If you use -O3 or -O2, then the loop is optimed to nothing. Because
the a++ never is used.
My question is :
1. On archlinux gcc 4.8.2, `gcc curses_test.c $(pkg-config --libs
--cflags ncurses) -o curses_test` and `gcc $(pkg-config --libs
--cflags ncurses) curses_test.c -o curses_test` works.
On cygwin it doesn't work, why?
2. what is wrong in openmp on cygwin? How can I fix it?
2014-03-14 20:42 GMT+08:00 rexdf Rexdf <weiyong....@gmail.com>:
> 1.the following code:
>
> #include <unistd.h>
> #include <curses.h>
> int main() {
> initscr();
> move(0, 0);
> addstr("hello, world\n");
> refresh();
> sleep(5);
> endwin();
> return 0;
> }
>
> The following is my shell command:
>
> $ pkg-config --libs --cflags ncurses
> -I/usr/include/ncurses -lncurses
>
> $ gcc $(pkg-config --libs --cflags ncurses) curses_test.c -o curses_test
> /tmp/cc74H73T.o:curses_test.c:(.text+0xf): undefined reference to `initscr'
> /tmp/cc74H73T.o:curses_test.c:(.text+0x14): undefined reference to
> `ncwrap_stdscr'
> /tmp/cc74H73T.o:curses_test.c:(.text+0x2c): undefined reference to `wmove'
> /tmp/cc74H73T.o:curses_test.c:(.text+0x31): undefined reference to
> `ncwrap_stdscr'
> /tmp/cc74H73T.o:curses_test.c:(.text+0x49): undefined reference to `waddnstr'
> /tmp/cc74H73T.o:curses_test.c:(.text+0x4e): undefined reference to
> `ncwrap_stdscr'
> /tmp/cc74H73T.o:curses_test.c:(.text+0x56): undefined reference to `wrefresh'
> /tmp/cc74H73T.o:curses_test.c:(.text+0x67): undefined reference to `endwin'
> /usr/lib/gcc/i686-pc-cygwin/4.8.2/../../../../i686-pc-cygwin/bin/ld:
> /tmp/cc74H73T.o: bad reloc address 0x20 in section `.eh_frame'
> /usr/lib/gcc/i686-pc-cygwin/4.8.2/../../../../i686-pc-cygwin/bin/ld:
> final link failed: Invalid operation
> collect2: error: ld returned 1 exit status
>
> $ gcc curses_test.c $(pkg-config --libs --cflags ncurses) -o curses_test
>
> $ ./curses_test.exe
>
>
> Do I have to put the source file before `-I/usr/include/ncurses
> -lncurses`? And why?
>
>
> 2.It's about OpenMP
> openmp_1.cpp
> #include <iostream>
> #include <time.h>
> void test()
> {
> int a = 0;
> for (int i=0;i<100000000;i++)
> a++;
> }
> int main()
> {
> clock_t t1 = clock();
> for (int i=0;i<8;i++)
> test();
> clock_t t2 = clock();
> std::cout<<"time: "<<t2-t1<<std::endl;
> }
>
> openmp_2.cpp
> #include <iostream>
> #include <time.h>
> void test()
> {
> int a = 0;
> for (int i=0;i<100000000;i++)
> a++;
> }
> int main()
> {
> clock_t t1 = clock();
> #pragma omp parallel for
> for (int i=0;i<8;i++)
> test();
> clock_t t2 = clock();
> std::cout<<"time: "<<t2-t1<<std::endl;
> }
>
> On cygwin
> g++ openmp_1.cpp -o openmp_1.exe
> g++ openmp_2.cpp -o opemnp_2.exe -fopenmp
> time 1937
> time 2297
>
> On MingW-g++ 4.8.1
> g++ openmp_1.cpp -o openmp_1.exe
> g++ openmp_2.cpp -o opemnp_2.exe -fopenmp
> time 1975
> time 492
>
> On Visual Studio 2013
> Openmp_2.cpp Project
> C/C++ --> Language --> Open MP Support yes(/openmp)
> time 1849
> time 321
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