Smali file is corrupted after writing with fputs()

Question

Goal : i want to inject a one line code in smali

My current approach

I am reading smali file with getline() and writing whole content to a new file with some changes.

Here is code

while(getline(&line,&line_size,smaliFile) != -1) {
        if (FLAG) {
            if (strstr(line,"return-void") != NULL) {
                fputs("    invoke-static {}, Lcom/hax/util/MainService;->start()V\n\n",smali_tmp);
                fputs("    return-void\n",smali_tmp);
                FLAG=0;
                continue;
            }
        }
        fputs(line,smali_tmp);
    }

    if (FLAG) {
        fprintf(stderr,"can't find hook point !\n");
        exit(1);
    }

Where smaliFile is original smali file and smali_tmp is new patched smali file.

And if i am doing same thing manually then it's recompiling as expected

Here is error while recompiling

Smali.smali[1771,7] mismatched input '' expecting END_METHOD_DIRECTIVE         
Could not smali file: com/gbwhatsapp/Smali.smali

But when i am looking into corrupted file at location 1771,7 with vim , there is no any unusual thing or character present.

I also used diff command to check changes and diff outputs only one line which was added by me.