如题所述
第1个回答 2022-10-26
答: -1 是个最大的负整数，0-1=-1， -1在计算机中用补码表示。
-1的表现形式与内存类型相关，通常，-1用32bits 补码表示(0xffffffff)，根据上下文，
也可能被变为8bits(0xff), 16bits(0xffff), 64bits
看代码：
#include
int main(int argc, char *argv[])
{
int a = -1; // 默认的, -1 为4byes 0xffffffff
long int b = -1; // 根据上下文, 内存类型为8bytes, -1 被转换为 0xffffffffffffffff
char c = -1; // 根据上下文， 内存类型为1bytes, -1 被转换为 0xff
short d = -1; // 根据上下文， 内存类型为2bytes, -1 被转换为 0xffff
printf("%d %xn",-1,-1); // nature
printf("%ld %lxn",(long int)-1,(long int)-1); // -1 被强制转换为64bits(-1)
// char 数据类型用32bits 表示，不会触发数据变换, 上下文要求传递函数参数，故仍用32bits 表示。
printf("%c %xn",(char)-1,(char)-1);
printf("size of int: %dn",sizeof(int));
printf("size of long int: %dn",sizeof(long int));
return 0;
}
看 gdb 反汇编代码， 加深理解！
(gdb) disassemble /m main
Dump of assembler code for function main:
4{
0x00000000004004c4 :push %rbp
0x00000000004004c5 :mov %rsp,%rbp
0x00000000004004c8 :sub $0x30,%rsp
0x00000000004004cc :mov %edi,-0x24(%rbp)
0x00000000004004cf :mov %rsi,-0x30(%rbp)
5int a = -1;// 默认的,-1 为4byes 0xffffffff
=> 0x00000000004004d3 :movl $0xffffffff,-0x14(%rbp)
6long int b = -1; // 根据上下文, 内存类型为8bytes, -1 被转换为 0xffffffffffffffff
0x00000000004004da :movq $0xffffffffffffffff,-0x10(%rbp)
7char c = -1;// 根据上下文， 内存类型为4bytes, -1 被转换为 0xff
0x00000000004004e2 :movb $0xff,-0x3(%rbp)
8short d = -1;// 根据上下文， 内存类型为1bytes, -1 被转换为 0xffff
0x00000000004004e6 :movw $0xffff,-0x2(%rbp)
9printf("%d %xn",-1,-1);// nature
0x00000000004004ec :mov $0x400678,%eax
0x00000000004004f1 :mov $0xffffffff,%edx
0x00000000004004f6 :mov $0xffffffff,%esi
---Type to continue, or q to quit---
0x00000000004004fb :mov %rax,%rdi
0x00000000004004fe :mov $0x0,%eax
0x0000000000400503 :callq 0x4003b8
10printf("%ld %lxn",(long int)-1,(long int)-1); // -1 被强制转换为64bits(-1)
0x0000000000400508 :mov $0x40067f,%eax
0x000000000040050d :mov $0xffffffffffffffff,%rdx
0x0000000000400514 :mov $0xffffffffffffffff,%rsi
0x000000000040051b :mov %rax,%rdi
0x000000000040051e :mov $0x0,%eax
0x0000000000400523 :callq 0x4003b8
11// char 数据类型用32bits 表示，不会触发数据变换, 上下文要求传递函数参数，故仍用32bits 表示。
12printf("%c %xn",(char)-1,(char)-1);
0x0000000000400528 :mov $0x400688,%eax
0x000000000040052d :mov $0xffffffff,%edx
0x0000000000400532 :mov $0xffffffff,%esi
0x0000000000400537 :mov %rax,%rdi
0x000000000040053a :mov $0x0,%eax
0x000000000040053f :callq 0x4003b8
13printf("size of int: %dn",sizeof(int));
0x0000000000400544 :mov $0x40068f,%eax
0x0000000000400549 :mov $0x4,%esi
0x000000000040054e :mov %rax,%rdi
0x0000000000400551 :mov $0x0,%eax
0x0000000000400556 :callq 0x4003b8
14printf("size of long int: %dn",sizeof(long int));
0x000000000040055b :mov $0x4006a0,%eax
0x0000000000400560 :mov $0x8,%esi
0x0000000000400565 :mov %rax,%rdi
0x0000000000400568 :mov $0x0,%eax
0x000000000040056d :callq 0x4003b8
15return 0;
0x0000000000400572 :mov $0x0,%eax
16}
0x0000000000400577 :leaveq
0x0000000000400578 :retq
End of assembler dump.
