unsigned int u = 1234;
int i = -5678;
unsigned int result = u + i;
What implicit conversions are going on here, and is this code safe for all values of u and i? (Safe, in the sense that even though result in this example will overflow to some huge positive number, I could cast it back to an int and get the real result.)
Answer:
Short Answer
Your i will be converted to an unsigned integer by adding UINT_MAX + 1, then the addition will be carried out with the unsigned values, resulting in a large result (depending on the values of u andi).
Long Answer
According to the C99 Standard:
6.3.1.8 Usual arithmetic conversions
- If both operands have the same type, then no further conversion is needed.
- Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank is converted to the type of the operand with greater rank.
- Otherwise, if the operand that has unsigned integer type has rank greater or equal to the rank of the type of the other operand, then the operand with signed integer type is converted to the type of the operand with unsigned integer type.
- Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, then the operand with unsigned integer type is converted to the type of the operand with signed integer type.
- Otherwise, both operands are converted to the unsigned integer type corresponding to the type of the operand with signed integer type.
In your case, we have one unsigned int (u) and signed int (i). Referring to (3) above, since both operands have the same rank, your i will need to be converted to an unsigned integer.
6.3.1.3 Signed and unsigned integers
- When a value with integer type is converted to another integer type other than _Bool, if the value can be represented by the new type, it is unchanged.
- Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.
- Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.
Now we need to refer to (2) above. Your i will be converted to an unsigned value by adding UINT_MAX + 1. So the result will depend on how UINT_MAX is defined on your implementation. It will be large, but it will not overflow, because:
6.2.5 (9)
A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.
Bonus: Arithmetic Conversion Semi-WTF
#include <stdio.h>
int main(void)
{
unsigned int plus_one = 1;
int minus_one = -1;
if(plus_one < minus_one)
printf("1 < -1");
else
printf("boring");
return 0;
}
You can use this link to try this online: http://codepad.org/yPhYCMFO
Bonus: Arithmetic Conversion Side Effect
Arithmetic conversion rules can be used to get the value of UINT_MAX by initializing an unsigned value to -1, ie:
unsigned int umax = -1; // umax set to UINT_MAX
This is guaranteed to be portable regardless of the signed number representation of the system because of the conversion rules described above. See this SO question for more information: Is it safe to use -1 to set all bits to true?
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ANSI C鏍囧噯鍙繚璇佷簡C瀹炵幇蹇呴』鑳藉鍖哄埆鍑哄墠6涓瓧絎︿笉鍚岀殑澶栭儴鍚嶇О銆傝屼笖榪欎釜瀹氫箟涓茍娌℃湁鍖哄垎澶у啓瀛楁瘝涓庡叾瀵瑰簲鐨勫皬鍐欏瓧姣嶃?/span>
鍥犳錛岀紪鍐欏彲縐繪紼嬪簭蹇呴』灝忓績榪欎竴鐐廣?br />
2.瀛楃鏄湁絎﹀彿鐨勬暣鏁幫紝榪樻槸鏃犵鍙風殑鏁存暟
鍙湁鎶婁竴涓瓧絎﹀艱漿鎹?span style="color: red; ">涓轟竴涓緝澶х殑鏁存暟鏃訛紝鎵嶉噸瑕併傚湪鍏朵粬鎯呭喌涓嬶紝緇撴灉閮芥槸錛氬浣欑殑浣嶈綆鍗曠殑“涓㈠純”銆?br />
鍦ㄨ漿鎹㈣繃紼嬩腑錛氬簲璇ュ皢瀛楃浣滀負鏈夌鍙鋒暟榪樻槸鏃犵鍙鋒暟錛?br />濡傛灉鏈夌鍙鳳紝緙栬瘧鍣ㄥ皢char鏁版嵁錛屾墿灞曞埌int鏃跺欙紝搴旇澶嶅埗絎﹀彿浣嶃?br />濡傛灉鏃犵鍙鳳紝緙栬瘧鍣ㄥ彧闇鍦ㄥ浣欑殑浣嶄笂濉厖0.
