|Fundamental C - Simple Strings|
|Written by Harry Fairhead|
|Sunday, 08 December 2019|
Page 3 of 3
Usually the discussion of avoiding string or more generally array or buffer overflow ends at this point, but in the real world things are more complex.
When programming in C you need to be aware of where array data comes from. In general, the arrays and strings that you create and consume are safe enough because you know their size and can ensure that they are null-terminated where necessary. This means that you can use null-terminated string functions if you want to. This includes any functions you might write - you don’t have to further protect a function that you know only you are going to use and in a responsible manner.
Where things get dangerous is when data is generated externally – user input, network data, file data, or anything that is not originated by your program. In this case you have to put an upper limit on the number of items of data you are prepared to accept. This usually means using strn functions as opposed to str functions and more generally specifying array sizes. It also means that you have to remember to check that an array access is within the array bounds for every access – this is inefficient but safe.
Of course, in the real world implementing such strategies is always much harder. For example, consider the network data problem. You set up an array to accept data from a device or a service which normally sends you 500 bytes. However, you have no guarantee that network problems or exceptional circumstances might not force it to send 750 bytes or more. In an ideal world you would simply allocate an array so huge that overrunning it was unlikely – you would still have to check that it wasn’t overrun, however. In the real world you generally can’t afford that sort of memory allocation, especially on small devices. So what can you do?
In most case the best solution is to divide the transaction into packets of data. Read in the first 500 bytes, process it and see if there is any more. In this way you can safely reuse the 500 bytes you have allocated to the array and still not miss any data that goes beyond this limit. Of course, any processing that you do has to be fast enough so that you can carry on reading the next 500 bytes without missing any data or even aborting the connection due to a timeout.
The exact details of implementing the repeated use of a small buffer to read in large amounts of data varies according to how the data transfer protocol works and what is to be done with the data, but a for loop and a test of the end of the data is generally what is required. In the case of limited resources it is often necessary to trade code for memory.
In the book but not in this extract:
Fundamental C: Getting Closer To The Machine
Now available as a paperback and ebook from Amazon.
Also see the companion volume: Applying C
Harry Fairhead is the author of Raspberry Pi IoT in C , Micro:bit IoT in C and Fundamental C: Getting Closer to the Machine. His latest book is Applying C For The IoT With Linux.
Remote C/C++ Development With NetBeans
Getting Started With C/C++ On The Micro:bit
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|Last Updated ( Monday, 09 December 2019 )|