实际函数：

outchar→PUTC→_IO_putc_unlocked→__overflow→_IO_OVERFLOW→vtable.__overflow→_IO_file_overflow→_IO_new_file_overflow→_IO_do_write→_IO_new_do_write→new_do_write _IO_SYSWRITE→_IO_new_file_write→__libc_write

outstring→PUT→_IO_sputn→_IO_XSPUTN→vtable.__xsputn→_IO_file_xsputn→_IO_new_file_xsputn→new_do_write→…