int main()
{
char c='a';
c=c+40;
// printf("%c\n", -1);
printf("c %d\n", c);
printf("unsigned c %u\n", (unsigned char)c);
}
unsigned c 137
濡傛灉緙栫▼鑰呭叧娉ㄤ竴涓渶楂樹綅鏄?鐨勫瓧絎︽槸姝ｈ繕鏄礋錛屽彲浠ヨ緗?span style="color: red; ">涓烘棤絎﹀彿瀛楃鏁?/span>銆傝繖鏍鋒墍鏈夌紪璇戝櫒閮戒細杞崲涓烘暣鏁版椂鍊欙紝濉厖涓?.
3.涓涓父瑙侀敊璇槸錛氬鏋渃鏄竴涓瓧絎﹀彉閲忥紝浣跨敤錛坲nsigned錛塩鍙互寰楀埌涓巆絳変環鐨勬棤絎﹀彿鏁存暟銆?span style="color: red; ">榪欐槸浼氬け璐ョ殑銆傚洜涓哄湪灝哻杞崲涓烘棤絎﹀彿鏁存暟鏃跺欙紝c灝嗛鍏堥鍏堣杞崲涓篿nt鍨嬫暣鏁般傝屾鏃跺彲鑳藉緱鍒伴潪棰勬湡鐨勭粨鏋溿?br />姝ｇ‘鏂規硶鏄?/span>錛?unsigned char )c錛岀洿鎺ヨ繘琛岃漿鎹€?br />
渚嬪涓婁釜渚嬪瓙涓紝鏈鍚庝竴鍙ユ敼涓?
閭ｄ箞緇撴灉鏄?
unsigned c 4294967177
4.縐諱綅榪愮畻絎?br /> 1.鍚戝彸縐諱綅鏃訛紝絀哄嚭鐨勪綅鐢?濉厖錛岃繕鏄敱絎﹀彿浣嶇殑鍓湰濉厖銆?br /> 2.濡傛灉鏄棤絎﹀彿鏁幫紝鐢?濉厖銆傚鏋滄槸鏈夌鍙鋒暟錛屾棦鍙互鐢?涔熷彲浠ョ敤絎﹀彿浣嶇殑鍓湰銆傦紙濡傛灉鍏蟲敞鍙崇Щ鏃跺欑┖鍑虹殑浣嶏紝鍙互澹版槑涓烘棤絎﹀彿綾誨瀷錛岄偅涔堢┖鍑虹殑浣嶉兘浼氳璁劇疆涓?錛?br />
濡傛灉琚Щ浣嶅璞′負n浣嶏紝閭ｄ箞縐諱綅璁℃暟蹇呴』澶т簬鎴栫瓑浜?錛岃屼弗鏍煎皬浜巒.
鍗充嬌C瀹炵幇灝嗙鍙蜂綅澶嶅埗鍒扮┖鍑虹殑浣嶄腑錛屾湁絎﹀彿鏁扮殑鍚戝彸縐諱綅錛?span style="color: red; ">涔熷茍涓嶇瓑浜庨櫎浠?鐨勬煇嬈″箓銆備緥濡傦紙-1錛?gt;>1緇撴灉涓?1錛岃屼笉鏄?1/2 == 0
5.闅忔満鏁版渶澶у鹼紝RAND_MAX鍦╨imits涓畾涔夈傛垜嫻嬭瘯緇撴灉絳変簬INT_MAX
6.闄ゆ硶榪愮畻鐨勬埅鏂?br />q=a/b;
r=a%b;
鍋囧畾b>0.
C璇█瀹氫箟鍙繚璇乹*b+r=a,浠ュ強a>=0 涓?b>0鏃訛紝淇濊瘉|r|<|b|浠ュ強r>=0.
