From C Puzzles
The expected output of the following C program is to print the elements in the array. But when actually run, it doesn't do so.| #include<stdio.h> #define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0])) int array[] = {23,34,12,17,204,99,16}; int main() { int d; for(d=-1;d <= (TOTAL_ELEMENTS-2);d++) printf("%d\n",array[d+1]); return 0; } |
d赋值为-1， 与宏NUM-2（值为5）相比较，这时d转化为无符号的数，因此条件为假，for循环未进入，若将NUM换为 const int就不会出现这样的情况。
延伸：在有符号和无符号的数比较时，有符号的数将先转化成无符号的数然后再比较。有一篇博文写的比较详细：
I thought the following program was a perfect C program. But on compiling, I found a silly mistake. Can you find it out (without compiling the program :-) ?
#includevoid OS_Solaris_print(){ printf("Solaris - Sun Microsystems\n");}void OS_Windows_print(){ printf("Windows - Microsoft\n");}void OS_HP-UX_print(){ printf("HP-UX - Hewlett Packard\n");}int main(){ int num; printf("Enter the number (1-3):\n"); scanf("%d",&num); switch(num) { case 1: OS_Solaris_print(); break; case 2: OS_Windows_print(); break; case 3: OS_HP-UX_print(); break; default: printf("Hmm! only 1-3 :-)\n"); break; } return 0;}
。。。。。。竟然只是变量名错了，OS_HP-UX_print 变量名中含-
The following program doesn't "seem" to print "hello-out". (Try executing it)
| #include <stdio.h> #include <unistd.h> int main() { while(1) { fprintf(stdout,"hello-out"); fprintf(stderr,"hello-err"); sleep(1); } return 0; } |
输出结果是：hello-errhello-out，stdout, stder分别表示标准输出，标准错误，默认都是将信息输出到终端上，在默认情况下，stdout是行缓冲的，他的输出会放在一个buffer里面，只有到换行的时候，才会输出到屏幕。而stderr是无缓冲的，会直接输 出，举例来说就是fprintf(stdout, "xxxx") 和 fprintf(stdout, "xxxx\n")，前者会憋住，直到遇到新行才会 一起输出。而fprintf(stderr, "xxxxx")，不管有么有\n，都输出。