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在VxWorks下，我为了知道读写一个4.5K文件的时间，做了如下两个实验：

实验1：将硬盘上的文件打开，接着读出，然后再写回该文件，最后关闭文件。

#define MAX_LENGTH 4610 int fd; char *flename = "Test.dat"; unsigned char buffer[MAX_LENGTH]; unsigned int readlength, writelength; struct timespec tp1, tp2, tp3, tp4, tp5, tp6; int i;

while ( 1 ) { readlength = 0; writelength = 0; bzero( buffer, MAX_LENGTH ); clock_gettime( CLOCK_REALTIME, &tp1 );

fd = open( flename, O_RDWR, 0 ); if ( ERROR == fd ) { printf( "Don't Open File!\n" ); return; } clock_gettime( CLOCK_REALTIME, &tp2 );

readlength = read( fd, buffer, MAX_LENGTH ); clock_gettime( CLOCK_REALTIME, &tp3 );

for ( i = 0; i < MAX_LENGTH; i++ ) { buffer = ~buffer; }

clock_gettime( CLOCK_REALTIME, &tp4 ); writelength = write( fd, buffer, MAX_LENGTH ); clock_gettime( CLOCK_REALTIME, &tp5 );

close( fd ); clock_gettime( CLOCK_REALTIME, &tp6 );

结果：第一次：OpenTime = 173000000(ns); ReadTime = 17000000(ns); WriteTime = 0(ns); CloseTime = 2000000(ns);

以后的每次：OpenTime = 0(ns); ReadTime = 0(ns); WriteTime = 0(ns); CloseTime = 1000000(ns);

实验2：将硬盘上的文件打开，接着读出，最后关闭文件。

#define MAX_LENGTH 4610 int fd; char *flename = "Test.dat"; unsigned char buffer[MAX_LENGTH]; unsigned int readlength, writelength; struct timespec tp1, tp2, tp3, tp4, tp5, tp6; int i;

while ( 1 ) { readlength = 0; writelength = 0; bzero( buffer, MAX_LENGTH ); clock_gettime( CLOCK_REALTIME, &tp1 );

fd = open( flename, O_RDWR, 0 ); if ( ERROR == fd ) { printf( "Don't Open File!\n" ); return; } clock_gettime( CLOCK_REALTIME, &tp2 );

readlength = read( fd, buffer, MAX_LENGTH ); clock_gettime( CLOCK_REALTIME, &tp3 );

close( fd ); clock_gettime( CLOCK_REALTIME, &tp6 );

结果：第一次：OpenTime = 185000000(ns); ReadTime = 17000000(ns); CloseTime = 0(ns);

以后的每次：OpenTime = 2000000(ns); ReadTime = 5000000(ns); CloseTime = 0(ns);

疑问：

1、为什么在加入了write后，相反的所用的整个时间竟然变短了？？？

2、为什么在加入了write后，Open、Read、Write都不需要时间，而Close却需要很长的时间？相反的，在只有Read的代码中，Open、Read是需要时间开销的，但Close却不需要时间呢？