http://blog.csdn.net/zhanglianpin/article/details/44779009
在Keil C51 中使用printf ，首先需要重新實現 putchar(char c)函數。此函數在
<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">putchar</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> c)
{
ES=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
SBUF = c;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span>(TI==<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
TI=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
ES=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>
我們先分析一下上面這個程序哈，
關閉串口中斷
發送單字節數據
等待發送完畢
清除TI標誌
開啓串口中斷
在main函數裏可以直接使用printf函數進行輸出了。
但是，我一直存在這樣一個疑惑：
<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> main()
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">unsigned</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> test1 = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">55</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"the test is %d\r\n"</span>,test1);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>
使用串口輸出的數值一直不對，我後來自己理解，%d是整型，而在Keil C51整型佔用2個byte，所以我一般的解決辦法是做一次強制類型轉換：
<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> main()
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">unsigned</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> test1 = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">55</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"the test is %d\r\n"</span>,(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>)test1);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>
後來閱讀Keil C51的幫助手冊：
得到這樣一條信息：
| 格式 | 含義 | 針對類型 |
|---|---|---|
| %d | 兩個字節變量 | int |
| %bd | 單字節變量 | char |
| %ld | 四字節變量 | long int |
所以上面的問題的另一個解決方案是：
<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> main()
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">unsigned</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">char</span> test1 = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">55</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">printf</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"the test is %bd\r\n"</span>,test1);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>
下面附上Keil C51手冊內容。
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 the putchar function. The fmtstr argument 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 additional arguments are included in the printf function call.
The format string is read from left to right. The first format specification encountered references the first argument after fmtstr and converts and outputs it using the format specification. The second format specification accesses the second argument 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 by fmtstr.
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 and aaaa 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. The precision 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
<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Example <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">#include <stdio.h></span>
void tst_printf (void) {
char a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12365</span>;
long c = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0x7FFFFFFF</span>;
unsigned char <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">x</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'A'</span>;
unsigned <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">y</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">54321</span>;
unsigned long z = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0x4A6F6E00</span>;
float f = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10.0</span>;
float g = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">22.95</span>;
char buf [] = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Test String"</span>;
char <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">*p</span> = buf;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">printf</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"char <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%bd</span> int <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%d</span> long <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%ld</span>\n"</span>,a,b,c);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">printf</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Uchar <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%bu</span> Uint <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%u</span> Ulong <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%lu</span>\n"</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">x</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">y</span>,z);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">printf</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"xchar <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%bx</span> xint <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%x</span> xlong <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%lx</span>\n"</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">x</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">y</span>,z);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">printf</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"String <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%s</span> is at address <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%p</span>\n"</span>,buf,p);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">printf</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%f</span> != <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%g</span>\n"</span>, f, g);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">printf</span> (<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%*</span>f != <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%*</span>g\n"</span>, (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>)<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>, f, (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span>)<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>, g);
}</code>printf
| Summary | #include <stdio.h> int printf ( const char *<em>fmtstr</em> /* format string */ <[>, <em>arguments</em> ... <]>); /* 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 theputcharfunction. The fmtstr argument 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 enougharguments for the format specifications or if the argument types do not match those specified byfmtstr. Format specifications have the following general format: % <[><em>flags</em><]> <[><em>width</em><]> <[>.<em>precision</em><]> <[>{b|B|l|L}<]> <em>type</em>
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.
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.
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.
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.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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);
} |
Note
C51: PRINTF GIVES WRONG VALUES
Information in this article applies to:
- C51 All Versions
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.
printf 打印signed char 與unsigned char 之區別
先看一下例子
movsbl、movzbl的區別：
%dh = 8D
%eax = 98765432
movb %dh,%al //%eax = 9876548D
movsbl %dh,%eax //%eax = FFFFFF8D
movzbl %dh,%eax //%eax = 0000008D
如果 我們要一個字節一個字節的打印內存的信息的時候，還是需要使用unsigned char 作爲指針進行打印比較好。
%x是無符號整數的16進制，是無符號！而你的printf傳遞的實際是字符型，於是有一個類型提升的問題。p是無符號數，那麼0xff提升爲無符號整數仍然是0xff，而p1是有符號的，0xff實際的值是-1.那麼對應的無符號整數仍然應該爲-1的對應值，也就是0xffffffff。這纔是根本原因。