(gdb)
-1的表现形式与内存类型相关，通常，-1用32bits 补码表示(0xffffffff)，根据上下文，
也可能被变为8bits(0xff), 16bits(0xffff), 64bits
看代码：
#include
int main(int argc, char *argv[])
{
int a = -1; // 默认的, -1 为4byes 0xffffffff
long int b = -1; // 根据上下文, 内存类型为8bytes, -1 被转换为 0xffffffffffffffff
char c = -1; // 根据上下文， 内存类型为1bytes, -1 被转换为 0xff
short d = -1; // 根据上下文， 内存类型为2bytes, -1 被转换为 0xffff
printf("%d %xn",-1,-1); // nature
printf("%ld %lxn",(long int)-1,(long int)-1); // -1 被强制转换为64bits(-1)
// char 数据类型用32bits 表示，不会触发数据变换, 上下文要求传递函数参数，故仍用32bits 表示。
printf("%c %xn",(char)-1,(char)-1);
printf("size of int: %dn",sizeof(int));
printf("size of long int: %dn",sizeof(long int));
return 0;
}
看 gdb 反汇编代码， 加深理解！
(gdb) disassemble /m main
Dump of assembler code for function main:
4{
0x00000000004004c4 :push %rbp
0x00000000004004c5 :mov %rsp,%rbp
0x00000000004004c8 :sub $0x30,%rsp
0x00000000004004cc :mov %edi,-0x24(%rbp)
0x00000000004004cf :mov %rsi,-0x30(%rbp)
5int a = -1;// 默认的,-1 为4byes 0xffffffff
=> 0x00000000004004d3 :movl $0xffffffff,-0x14(%rbp)
6long int b = -1; // 根据上下文, 内存类型为8bytes, -1 被转换为 0xffffffffffffffff
0x00000000004004da :movq $0xffffffffffffffff,-0x10(%rbp)
7char c = -1;// 根据上下文， 内存类型为4bytes, -1 被转换为 0xff
0x00000000004004e2 :movb $0xff,-0x3(%rbp)
8short d = -1;// 根据上下文， 内存类型为1bytes, -1 被转换为 0xffff
0x00000000004004e6 :movw $0xffff,-0x2(%rbp)
9printf("%d %xn",-1,-1);// nature
0x00000000004004ec :mov $0x400678,%eax
0x00000000004004f1 :mov $0xffffffff,%edx
0x00000000004004f6 :mov $0xffffffff,%esi
---Type to continue, or q to quit---
0x00000000004004fb :mov %rax,%rdi
0x00000000004004fe :mov $0x0,%eax
0x0000000000400503 :callq 0x4003b8
10printf("%ld %lxn",(long int)-1,(long int)-1); // -1 被强制转换为64bits(-1)
0x0000000000400508 :mov $0x40067f,%eax
0x000000000040050d :mov $0xffffffffffffffff,%rdx
0x0000000000400514 :mov $0xffffffffffffffff,%rsi
0x000000000040051b :mov %rax,%rdi
0x000000000040051e :mov $0x0,%eax
0x0000000000400523 :callq 0x4003b8
11// char 数据类型用32bits 表示，不会触发数据变换, 上下文要求传递函数参数，故仍用32bits 表示。
12printf("%c %xn",(char)-1,(char)-1);
0x0000000000400528 :mov $0x400688,%eax
0x000000000040052d :mov $0xffffffff,%edx
0x0000000000400532 :mov $0xffffffff,%esi
0x0000000000400537 :mov %rax,%rdi
0x000000000040053a :mov $0x0,%eax
0x000000000040053f :callq 0x4003b8
13printf("size of int: %dn",sizeof(int));
0x0000000000400544 :mov $0x40068f,%eax
0x0000000000400549 :mov $0x4,%esi
0x000000000040054e :mov %rax,%rdi
0x0000000000400551 :mov $0x0,%eax
0x0000000000400556 :callq 0x4003b8
14printf("size of long int: %dn",sizeof(long int));
0x000000000040055b :mov $0x4006a0,%eax
0x0000000000400560 :mov $0x8,%esi
0x0000000000400565 :mov %rax,%rdi
0x0000000000400568 :mov $0x0,%eax
0x000000000040056d :callq 0x4003b8
15return 0;
0x0000000000400572 :mov $0x0,%eax
16}
0x0000000000400577 :leaveq
0x0000000000400578 :retq
End of assembler dump.
(gdb)
