c语言打开文件前,如何判断该文件是否已经被打开

用_access函数判断，再次打开时的情况要看你第一次的打开方式了，如果上次用的是非独占打开，那没问题，如果是独占打开，会打开失败

Example

/* ACCESS.C: This example uses _access to check the
* file named "ACCESS.C" to see if it exists and if
* writing is allowed.
*/

#include <io.h>
#include <stdio.h>
#include <stdlib.h>

void main( void )
{
/* Check for existence */
if( (_access( "ACCESS.C", 0 )) != -1 )
{
printf( "File ACCESS.C exists\n" );
/* Check for write permission */
if( (_access( "ACCESS.C", 2 )) != -1 )
printf( "File ACCESS.C has write permission\n" );
}
}

打开文件要用FOPEN吧
FILE *fp;
fp = fopen( "Student.dat", "wb" );
先判断这个指针是否为NULL就行了啊~

判断能否再次打开就行了！！
if ( fp == 0 )
fprintf (stderr, "Eorro");