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Using what we learned from the prototype, we now want to build a library worthy of general use The most obvious requirement is that we must make c s v g e t l i ne more robust so it will handle long lines or many fields; it must also be more careful in the parsing of fields To create an interface that others can use, we must consider the issues listed at the beginning of this chapter: interfaces, information hiding, resource management, and error handling The interplay among these strongly affects the design Our separation of these issues is a bit arbitrary, since they are interrelated
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Interface We decided on three basic operations: c h a r c-csvgetl ine(F1LE c - ) : read a new CSV line c h a r c - c s v f i e l d ( i n t n): return the n-th field of the current line i n t csvnf i e l d (voi d): return the number of fields on the current line What function value should c s v g e t l i ne return It is desirable to return as much useful information as convenient, which suggests returning the number of fields, as in the prototype But then the number of fields must be computed even if the fields aren't being used Another possible value is the input line length, which is affected by whether the trailing newline is preserved After several experiments, we decided that c s v g e t l i n e will return a pointer to the original line of input, or NULL if end of file has been reached We will remove the newline at the end of the line returned by c s v g e t l i ne, since it can easily be restored if necessary The definition of a field is complicated; we have tried to match what we observe empirically in spreadsheets and other programs A field is a sequence of zero or more characters Fields are separated by commas Leading and trailing blanks are preserved A field may be enclosed in double-quote characters, in which case it may contain commas A quoted field may contain double-quote characters, which are represented by a doubled double-quote; the CSV field " x " " y W defines the string x"y Fields may be empty; a field specified as "" is empty, and identical to one specified by adjacent commas Fields are numbered from zero What if the user asks for a non-existent field by calling c s v f i e l d(-1) or c s v f i e l d (100000) We could return " " (the empty string) because this can be printed and compared; programs that process variable numbers of fields would not have to take special precautions to deal with non-existent ones But that choice provides no way to distinguish empty from non-existent A second choice would be to print an error message or even abort; we will discuss shortly why this is
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not desirable We decided to return NULL, the conventional value for a non-existent string in C
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of fields To achieve this, either the caller must provide the memory or the callee (the library) must allocate it The caller of the library function f g e t s passes in an array and a maximum size If the line is longer than the buffer, it is broken into pieces This behavior is unsatisfactory for the CSV interface, so our library will allocate memory as it discovers that more is needed Thus only c s v g e t l i ne knows about memory management; nothing about the way that it organizes memory is accessible from outside The best way to provide that isolation is through a function interface: c s v g e t l i ne reads the next line, no matter how big, c s v f i e l d ( n ) returns a pointer to the bytes of the n-th field of the current line, and csvnf i e l d returns the number of fields on the current line W e will have to grow memory as longer lines or more fields arrive Details of how that is done are hidden in the c s v functions; no other part of the program knows how this works, for instance whether the library uses small arrays that grow, or very large arrays, or something completely different Nor does the interface reveal when memory is freed If the user calls only c s v g e t l i ne, there's no need to split into fields; lines can be split on demand Whether field-splitting is eager (done right away when the line is read) or lazy (done only when a field or count is needed) or very lazy (only the requested field is split) is another implementation detail hidden from the user
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