网站开发设计中的收获,17网站一起做网批,阿里云怎么建网站,英才网C语言的类型转换很重要#xff0c;经常出现#xff0c;但是往往不被人注意#xff0c;而在汇编代码当中就暴露无遗了。
如下列代码#xff1a;
char ch;
while ((ch getchar()) ! #) putchar(ch);
反汇编后#xff1a;
.text:00401006 mov…C语言的类型转换很重要经常出现但是往往不被人注意而在汇编代码当中就暴露无遗了。
如下列代码
char ch;
while ((ch getchar()) ! #) putchar(ch);
反汇编后
.text:00401006 mov eax, stru_407030._cnt .text:0040100B sub eax, 1 .text:0040100E mov stru_407030._cnt, eax .text:00401013 cmp stru_407030._cnt, 0 .text:0040101A jl short loc_40103D .text:0040101C mov ecx, stru_407030._ptr .text:00401022 movsx edx, byte ptr [ecx] .text:00401025 and edx, 0FFh .text:0040102B mov [ebpvar_8], edx .text:0040102E mov eax, stru_407030._ptr .text:00401033 add eax, 1 .text:00401036 mov stru_407030._ptr, eax .text:0040103B jmp short loc_40104D .text:0040103D ; --------------------------------------------------------------------------- .text:0040103D .text:0040103D loc_40103D: ; CODE XREF: _main1Aj .text:0040103D push offset stru_407030 ; File .text:00401042 call __filbuf .text:00401047 add esp, 4 .text:0040104A mov [ebpvar_8], eax ; 这里是重点 注意getchar()的返回类型是int要赋给char需要类型转换
;以上都是getchar()的汇编代码在很多实现中getchar()只是一个macro所以才会展开这么多代码
.text:0040104D .text:0040104D loc_40104D: ; CODE XREF: _main3Bj .text:0040104D mov cl, byte ptr [ebpvar_8] .text:00401050 mov [ebpvar_4], cl .text:00401053 movsx edx, [ebpvar_4] ;同样0x23h是int类型和它比较也要做转换所以要用带符号扩展的movsx .text:00401057 cmp edx, 23h .text:0040105A jz short loc_4010B4 ;以下是putchar()的代码
.text:0040105C mov eax, File._cnt .text:00401061 sub eax, 1 .text:00401064 mov File._cnt, eax .text:00401069 cmp File._cnt, 0 .text:00401070 jl short loc_40109A .text:00401072 mov ecx, File._ptr .text:00401078 mov dl, [ebpvar_4] .text:0040107B mov [ecx], dl .text:0040107D movsx eax, [ebpvar_4] .text:00401081 and eax, 0FFh .text:00401086 mov [ebpvar_C], eax .text:00401089 mov ecx, File._ptr .text:0040108F add ecx, 1 .text:00401092 mov File._ptr, ecx .text:00401098 jmp short loc_4010AF .text:0040109A ; --------------------------------------------------------------------------- .text:0040109A .text:0040109A loc_40109A: ; CODE XREF: _main70j .text:0040109A push offset File ; File .text:0040109F movsx edx, [ebpvar_4] .text:004010A3 push edx ; Ch .text:004010A4 call __flsbuf .text:004010A9 add esp, 8 .text:004010AC mov [ebpvar_C], eax .text:004010AF .text:004010AF loc_4010AF: ; CODE XREF: _main98j .text:004010AF jmp loc_401006