錛堝鏋渁<0, 閭ｄ箞r涔熷彲鑳藉皬浜?錛?/span>
渚嬪錛?br />
// Start typing your code here
cout<<(-3)/2<<endl;
return 0;
}
]]>
錛?閬垮厤鍑芥暟璋冪敤寮閿銆?br />
3.瀹忓畾涔夋敞鎰忕偣
1.涓嶈兘蹇借瀹氫箟涓殑絀烘牸
2.鏈濂藉皢瀹忓畾涔変腑姣忎釜鍙傛暟閮界敤鎷彿鎷搗鏉ャ傛暣涓〃杈懼紡鐨勭粨鏋滈〉鐢ㄦ嫭鍙鋒嫭璧鋒潵銆?br /> 3.紜繚璋冪敤瀹忕殑鍙傛暟涓紝涓嶅瓨鍦ㄦ湁鍓綔鐢ㄧ殑浠ｇ爜
4.assert瀹忋傚彲浠ュ湪鍑洪敊淇℃伅涓寘鍚枃浠跺悕鍜屾柇璦澶辮觸澶勭殑琛屽彿銆傚緢鏈夌敤銆?br />
5.瀹忓茍涓嶆槸綾誨瀷瀹氫箟銆?br />#define T1 struct foo *
typedef struct foo * T2;
T1 a,b; //struct foo * a, b;
T2 a,b; //a ,b閮芥槸鎸囧悜緇撴瀯鐨勬寚閽堛?img src ="http://www.shnenglu.com/chenglong7997/aggbug/180104.html" width = "1" height = "1" />
]]>
getchar榪斿洖鏁村艦
int main()
{
char c;
while( (c=getchar())!=EOF )
putchar(c);
}
搴旇灝哻澹版槑涓篿nt銆傚惁鍒欙紝c鍙兘鏃犳硶瀹逛笅EOF
2.鏇存柊鏂囦歡
璇繪搷浣滀箣鍚庯紝鏂囦歡鎸囬拡浼氬亸縐諱竴孌點傝繖鏃跺欙紝璁叉枃浠舵洿鏂板悗錛屽啓鍏ユ簮鏂囦歡涔嬪墠錛屽簲璇seek璁叉枃浠舵寚閽堣皟鍥炲幓銆?br />
3.浣跨敤setbuf璁劇疆杈撳嚭鐨勭紦鍐插尯澶у皬銆傚彲浠ユ槸stdout鍜宖ile
4.姝ｇ‘浣跨敤errno媯嫻嬮敊璇?br />errno=0;
/*璋冪敤搴撳嚱鏁?/
if(榪斿洖鐨勯敊璇? //榪欎釜閿欒鍊煎彲鑳戒笉鏄敱褰撳墠榪欎釜鍑芥暟寮曡搗鐨勩傝屾槸鐢卞綋鍓嶅嚱鏁幫紝鍙堣皟鐢ㄧ殑鍙﹀涓涓嚱鏁板紩璧風殑銆?br /> 媯鏌rrno;
5.signal澶勭悊鍑芥暟鍞竴瀹夊叏錛屽彲縐繪鐨勬搷浣滃氨鏄墦鍗頒竴鏉￠敊璇俊鎭紝鐒跺悗浣跨敤longjmp鎴栬卐xit绔嬪嵆閫鍑虹▼搴忋?br />
]]>
2.extern int n;
鍦ㄤ袱澶栦竴涓枃浠朵腑錛?long n錛?br />榪欐槸涓涓棤鏁堢殑紼嬪簭錛屽洜涓哄悓涓涓閮ㄥ彉閲忓悕鍦ㄤ袱涓笉鍚岀殑鏂囦歡涓澹版槑涓轟笉鍚岀殑綾誨瀷銆傜劧鍚庡ぇ澶氭暟c璇█瀹炵幇涓嶈兘媯嫻嬪嚭榪欑閿欒銆?br />
