Output redirected to a file and file being already closed, for one. Or anyway, stdout being closed midway due to many other possible reasons: this is (probably)

Output redirected to a file and file being already closed, for one. Or anyway, stdout being closed midway due to many other possible reasons: this is (probably) why printf returns a number of bytes written or -1, for example: http://www.cplusplus.com/reference/cstdio/printf/ http://www.cplusplus.com/reference/cstdio/printf/