invalid lvalue in assignment的问题

[chenhx666666]

C primer plus上的一个程序，对in赋值时使用gcc总是报错invalid lvalue in assignment，查了一下网上的说法，说gcc不再支持强制类型转换，但是这里哪里涉及到强制类型转换了呢？另外为什么out没有出现这种问题呢？

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define LEN 40

int main(int argc,char*argv[])
{
FILE *in,*out;
int ch;
char name[LEN];
int count=0;

if(argc<2)
{
fprintf(stderr,"usage:%s filename\n",argv[0]);
exit(1);
}

if((in=fopen(argv[1],"r"))=NULL)
{
fprintf(stderr,"I couldn't open the file\"%s\"\n",argv[1]);
exit(2);
}

strcpy(name,argv[1]);
strcat(name,".red");
if((out=fopen(name,"w"))==NULL)
{
fprintf(stderr,"Can't creat output file.");
exit(3);
}

while((ch=getc(in))!=EOF)
{
if(count++%3==0)
putc(ch,out);
}

if(fclose(in)!=0||fclose(out)!=0)
fprintf(stderr,"Error in closing files.\n");

return 0;
}

[luciferisnotsatan]

哪里强转了？

[ri_aje]

把这句

if((in=fopen(argv[1],"r"))=NULL)

改成这样

if((in=fopen(argv[1],"r"))==NULL)
// ------------------------^-----
//                         |