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3.5 Byte Manipulation Functions
There are two groups of functions that operate on multibyte fields, without interpreting the data, and without assuming that the data is a null-terminated C string. We need these types of functions when dealing with socket address structures because we need to manipulate fields such as IP addresses, which can contain bytes of 0, but are not C character strings. The functions beginning with str (for string), defined by including the <string.h> header, deal with null-terminated C character strings.
The first group of functions, whose names begin with b (for byte), are from 4.2BSD and are still provided by almost any system that supports the socket functions. The second group of functions, whose names begin with mem (for memory), are from the ANSI C standard and are provided with any system that supports an ANSI C library.
We first show the Berkeley-derived functions, although the only one we use in this text is bzero. (We use it because it has only two arguments and is easier to remember than the three-argument memset function, as explained on p. 8.) You may encounter the other two functions, bcopy and bcmp, in existing applications.
bzero sets the specified number of bytes to 0 in the destination. We often use this function to initialize a socket address structure to 0. bcopy moves the specified number of bytes from the source to the destination. bcmp compares two arbitrary byte strings. The return value is zero if the two byte strings are identical; otherwise, it is nonzero.
The following functions are the ANSI C functions:
memset sets the specified number of bytes to the value c in the destination. memcpy is similar to bcopy, but the order of the two pointer arguments is swapped. bcopy correctly handles overlapping fields, while the behavior of memcpy is undefined if the source and destination overlap. The ANSI C memmove function must be used when the fields overlap.
memcmp compares two arbitrary byte strings and returns 0 if they are identical. If not identical, the return value is either greater than 0 or less than 0, depending on whether the first unequal byte pointed to by ptr1 is greater than or less than the corresponding byte pointed to by ptr2. The comparison is done assuming the two unequal bytes are unsigned chars.
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