Reading Stdin in chunks... (possibly with scanf?)

Question

I have a linux box.

On this linux box, there is a program.

In this program, I have a loop like this:

int num=*"what num needs to be"*;
char proc[num];

int result;
while (1) {
result=scanf("%[^'&']%s",proc);
printf("And proc is: %s\n",proc);
printf("Result counter was: %i\n",result)
if (result == 0) break;
}

scanf("%[^'&'],%s",proc)
printf("post lop result is: %s", proc);

As you may have guessed, stdin contains data I need delineated by the '&' character.

As I'm hoping someone more skilled than me has guessed, the output looks something like:

And proc is: *first delineated section*
Result counter was: 1
And proc is: *first delineated section*
Result counter was: 0
post loop result is: *first delineated section*

I thought that scanf was supposed to consume the part of stdin it has already read. Why isn't it doing this?

Also, FYI: this is being run on a very cheap, slow server. Volume may or may not become more than slight. Efficiency is thus a plus, I'm open to however someone might suggest I do this....

Thanks!

Jonathan Leffler Jonathan Leffler · Accepted Answer · 2013-06-12T20:55:54

The scanset does not need two single quotes in it — one is sufficient if you want to break on a single quote, but I suspect you only want to stop on &, and the code below assumes that too. Once you've read up to the first &, you need some code to read the &. You need to check the result of scanf() before using the data it returned.

Hence:

int num = 100;
char proc[num];

int result;
while ((result = scanf("%99[^&]", proc)) == 1)
{
    printf("And proc is: <<%s>>\n", proc);
    printf("Result counter was: %i\n", result);
    int c;
    while ((c = getchar()) != EOF && c != '&')
        ;
}

You also need to decide whether newlines mark the end of a field too...if they do, then:

int num = 100;
char proc[num];

int result;
while ((result = scanf("%99[^&\n]", proc)) == 1)
{
    printf("And proc is: <<%s>>\n", proc);
    printf("Result counter was: %i\n", result);
    int c;
    while ((c = getchar()) != EOF)
    {
        if (c != '&' && c != '\n')
        {
            ungetc(c, stdin);
            break;
        }
    }
}

Note the use of %99[...] to prevent buffer overflows. The angle brackets <<%s>> simply mark the start and end of the string; they make trailing blanks and tabs visible, for example.

The code assumes you have a C99 compiler that allows variable declarations midway through a block of code. If not, move int c; to the top of the loop

Reading Stdin in chunks... (possibly with scanf?)

2 Answers