select的fd超过1024将会非常危险------FD_SET导致core dump

         关于linux select无须多说， 来看代码：

 

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/time.h>
#include<sys/types.h>

int main(int argc, char *argv[])
{
	if(argc != 2)
	{
		printf("para error\n");
		return -1;
	}
	
    struct timeval tv;              // 超时时间
	tv.tv_sec = 10;
    tv.tv_usec = 500;               // 注意单位是微秒

    fd_set rdfds;
    FD_ZERO(&rdfds);                // 描述集初始化
	
	unsigned int n = atoi(argv[1]);
	printf("n is %u\n", n);
	for(unsigned int i = 0; i < n; i++)
	{
		FD_SET(i, &rdfds); 
	}
	
	printf("to select\n");
	select(n, &rdfds, NULL, NULL, &tv); 

	return 0;
}

       执行结果：

 

 

xxxxxx:~/network> g++ -g server.cpp 
server.cpp:34:2: warning: no newline at end of file
xxxxxx:~/network> ./a.out 2000
n is 2000
Segmentation fault (core dumped)
xxxxxx:~/network> gdb a.out core 
GNU gdb 6.6
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welcome to change it and/or distribute copies of it under certain conditions.
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There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i586-suse-linux"...
Using host libthread_db library "/lib/libthread_db.so.1".

warning: Can't read pathname for load map: Input/output error.
Reading symbols from /lib/libonion.so...done.
Loaded symbols for /lib/libonion.so
Reading symbols from /usr/lib/libstdc++.so.6...done.
Loaded symbols for /usr/lib/libstdc++.so.6
Reading symbols from /lib/libm.so.6...done.
Loaded symbols for /lib/libm.so.6
Reading symbols from /lib/libgcc_s.so.1...done.
Loaded symbols for /lib/libgcc_s.so.1
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/libdl.so.2...done.
Loaded symbols for /lib/libdl.so.2
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
Core was generated by `
Program terminated with signal 11, Segmentation fault.
#0  0x080485af in main (argc=-1, argv=0xffffffff) at server.cpp:27
27                      FD_SET(i, &rdfds); 
(gdb) 

        可见， 程序core在FD_SET处， 为什么呢， 看看linux源码中关于FD_SET的部分吧：

 

 

static __inline__ void __FD_SET(unsigned long fd, __kernel_fd_set *fdsetp)
{
    unsigned long _tmp = fd / __NFDBITS;
    unsigned long _rem = fd % __NFDBITS;
    fdsetp->fds_bits[_tmp] |= (1UL<<_rem);
}

#define __NFDBITS    (8 * sizeof(unsigned long))

typedef struct {
    unsigned long fds_bits [__FDSET_LONGS];
} __kernel_fd_set;

#define __FDSET_LONGS   (__FD_SETSIZE/__NFDBITS)

#define __FD_SETSIZE    1024

        一幕撩人， 一目了然。