3.涓涓▼搴忕敱澶氫釜妯″潡緇勬垚錛屾瘡涓ā鍧楅兘闇瑕佺煡閬撲竴涓壒瀹氱殑鏂囦歡鍚嶃傛垜浠笇鏈涜兘澶熷仛鍒板彧鍦ㄤ竴澶勬敼鍔ㄨ繖涓枃浠跺悕錛屾墍鏈夋ā鍧椾腑鐨勬枃浠跺悕灝辮兘鍚屾椂寰楀埌鏇存柊銆?br />鍙互錛屽厛鍒涘緩涓涓枃浠訛紝鍙仛file.h錛屽畠鍖呭惈浜嗗０鏄?span style="color: red; ">extern char filename[];
闇瑕佺敤鍒板閮ㄥ璞ilename鐨勬瘡涓猚婧愭枃浠墮兘搴旇鍔犱笂錛?#include "file.h"錛?br />鏈鍚庨夋嫨涓涓狢婧愭枃浠訛紝鍦ㄥ叾涓粰鍑篺ilename鐨勫垵濮嬪箋傚鍦╢ile.c涓?br />#include "file.h";
char filename[]="/etc/passwd";
榪欐牱灝變繚璇佷簡filename鐨勭被鍨嬫槸姝ｇ‘鐨勩傝В鍐充簡2涓殑闂銆?img src ="http://www.shnenglu.com/chenglong7997/aggbug/180101.html" width = "1" height = "1" />
]]>
c璇█涓瓨鍦ㄤ袱綾繪暣鏁扮畻鏈繍綆楋紝鏈夌鍙瘋繍綆楀拰鏃犵鍙瘋繍綆椼?span style="color: #ff00ff; ">鍦ㄦ棤絎﹀彿榪愮畻閲岋紝娌℃湁浜嗙鍙蜂綅錛屾墍浠ユ槸娌℃湁婧㈠嚭鐨勬蹇電殑銆?/p>
鎵鏈夌殑鏃犵鍙?span style="color: red; ">榪愮畻閮芥槸浠?鐨刵嬈℃柟涓烘ā銆傚鏋滅畻鏈繍綆楃鐨勪竴涓搷浣滄暟鏄湁絎﹀彿涔︼紝鍙︿竴涓槸鏃犵鍙鋒暟錛岄偅涔堟湁絎﹀彿鏁?/p>
浼氳杞崲涓烘棤絎﹀彿鏁?/span>(琛ㄧず鑼冨洿灝忕殑鎬繪槸琚漿鎹負琛ㄧず鑼冨洿澶х殑)錛岄偅涔堟孩鍑轟篃涓嶄細鍙戠敓銆備絾鏄紝褰撲袱涓搷浣滄暟閮芥槸鏈夌鍙鋒暟
鏃訛紝婧㈠嚭灝辨湁鍙兘鍙戠敓銆傝屼笖婧㈠嚭鐨勭粨鏋滄槸鏈畾涔夌殑銆?span style="color: red; ">褰撲竴涓繍綆楃殑緇撴灉鍙戠敓婧㈠嚭鏃訛紝浠諱綍鍋囪閮芥槸涓嶅畨鍏ㄧ殑銆?/span>
渚嬪錛屽亣瀹歛鍜宐鏄袱涓潪璐熺殑鏁村瀷鍙橀噺(鏈夌鍙?錛屾垜浠渶瑕佹鏌+b鏄惁婧㈠嚭錛屼竴縐嶆兂褰撶劧鐨勬柟寮忔槸:
if (a + b < 0)
婧㈠嚭錛?/p>
