s ", str, str, str);
sprintf (buff_out + len, "%s %s", str, str);

MORE INFORMATION

• Refer to printf in the Cx51 User's Guide.