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• wonder about some #define tricks

  - by kingkai

  Whlie reading codes of my group project, I come across many DEFINEs, and some of them seems strange. To generalize it, please look at the following 2 examples. Example 1: #define SNPRINTF(dst, fmt, arg...) snprintf(dst, sizeof(dst), fmt, ##arg) what does "##" means in this circumstance? I've tried to delete both of them, and write codes like "char buf[1024]; SNPRINTF(buf,"%s,%s","abcd","efg");" which produced the same result. So "##" seems no use and no harm to me. Example 2: #define CLOSE(fd) do { \ if (-1 != (fd)) { \ close(fd); \ (fd) = -1; \ } \ } while (0) Necessary to stuff the inner code to the do{}while(0) statement? what's the use? Thanks!

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• dynamical binding or switch/case?

  - by kingkai

  A scene like this: I've different of objects do the similar operation as respective func() implements. There're 2 kinds of solution for func_manager() to call func() according to different objects Solution 1: Use virtual function character specified in c++. func_manager works differently accroding to different object point pass in. class Object{ virtual void func() = 0; } class Object_A : public Object{ void func() {}; } class Object_B : public Object{ void func() {}; } void func_manager(Object* a) { a->func(); } Solution 2: Use plain switch/case. func_manager works differently accroding to different type pass in typedef _type_t { TYPE_A, TYPE_B }type_t; void func_by_a() { // do as func() in Object_A } void func_by_b() { // do as func() in Object_A } void func_manager(type_t type) { switch(type){ case TYPE_A: func_by_a(); break; case TYPE_B: func_by_b(); default: break; } } My Question are 2: 1. at the view point of DESIGN PATTERN, which one is better? 2. at the view point of RUNTIME EFFCIENCE, which one is better? Especailly as the kinds of Object increases, may be up to 10-15 total, which one's overhead oversteps the other? I don't know how switch/case implements innerly, just a bunch of if/else? Thanks very much!

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• why malloc+memset slower than calloc?

  - by kingkai

  It's known that calloc differentiates itself with malloc in which it initializes the memory alloted. With calloc, the memory is set to zero. With malloc, the memory is not cleared. So in everyday work, i regard calloc as malloc+memset. Incidentally, for fun, i wrote the following codes for benchmark. The result is confused. Code 1: #include<stdio.h> #include<stdlib.h> #define BLOCK_SIZE 1024*1024*256 int main() { int i=0; char *buf[10]; while(i<10) { buf[i] = (char*)calloc(1,BLOCK_SIZE); i++; } } time ./a.out real 0m0.287s user 0m0.095s sys 0m0.192s Code 2: #include<stdio.h> #include<stdlib.h> #include<string.h> #define BLOCK_SIZE 1024*1024*256 int main() { int i=0; char *buf[10]; while(i<10) { buf[i] = (char*)malloc(BLOCK_SIZE); memset(buf[i],'\0',BLOCK_SIZE); i++; } } time ./a.out real 0m2.693s user 0m0.973s sys 0m1.721s Repalce memset with bzero(buf[i],BLOCK_SIZE) in Code 2 produce the result alike. My Question is that why malloc+memset is so much slower than calloc? How can calloc do that ? Thanks!

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• Can func get the lineno who call itself? (C/C++)

  - by kingkai

  Hi, I've a problem , as the following code discribe itself. 1 #include<stdlib.h> 2 #include<stdio.h> 3 void log() 4 { 5 printf("Log [Line:%d]\n",__LINE__); 6 } 7 int main() 8 { 9 log(); 10 log(); 11 } The expected result is Log [Line:9] Log [Line:10] But, the fact is Log [Line:5] Log [Line:5] No surprising, LINE has been substituted at the pre-process stage as 5. My Question is, how to design the log function to get the expected result? Thanks!

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