Question

This program is really getting on my nerves:

I am trying to read a line from a file with following information:

512 MB 136.186.99.1 00-25-B3-0B-31-29

which is in the format of double string string string

and the code I’m using is

fscanf(filePtr, "%lf %s %s %s", &Computer[i].ram, Computer[i].ram_unit, Computer[i].MACNo, Computer[i].IPV4);

but when I print Computer[i].ram_unit I get:

MB136.186.99.1

Please help me to find out what I’m doing wrong. Let me know if you like me to paste the entire code.

Thanks

Answer 1

First, you have MACNo and IPV4 reversed in your fscanf relative to the sample input.

Can’t tell for sure without seeing the structure definition, but it looks like a possible array overrun. For example, if your Computer was defined like this:

struct ComputerType {
    double ram;
    char ram_unit[2];  /* This needs to be 3 (assuming unit is always 2 chars long) */
    char IPV4[16];
    char MACNo[17];
};

when you read “MB” into ram_unit, you could end up having

ram_unit[0] = 'M'
ram_unit[1] = 'B'
IPV4[0] = '\0'

And then when you read in the IP address into IPV4 that makes it

ram_unit[0] = 'M'
ram_unit[1] = 'B'
IPV4[0] = '1'
IPV4[1] = '3'
[etc]
IPV4[10] = '1'
IPV4[11] = '\0'

When you go to print out ram_unit, the print function will start at the memory location &ram_unit[0] and keep on printing until it sees a NULL. But since the NULL ended up in IPV4[0] and that got overwritten when you read in the IP address, it won’t stop printing until it gets to the NULL at IPV4[11] and so you get the unexpected concatenation.