CS360 Midterm Exam: March 2, 2004. Question 1

Behold the following system call:

int send_to_card(char *ptr, int nbytes);

This sends the bytes specified by ptr and nbytes to a special card on your computer. Nbytes must be less than 4096. It returns 0 on success, and -1 on failure.

Your job is to write:

int bsend_to_card(char *ptr, int nbytes);

bsend_to_card() gives you two features:

It performs buffering, so that calls where nybtes is small will typically not be penalized with system call overhead.
It allows nbytes to be greater than 4096 bytes.

bsend_to_card() will work as follows. It will manage a 4096-byte buffer in the following way:

If bsend_to_card() is called, and the specified memory will fit in the buffer, it copies the specified memory to the buffer and returns.
If bsend_to_card() is called when the buffer is not empty, and the and specified memory will not fit into the buffer, then the buffer is flushed by calling send_to_card() on the bytes in the buffer, and the process is repeated with an empty buffer.
If bsend_to_card() is called when the buffer is empty, and the specified memory will not fit into the buffer, then send_to_card() is called with the first 4096 bytes of the specified memory, and the process is repeated with the remaining bytes of the specified memory.
If bsend_to_card() is called with nbytes equal to zero, then the buffer is flushed.

As an example, suppose the buffer holds 1000 bytes, and bsend_to_card(0x10000, 8192) is called. bsend_to_card() will do the following things:

Since the specified memory will not fit into the buffer, and the buffer is not empty, the buffer will be flushed by a call to send_to_card(buffer, 1000).
Since the specified memory will not fit into an empty buffer, send_to_card(0x10000, 4096) is called.
The remaining 4096 bytes will fit into the empty buffer, so they are copied there, and bsend_to_card() returns zero.

If send_to_card() ever returns -1, bsend_to_card() should instantly return -1. Otherwise, bsend_to_card() should return 0 when it is done.

You will need global variables for this. I am including an answer sheet for this problem that includes the proper declarations for global variables. You may add more if you want, but you do not have to.