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File input and output

File input and output. CS101 2012.1. File. A file is a 1d array of bytes on disk Can grow and shrink at the end, like a vector Interpreting the bytes is the business of the program using the file Bytes in a text (or .cpp) file are assumed to be ASCII code

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File input and output

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  1. File input and output CS101 2012.1

  2. File • A file is a 1d array of bytes on disk • Can grow and shrink at the end, like a vector • Interpreting the bytes is the business of the program using the file • Bytes in a text (or .cpp) file are assumed to be ASCII code • Bytes in a.out are assumed to be compiled executable code • The hexdump utility to view bytes in a file Chakrabarti

  3. ofstream #include <fstream> using namespace std; ofstream of(“/path/to/file”); of << 2 << 3 << (2-3) << endl; of.close(); If file does not exist, it is created empty. If it exists, it is truncated to zero bytes. A write cursor is initialized to 0. On Linux, 0x0a (linefeed) appended. On Windows, 0x0d (carriage return), then 0x0a are appended. Write cursor increased by 1 or 2. Integer 3 is translated to string “3”. The ASCII code byte for char `3’ (0x33) is appended to file. Write cursor is incremented. Chakrabarti

  4. ios flags to open fstream • ios::binaryOpen in binary mode (no CRLF patch ups) • ios::atePosition write cursor at end of file (as against beginning by default) • ios::truncTruncate file, similar to vec.resize(0)Create file if it does not exist already • ios::in and ios::outOpen for read, write, both Chakrabarti

  5. fstream: read and write fstream fs(“/path/to/file”, ios::binary | ios::in | ios::out | ios::trunc); char buf[100]; fs.read(buf, 100); // read/get cursor fs.write(buf, 100); // write/put cursor This block will be copied to or from the file (what part of the file?) buf Data blockin RAM 100 bytes Chakrabarti

  6. Get and put cursors px gx • Every read advances gx; every write advances px • No ordering beween gx and px in general • Can also be set using seekg and seekp • And current position known using tellg and tellp • Writing may extend the file • Reading past end sets EOF on fstream Chakrabarti

  7. tellg, tellp, seekg, seekp fstream fs(“/path/to/file”, ios::binary | ios::in | ios::out | ios::trunc); // getting cursor positions pos_type gx = fs.tellg() pos_type px = fs.tellp(); // setting cursor positions fs.seekg(gx); fs.seekp(px); Like integers are used to index arrays and matrices, pos_type is used to index bytes in a file Chakrabarti

  8. A vector on a disk file • Does not take up any heap space • Can be deathly slow class FileVector { public: FileVector(char *fname, int siz); void put(int ix, int val); int get(int ix) const; int size() const; }; Chakrabarti

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