用rand()产生相同随机数的原因

/* Copyright (C) 1992 Free Software Foundation, Inc. This file is part of the GNU C Library. The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Library General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. The GNU C Library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details. You should have received a copy of the GNU Library General Public License along with the GNU C Library; see the file COPYING.LIB. If not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */ #include <ansidecl.h> #include <stdlib.h> #include <stdio.h> int DEFUN_VOID(main) { int i1, i2; int j1, j2; /* The C standard says that "If rand is called before any calls to srand have been made, the same sequence shall be generated as when srand is first called with a seed value of 1." */ i1 = rand(); i2 = rand(); srand (1); j1 = rand(); j2 = rand(); if (i1 < 0 || i2 < 0 || j1 < 0 || j2 < 0) { puts ("Test FAILED!"); } if (j1 == i1 && j2 == i2) { puts ("Test succeeded."); return 0; } else { if (j1 != i1) printf ("%d != %d\n", j1, i1); if (j2 != i2) printf ("%d != %d\n", j2, i2); puts ("Test FAILED!"); return 1; } }这是glib-2.06/stdlib/testrand.c中的代码。

很早以前写生成随机数的程序，总是忘了先srand()指定一个种子，所以生成的数列都是一样的。

今天看一些库函数的源代码，一不小心看到stdlib文件夹里竟然有这段测试随机生成相同不相同的代码。

/* The C standard says that "If rand is called before any calls to srand have been made, the same sequence shall be generated as when srand is first called with a seed value of 1." */这句话保证了不指定srand(）的话必须生成相同！