December
1998 QUESTION 2 Total Marks: 20 Marks |
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In Unix, a file descriptor set (fd_set) is a data-structure in which each bit
represents the availability of a different device in the system. In a system that only has
a maximum of 32 distinct devices, a file descriptor set can be represented by data-type fd_set below. For example, if devices zero,
three, and four becomes available, then this will be represented by an fd_set value of 25
(i.e., the binary value 00000000000000000000000000011001). The following question will
require the definition of the following C functions: typedef unsigned int fd_set; fd_set FD_SET(fd_set p, int n);
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| (a) | What would be returned if the following expressions were evaluated: | [5] |
| (i) 2 & 3 | ||
| (ii) 3 | 9 | ||
| (iii) 21 & (~4) | ||
| (iv) 1 << 3 | ||
| (v) 21 >> 2
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| (b) | Define a macro FD_ZERO, that takes a single argument that
represents a fd_set, which
initialise each bit of the file descriptor set to 0.
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[2] |
| (c) | Define a function FD_SET, that returns as it's result a fd_set that
augments the value represented by p such that the nth
bit (the rightmost bit is bi 0) of fd_set is set to one, i.e., fd_set(5,1) should return 7.
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[4] |
| (d) | Define a function FD_CLR, that returns as it's result a fd_set that
augments the value represented by p such that the nth
bit (the rightmost bit is bit 0) of the file descriptor set to one, i.e., fd_clr(5,0) should return 4.
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[3] |
| (e) | Define a function FD_ISSET, that returns zero if the nth bit (the rightmost bit is bit 0) of the file
descriptor set is zero, and a non-zero value otherwise, i.e., fd_isset(5,2) returns an integer that is not zero, whereas fd_isset(5,1) return 0.
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[2] |
| (f) | Define a function FDS_ACTIVE, that returns the
number of bits that are set within the file descriptor set.
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[4] |