程序定义了fp1和fp2两个文件指针，分别指向源文件和目标文件，把文件1中的字符，经过加密送到文件2

#include
#include
#include
void main()
{
FILE *fp1,*fp2;
char fl1[20],fl2[20];
char c;
printf("please input source file name:\n");
scanf("%s",fl1);
printf("please input target file name:\n");
scanf("%s",fl2);
if((fp1=fopen("fl1","r"))==NULL)
{
printf("cannot open source file\n");
exit(1);
}
if((fp2=fopen("fl2","w"))==NULL)
{
printf("cannot open target file\n");
exit(1);
}
while(!feof(fp1))
{
int ch=0;
int n=0;
c=fgetc(fp1);
if((c>='a'&&ch<='z')||(c>='A'&&ch<='Z'))
{
c=c+1;
if(c>'z'&&c<='Z'+n||c>'z')c=c-26;
}
fputc(c,fp2);
}
fclose(fp1);
fclose(fp2);
}

你的程序需要进行以下修改：
if((fp2=fopen("fl2","w"))==NULL)改成if((fp2=fopen("fl2","w+"))==NULL)
printf("cannot open target file\n");改成fclose(fp1); printf("cannot open target file\n");

question? 再看看别人怎么说的。