C

How computers actually work

C

Memory, pointers, the stack, malloc. The language under your operating system, compiled for real in your browser.

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  1. 01 Hello, compilerWrite your first C program and find out what a compiler really does: it turns your text into machine code the CPU can run. 2 exercises
  2. 02 Variables are bytesIn C every variable is a fixed number of bytes in memory. Learn int, char, double and long long, and measure them with sizeof. 2 exercises
  3. 03 Numbers are just bitsHow integers are stored in binary, why an unsigned char wraps from 255 to 0, and how to print numbers in hex with printf. 2 exercises
  4. 04 Decisions and loopsMake C programs choose with if and else, and repeat work with for and while loops. Includes FizzBuzz and reading input. 2 exercises
  5. 05 Functions and the stackHow C functions get their own stack frame, why local variables vanish after return, and how recursion stacks frames on top of each other. 2 exercises
  6. 06 Arrays in memoryC arrays are blocks of same-sized elements packed side by side in memory. Learn indexing, sizeof tricks and why C never checks bounds. 2 exercises
  7. 07 Strings are char arraysC has no string type. A string is an array of chars ending in a zero byte. Write your own strlen and change letters by hand. 2 exercises
  8. 08 Pointers and addressesA pointer is a variable that holds a memory address. Learn what & and * do in C, with no magic and no fear. 2 exercises
  9. 09 Pass by value, pass a pointerC always copies arguments into the function. Learn why a naive swap fails and how passing pointers lets a function change your variables. 2 exercises
  10. 10 Pointer arithmeticAdding 1 to a pointer moves it by one whole element. See why arr[i] is really *(arr + i) and how arrays decay into pointers in C. 2 exercises
  11. 11 Structs and paddingGroup related values into one C struct, use the dot and arrow operators, and see why sizeof a struct can be bigger than its parts. 2 exercises
  12. 12 The heap and mallocAsk for memory at runtime with malloc, give it back with free, build a dynamic array in C and learn why memory leaks happen. 2 exercises

Projects for this track

FreeC · 40 min

Bit printer

Print any number in binary using shifts and masks. Once you can see the bits, overflow stops being magic.

  1. Write void print_bits(unsigned char x).
  2. Loop from bit 7 down to 0 and print (x >> i) & 1.
  3. Print 5, 255 and what 255 + 1 becomes in an unsigned char.
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FreeC · 40 min

Reverse a string in place

No library help: find the end of a string yourself, then swap characters from both ends until they meet. The best pointer practice there is.

  1. Write int my_strlen(const char *s) that counts characters up to the zero byte.
  2. Write void reverse(char *s) with one pointer at the start and one at the end, swapping as they move inwards.
  3. Test it on "hello", on a one-letter string and on an empty string.
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Pro, coming soonC · 4 hours

Write your own malloc

Carve a big byte array into blocks, track what is free, and hand out memory yourself. The best way to understand the heap is to be the heap.

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