In this assignment, you will use fread, fwrite, and fseek to implement operations on fixed-length records in a binary data file.
As usual, you should aim for reasonably efficient algorithms and reasonably well-organized, well-factored, comprehensible code.
Code correctness (mostly auto-marking) is worth 90% of the marks; code quality is worth 10%.
The Solarflares Tea And Lemonade Company maintains binary data files that have fixed-length records of customer names and loyalty points (called sunspots). The two operations they need you to implement are looking up sunspots and updating sunspots of a customer.
Each record takes 32 bytes and conforms to this struct on x86-64 (in record.h):
#define NAME_LEN_MAX 29
typedef struct record {
unsigned char name_len; // length of name
char name[NAME_LEN_MAX]; // name; NOT nul-terminated! use name_len above
unsigned short sunspots; // 2 bytes on x86-64
} record;I need to emphasize again: The name is not NUL-terminated like C
strings! You must use name_len for its length. The name can
really be as long as NAME_LEN_MAX; and when not, the unused space
contains arbitrary data—make no assumption.
And yet! The functions you will implement take C NUL-terminated strings as arguments. You will need to bridge the gap.
The data file is 0 or more records consecutively, and not sorted in
any particular order. The provided sample.dat has 3 records. A good way
to display it is hexdump -C sample.dat:
$ hexdump -C sample.dat
00000000 0e 44 65 6e 6e 69 73 20 52 69 74 63 68 69 65 2d |.Dennis Ritchie-|
00000010 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 86 07 |--------------..|
00000020 0a 41 72 63 68 69 6d 65 64 65 73 2d 2d 2d 2d 2d |.Archimedes-----|
00000030 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 22 01 |--------------".|
00000040 0b 41 6c 61 6e 20 54 75 72 69 6e 67 2d 2d 2d 2d |.Alan Turing----|
00000050 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 2d 60 ea |--------------`.|
00000060Leftmost column: file position (hexadecimal). Middle chunk: bytes in hexadecimal. Right column: bytes as characters, so whenever there should be a string, you can read it. It’s 16 bytes per line, nicely two lines (32 bytes) per record.
Here are the records explained:
Name: Dennis Ritchie (length 14), 1926 sunspots (hex 786, x86-64 stores multi-byte numbers in reverse byte order, the two bytes are 86, 07.)
Name: Archimedes (length 10), 290 sunspots.
Name: Alan Turing (length 11), 60,000 sunspots.
You should write more code than you hand in. Two reasons: Exercises, and sometimes you need to make your own diagnostic tools. Here is 1 token mark to give you a nudge.
Complete and hand in print.c to read a data file and print all records in human format; print in the same order as in the file. You can then use it to help verify your work in the rest of the assignment. A sample of the exact output format is in sample-print.txt.
You will need: How to adapt printf %s to print a string
that has a known length but no NUL. How to find out: In the man page or
on https://en.cppreference.com/w/c/io/fprintf, look
for:
%sname_len field is a variable and it’s different every
timeThere are two access operations to implement (hand in record.c):
int get_sunspots(FILE *f, const char *name, unsigned short *psunspots);Briefly: Get the sunspots field from the record that has the given name. There are more conditions, especially how to store the answer, and what if there is no such record; see the comments in record.h.
void set_sunspots(FILE *f, const char *name, unsigned short sunspots);Briefly: Look for the record that has the given name, change the sunspots field to the given number. If there is no such record, write a new one at the end of the file. There are more conditions, see the comments in record.h.
You may assume that f has already been opened
appropriately. You may assume that the name argument is at most
NAME_LEN_MAX bytes long (if not counting NUL termination).
A sample user program sample-main.c is provided to read Ritchie’s sunspots, add a new record for a new name, and change Archimedes’s sunspots.
Important: If you can’t implement one of the functions, you must still provide it with an essentially empty body free of compile-time errors, like in the starter code. Otherwise I can’t test the other one you can implement. A simple test: You can build an executable from your file with sample-main.c.
gcc -O2 -Wall record.c sample-main.cExcept for stated requirements, error handling is optional for simplicity. If you have debugging/error messages, send them to stderr only.