Reading input from fgetc() and printing with printf()

Question

Okay, I'm sure there's something I'm missing here, but I have no idea what it is and I'm hoping someone could help me figure it out.

I'm reading in input from the command line and was writing a function that uses fgetc() to do so. However, a seemingly superficial change in the function causes it to behave completely differently.

This is the main() function:

while(1)
{
     char* cmd = malloc(sizeof(char) * 80);
     printf("Enter command: ");
     read_flush(cmd, 80);
     printf("%s\n", cmd);
     free(cmd);
}

And this is one of the versions of read_flush():

int read_flush(char* buffer, int count)
{
    int i, c;
    for(i = 0; i < count; i++)
    {
        c = fgetc(stdin);
        if(c == '\n' || c == EOF)
            return i;
        buffer[i] = (char)c;
    }

    return i;
}

This one works fine. You type in input and it will spit it back out. However, this next version causes main to just print "Enter command:" over and over without giving the user a chance to enter input.

int read_flush(char* buffer, int count)
{
    int i, c;
    while(i < count)    
    {
        c = fgetc(stdin);
        if(c == '\n' || c == EOF)
            return i;
        buffer[i] = (char)c;
        i++;
    }

    return i;
}

What subtlety with fgetc() am I missing here?

score 1 · Answer 1 · answered Feb 23 '12 at 05:21

1

Try initializing i in your second read_flush implementation.

answered Feb 23 '12 at 05:21

Adam S

3,065
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Wow, whoops. Yeah, that was all it was. – user1227489 Feb 23 '12 at 05:28
It's good to get in the habit of always initializing your variables, especially when working with pointers. It will save you many headaches down the road. – Adam S Feb 23 '12 at 14:41

score 1 · Answer 2 · answered Feb 23 '12 at 05:22

1

In the second version it looks like you are not initializing i to zero like you do in the first version. So it could be starting with a garbage value larger than count, and thus the loop never executes.

answered Feb 23 '12 at 05:22

Brian Rogers

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score 1 · Answer 3 · answered Feb 23 '12 at 05:24

1

Both versions have the same error. You don't add a NUL on the end of the string. malloc does not initialize the memory that it returns.

answered Feb 23 '12 at 05:24

stark

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Reading input from fgetc() and printing with printf()

3 Answers3