C语言 char型也是分有符号数和无符号数的
unsigned char型表示无符号数, 取值范围是 0~255.
signed char型表示无符号数, 取值范围是 -128~127.
在x86平台上, gcc规定不带'unsigned'或'signed'关键字的char是有符号的. 参见以下代码 :
#include<stdio.h>
int main(void)
{
printf("(signed char)200: %d\n", (signed char)200);
printf("(unsigned char)200: %d\n", (unsigned char)200);
printf("(char)200: %d\n", (char)200); // gcc定义char型是有符号的。
return 0;
}
打印结果为:
(signed char)200: -56
(unsigned char)200: 200
(char)200: -56
利用objdump工具对其进行反汇编, 观察向printf函数传递的第二个参数, 节选部分汇编代码:
printf("(signed char)200: %d\n", (signed char)200);
80483cd: b8 e0 84 04 08 mov $0x80484e0,%eax
80483d2: c7 44 24 04 c8 ff ff movl $0xffffffc8,0x4(%esp)
80483d9: ff
80483da: 89 04 24 mov %eax,(%esp)
80483dd: e8 12 ff ff ff call 80482f4 <printf@plt>
printf("(unsigned char)200: %d\n", (unsigned char)200);
80483e2: b8 f6 84 04 08 mov $0x80484f6,%eax
80483e7: c7 44 24 04 c8 00 00 movl $0xc8,0x4(%esp)
80483ee: 00
80483ef: 89 04 24 mov %eax,(%esp)
80483f2: e8 fd fe ff ff call 80482f4 <printf@plt>
printf("(char)200: %d\n", (char)200); // gcc定义char型是有符号的。
80483f7: b8 0e 85 04 08 mov $0x804850e,%eax
80483fc: c7 44 24 04 c8 ff ff movl $0xffffffc8,0x4(%esp)
8048403: ff
8048404: 89 04 24 mov %eax,(%esp)
8048407: e8 e8 fe ff ff call 80482f4 <printf@plt>
还有一个问题需要注意, 从汇编代码可以看到, 实际上用了四个字节存储char类型的参量, 即: 像printf这种形参类型未知的函数, 调用函数时编译器会自动对相应的实参做Integer Promotion.
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