为什么 fgets 持续地返回 NULL？

I'm trying to write a program that can open all given files in a directory, and output the files with some changes to another directory given a path specified by the user. I made a function that will be called after fork() for each filename encountered in the current directory, and the output will have the same name in this new directory. I have looked all over, and I still can't find a reason as to why fgets() keeps returning NULL.

void sorter(char *fileName, char *directory, char*   newName){

    FILE *edit = fopen(fileName, "r");
    char buf[700];
    char *bufp = buf;
    char *fLine = fgets(bufp, sizeof(buf), edit);

    if (edit == NULL){
        exit(EXIT_FAILURE);
    }

    printf("%s\n", fLine);
    chdir(directory);
    FILE *output=fopen(newName, "w");   
    while(fLine){
        fprintf(output, fLine);
        fprintf(output, "done");    
    }   
    fclose(output);
    fclose(edit);
}