How can i find specific char in file with file_pointer?

I want to make simple login,register system. and register part is done, but login part is little confuse..

Login with Txt file. Filename - ID, file including "Nickname|PW"

then scanf ID first, if id exist -> get PW and open ID txt file to compare pw.

I'm tried fgets and fread and more... but i dont know exactly how to use them

int ckk; // position of "|"

ck_pw = fopen(fpp_id, "r"); //open ID txt file to read including text

for (int j = 0;; j++) { // count where is "|"
    char buf; // buffer
    buf = '\0'; //reset buf
    fread(buf, j, 1, ck_pw);
    if (buf == '|') { //find "|"
        ckk = j + 1;
        break;
    }
}

fseek(ck_pw, ckk, SEEK_CUR); //move cursor to "|"

for (int k = 0;; k++) { //read text before space(NULL)
    fread(pw_ck, k, 1, ck_pw);
    if (pw_ck[k] == NULL) {
        break;
    }
}    

I used an uninitialized "ck_pw" local variable. in Visual studio 2017