Preventing buffer overflow when using fscanf

0
votes

I'm using fscanf to read some values from a CSV file and I want to ensure that the data read into the values will not be too large and cause a buffer overflow.

My csv file has the format int,string,string and my code to read is below (I will fix the while condition later):

while(fscanf(f, "%d,%[^,],%[^,]", &inArray[i].ID, inArray[i].label, inArray[i].brand)/*insert while condition here*/

When using scanf I would specify the length like so to prevent overflow: scanf("%20f", example);

But if I try the same with the above: while(fscanf(f, "%d,%20[^,],%10[^,]", &inArray[i].ID, inArray[i].label, inArray[i].brand)/*insert while condition here*/

I get a crash when the code executes.

3
c++inputbuffer-overflowscanf
Have you tried debugging? Is it crashing in the first iteration of your loop? How do you initialize i? - Hulk
If you want to learn the C++ way of reading formatted input, you probably want to have a look at cppreference on istream operator >> - Hulk

3 Answers

3
votes

Try fscanf_s, this function has security enhancements. http://msdn.microsoft.com/en-us/library/6ybhk9kc(v=vs.90).aspx

2
votes

You can't do that with fprintf when reading characters.

I would read the whole line first, e.g., with getline(), locate the separators (or tokenize the line), and then parse the individual elements.

Btw., the reason for you crash might also be a wrong definition/initialization of inArray.

1
votes

OP likely used the wrong width in the fscanf().

Although OP did not post details about inArray[i] let's assume it was 

struct {
  int ID;
  char label[20];
  char brand[10];
} inArray[100];

The format should then be

"%d,%19[^,],%9[^,]"

The width of 19 needs to be 1 less than the size of the destination, thus allowing a spot for the '\0'.