Let's first start refreshing your memory with a couple of small interesting programs in C.
Because you are always going to be dealing with Executables compiled in C and C++; A better understanding and grip on these languages will help you a lot with solving challenges in CTFs. Also keep in mind that a lot of tools are made with C. A great grip of C will help you contribute to open source tools and maybe build a few yourself. PS : You have a easier time at college. :)
Assignment: C language¶
If you have not learned C before, don't worry this is your chance to :)
Try taking up a C refresher such that you have:
- Good idea of Loops(while, for, switch) and control statements.
- Good grip over functions and functions calls.
- Basic idea about pointers (Syntax, Usage).
- Basic knowledge on Structures and objects.
- Basic File I/O.
If you are familiar with all these steps lets just complete a few simple C programming questions. These questions may need you to search around on the internet and also a bit of testing and tweaking.
Pointers are pretty important in c. A thing clarifying pointers would be nice. So try to implement the python
int()function in C. Like
atoi(char* ptr, int base)to return the integer stored at the pointer ptr.
- The function accepts a pointer to a string and an integer mode as arguments and outputs the appropriate number.
- the format should be
Int(char* numstring, int base)
Example: Int(‘234234234’, 10) Output: 234234234 Int(‘64’, 16) Output: 40
- Banned functions: ato[--anything]
Implement a function “
print_this” in C which is the equivalent of the printf in C.
- The function should accept a format string and the number of arguments as specified by the format string.
- The function should be able to accept and process:
- Strings -eg
print_this(“%s”,ptr)should print ptr as a string
- Hex numbers -eg
print_this(“%x”, 10)should print
- Octal numbers
- Binary numbers
- Strings -eg
%dshould all be handled
- Numbers before like
%02dshould be used to specify bytes printed on the screen.
%$should be treated as a illegal format string that cannot be used and
print_thisshould print nothing and return an error. An
EACCESSerror must be returned.
- Should print other illegal escape sequences directly
- The function should not use printf. It can use putc to print to the screen.
- The function should be available to use by other programs. Write a header file so that other functions can import this function.
- Macros must be used to denote any integer values used inside thefunction.
- The function should also return proper values to denote that the execution happened properly
- Also make sure that the number of arguments passed is same as the number of the format strings present. Incase this is not same the function should return some error.
- Feel free to add any good modifications that you feel will bebeneficial for yourself and the community.