瀹為檯涓婏紝鍦ㄧ幇瀹炰笘鐣岄噷錛岃繖騫朵笉鑳芥甯歌繍琛屻傚綋a+b紜疄鍙戠敓婧㈠嚭鏃訛紝鎵鏈夊叧浜庣粨鏋滃浣曠殑鍋囪鍧囦笉鍙潬銆傛瘮濡傦紝鍦ㄦ煇浜?/p>
鏈哄櫒鐨刢pu錛屽姞娉曡繍綆楀皢璁劇疆涓涓唴閮ㄥ瘎瀛樺櫒涓哄洓縐嶇姸鎬?姝ｏ紝璐燂紝闆跺拰婧㈠嚭銆傚湪榪欑鏈哄櫒涓婏紝c緙栬瘧鍣ㄥ畬鍏ㄦ湁鐞嗙敱瀹炵幇浠ヤ笂
鐨勪緥瀛愶紝浣垮緱a+b榪斿洖鐨勪笉鏄礋錛岃屾槸榪欎釜鍐呭瓨瀵勫瓨鍣ㄧ殑婧㈠嚭鐘舵併傛樉鐒訛紝if鐨勫垽鏂細澶辮觸銆?/p>
涓縐嶆紜殑鏂瑰紡鏄皢a鍜宐閮藉己鍒惰漿鎹負鏃犵鍙鋒暣鏁幫細
if ( (unsigned)a + (unsigned)b > INT_MAX)
婧㈠嚭;
榪欓噷鐨刬nt_max鍊間負鏈夌鍙鋒暣鍨嬬殑鏈澶у箋傚湪涓鑸殑緙栬瘧鍣ㄩ噷鏄竴涓瀹氫箟鐨勫父閲忋侫NSI C鍦╨imits閲屽畾涔変簡INT_MAX錛屽間負
2鐨?1嬈℃柟-1.
涓嶉渶瑕佺敤鍒版棤絎﹀彿綆楁暟榪愮畻鐨勫彟涓縐嶅彲琛屾柟娉曟槸錛?/p>
if (a > INT_MAX - b )
婧㈠嚭錛?nbsp;
]]>
]]>
鍥犳錛宨nt *p=a; //right
int *p=&a; //error, (&a宸茬粡鏄竴涓寚鍚戞暣涓暟緇勭殑鎸囬拡)
2.涓簃ain鍑芥暟鎻愪緵榪斿洖鍊?br />main錛堬級
{}
闅愬惈鐫main榪斿洖鏁存暟錛?span style="color: red; ">涓涓繑鍥炴暣鏁扮殑鍑芥暟濡傛灉榪斿洖澶辮觸錛屽疄闄呬笂闅愬惈榪斿洖鏌愪釜“鍨冨溇”鏁存暟錛屽彧瑕佽鍊間笉琚敤鍒幫紝灝辨棤鍏崇揣瑕併?br />鐒惰岋紝鍦ㄦ煇浜涙儏鍐典笅錛宮ain鐨勮繑鍥炲煎嵈騫墮潪鏃犲叧绱ц錛屽ぇ澶氭暟C璇█瀹炵幇閫氳繃main鐨勮繑鍥炲鹼紝鏉ュ憡鐭ユ搷浣滅郴緇熻鍑芥暟鐨勬墽琛屾槸鎴愬姛榪樻槸澶?/span>璐?/span>銆傚鏋滀竴涓▼搴忕殑main鍑芥暟騫朵笉榪斿洖浠諱綍鍊鹼紝閭ｄ箞鏈夊彲鑳界湅涓婂幓鎵ц澶辮觸銆傛墍浠ユ渶濂芥彁渚涜繑鍥炲?br />
3.杈圭晫璁＄畻涓庝笉瀵圭О杈圭晫銆?br />閫傚悎c涓互涓嬫爣涓?寮濮嬬殑璁＄畻銆?/span>
]]>
/*浼氳褰撳仛娉ㄩ噴鐨勫紑濮嬨傚簲璇=x / *p;
鎴栬厃 = x/(*p);
2.濡傛灉涓涓暣褰㈠父閲忕涓涓瓧絎︿負鏁板瓧0.閭ｄ箞榪欎釜甯擱噺浼氳褰撳仛8榪涘埗鏁般?img src ="http://www.shnenglu.com/chenglong7997/aggbug/179858.html" width = "1" height = "1" />
]]>