參考：
在定義FILE * fp 之后，fopen的用法是: fp = fopen(filename,"w")。而對於fopen_s來說，還得定義另外一個變量errno_t err，然后err = fopen_s(&fp,filename,"w")。返回值的話，對於fopen來說，打開文件成功的話返回文件指針（賦值給fp），打開失敗則返回NULL值；對於fopen_s來說，打開文件成功返回0，失敗返回非0。
在vs編程中，經常會有這樣的警告：warning C4996: 'fopen': This function or variable may be unsafe. Consider using fopen_s instead. To disable deprecation, use_CRT_SECURE_NO_WARNINGS. See online help for details. 是因為 fopen_s比fopen多了溢出檢測，更安全一些。
#include <stdio.h>
FILE *stream, *stream2;
int main( void ) { int numclosed; errno_t err;
// Open for read (will fail if file "crt_fopen_s.c" does not exist) if( (err = fopen_s( &stream, "crt_fopen_s.c", "r" )) !=0 ) printf( "The file 'crt_fopen_s.c' was not opened\n" ); else printf( "The file 'crt_fopen_s.c' was opened\n" );
// Open for write if( (err = fopen_s( &stream2, "data2", "w+" )) != 0 ) printf( "The file 'data2' was not opened\n" ); else printf( "The file 'data2' was opened\n" );
// Close stream if it is not NULL if( stream) { if ( fclose( stream ) ) { printf( "The file 'crt_fopen_s.c' was not closed\n" ); } }
// All other files are closed: numclosed = _fcloseall( ); printf( "Number of files closed by _fcloseall: %u\n", numclosed ); }