Re: slurping in binary data

George wrote:
On Tue, 18 Nov 2008 12:26:05 GMT, James Kuyper wrote:

George wrote:
On Tue, 18 Nov 2008 00:59:20 -0800 (PST), Nick Keighley wrote:
...
assuming you mean the twenty lines at the beginning. Would fgets()
followed by

int line_num;
char data[32];

fscanf (line, "%d %32s", &line_num, data);

do the job?
Why fgets before scanf?

Key point to keep in mind here: I was thinking of sscanf(), not fscanf() (or scanf()). The fgets()/sscanf() combo is the best way I know of to read most text-format files.

Because scanf() treats newline characters the same way as any other whitespace character. This is usually not the way they should be handled. As a result, a single incorrectly formatted line can cause all following lines to be handled incorrectly, causing bugs that can be a real pain to track down.

How does fgets know to stop?

It stops at the first newline or when the buffer you've provided it is full, or at the end of the file, whichever comes first. The key point is the "newline" - that's what makes this approach more robust when reading line-oriented files.

Presumably, we want to start scanf'ing with '1'. Let me refresh you memory
of the data set. I call it george.txt to reflect my pseudonym.

1 0001000000000000001
2 0001000000000000001
3 10000011001000000000000001
4 10000011001000000000000001
5 10000011001000000000000001
6 10000011001000000000000001
7 10000011001000000000000001
8 10000011001000000000000001
9 10000011001000000000000001
10 10000011001000000000000001
11 100000001111100
12 100000001111100
13 100000001111100
14 1000001110110111100000000000001
15 1000001110110111100000000000001
16 1000001110110111100000000000001
17 1000001110110111100000000000001
18 1000001110110111100000000000001
19 1000001110110111100000000000001
20 0001000000000000001

It's 20 by forty. Given that I premise that the first datum is the line
number, do I still have to fgets?

If your input file is perfectly formatted, and your program is correctly written, there's no need. However, I think it's poor design to write code that fails catastrophically when given incorrect inputs. I believe in designing programs so they fail gracefully when given bad input. That means that they fail without undefined behavior, and with an informative error message, if possible. It's a lot harder to achieve that goal with fscanf() than it is with fgets()/sscanf().

What happens if the line number is missing from, for example, line 11? With fscanf(), it will try to interpret 100000001111100 as a decimal integer, and store it into the line number (with undefined behavior unless INT_MAX is larger than that value), and then put "12" into the data buffer. fscanf() will return a value of 2, indicating a successful read, because it has no way of noticing that anything went wrong. With fgets()/sscanf(), you can check whether sscanf()==2; if it does not, you immediately know there's a problem with the line.

Continuing processing despite a problem like that can be pointless, or mandatory, or anywhere in between those two extremes, depending upon your application. If you keep using fscanf(), it would attempt to read 100000001111100 as the line number and put "13" into the data buffer; it will stay out of sync with the actual lines until the end of the file, or the next incorrectly formatted line, whichever comes first.

With fgets()/sscanf(), fgets() will start cleanly at the next line, so sscanf() can do exactly what you need it to do; the combination of those two functions won't stay out of sync with the data, the way fscanf() would.
.

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