- HW03 is due soon.
- All or nothing tests, so TAs will look for extra partial credit.
- HW04 is available.
- We've been using the C standard library to access OS functionality,
but that's just calling functions made of normal code.
- In order to do things, these library functions need to call into the
- This is done with the syscall instruction, which works a lot like
the call instruction.
- The Linux kernel exposes about 300 syscalls.
List of Syscalls
Standard file descriptors:
Programming in C
- Let's write "prime", which prints the Nth prime number.
- Top-down design.
- Allocate an array for N primes.
- Pre-load first 3 primes.
- A number is-prime if it's not divisible by any prime up to sqrt(x).
- Makefile first.
- gcc command
- clean rule
- Local variables are "on the stack".
- Can allocate arrays "on the stack".
- Standard library functions and headers:
- stdio.h: printf
- stdlib.h: atol
- math.h: ceil, sqrt -- show manpage, talk about libm
- Casts. For numbers, these convert.
Arrays and Pointers
- Arrays in C are just like arrays in ASM. They're a contigous sequence of
memory starting at some address.
- Array variables store the address of the start of the array.
- C knows how big the type stored in array is.
- For arrays in ASM, we had to manually calculate the byte offsets.
- An array of words in C would be xs, xs, etc.
- Pointers are variables that store memory addresses with an associated type
so "int*" (int pointer) is the address of an int.
- C lets you do arthmetic with pointers.
- If you add one to an int pointer, it steps to the next int-sized chunk of memory.
So if p is a pointer to a 4 byte int, then (p + 1) is the address increased by
- Show sizeof: char, short, int, long, float, double, char*, long*
- Array varibles and pointers are the same thing with different notation.
- If pp is a pointer and aa is an array, both of the same type pointing to the same
address, then pp and *(pp + 3) are the same thing.
- Strangely, pp[ii] and ii[pp] are the same thing too.