(char*)malloc(sizeof(char))导致分割错误，如何?

The code works fine on most inputs, but for userID's whih are very long I get a segmentation fault. My question is, how can malloc cause a segmentation fault? simply allocating memory shouldn't cause this. I found the problem area by using printf() statements, it seem the malloc within my read_line() function is where the problem is because the second "read_line" does not print, but the first before the malloc does. The code works fine on most inputs, but for use