C Crash Course

C functions are typically split into header files (.h) where things are declared but not defined, and implementation files (.c) where they are defined. When we run the C compiler, a complex sequence of events is triggered with the usual successful outcome begin an executable file as illustrated at http://www.codingunit.com/

Compilation process

Compilation process

The preprocessor merges the contents of the header and implementation files, and also expands any macros. The compiler then translates these into low level object code (.o) for each file, and the linker then joins together the newly generated object code with pre-compiled object code from libraries to form an executable. Sometimes we just want to generate object code and save it as a library (e.g. so that we can use it in Python).

In [2]:

from IPython.display import Image

A tutorial example - coding a Fibonacci function in C

Python version

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def fib(n):
    a, b = 0,  1
    for i in range(n):
        a, b = a+b, a
    return a

In [4]:

fib(100)

Out[4]:

354224848179261915075

C version

Header file

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%%file fib.h

double fib(int n);

Writing fib.h

Implemetnation file

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%%file fib.c

double fib(int n) {
    double a = 0, b = 1;
    for (int i=0; i<n; i++) {
        double tmp = b;
        b = a;
        a += tmp;
     }
    return a;
}

Writing fib.c

Driver program

In [7]:

%%file main.c
#include <stdio.h> // for printf()
#include <stdlib.h> // for atoi())
#include "fib.h" // for fib()

int main(int argc, char* argv[]) {
    int n = atoi(argv[1]);
    printf("%f", fib(n));
}


Writing main.c

Makefile

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%%file Makefile

CC=clang
CFLAGS=-Wall

fib: main.o fib.o
     $(CC) $(CFLAGS) -o fib main.o fib.o

main.o: main.c fib.h
     $(CC) $(CFAGS) -c main.c

fib.o: fib.c
     $(CC) $(CFLAGS) -c fib.c

clean:
     rm -f *.o

Overwriting Makefile

Compile

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! make

clang  -c main.c
clang -Wall -c fib.c
clang -Wall -o fib main.o fib.o

Run executable file

In [10]:

! ./fib 100

354224848179261997056.000000

C Basics

The basic types are very simple - use int, float and double for numbers. In general, avoid float for plain C code as its lack of precision may bite you unless you are writing CUDA code. Strings are quite nasty to use in C - I would suggest doing all your string processing in Python ...

Structs are sort of like classes in Python

struct point {
    double x;
    double y;
    double z;
};

struct point p1 = {.x = 1, .y = 2, .z = 3};
struct point p2 = {1, 2, 3};
struct point p3;
p3.x = 1;
p3.y = 2;
p3.z = 3;

You can define your own types using typedef -.e.g.

#include <stdio.h>
struct point {
    double x;
    double y;
    double z;
};

typedef struct point point;

int main() {
    point p = {1, 2, 3};
    printf("%.2f, %.2f, %.2f", p.x, p.y, p.z);
};

Most of the operators in C are the same in Python, but an important difference is the increment/decrement operator. That is

int c = 10;
c++; // same as c = c + 1, i.e., c is now 11
c--; // same as c = c - 1, i.e.. c is now 10 again

There are two forms of the increment operator - postfix c++ and prefix ++c. Both increment the variable, but in an expression, the postfix version returns the value before the increment and the prefix returns the value after the increment.

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%%file increment.c
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int x = 3, y;
    y = x++; // x is incremented and y takes the value of x before incrementation
    printf("x = %d, y = %d\n", x, y);
    y = ++x; // x is incremented and y takes the value of x after incrementation
    printf("x = %d, y = %d\n", x, y);
}

Writing increment.c

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%%bash

clang -Wall increment.c -o increment
./increment

x = 4, y = 3
x = 5, y = 5

Ternary operator

The ternary operator expr = condition ? expr1 : expr2 allows an if-else statement to be put in a single line. In English, this says that if condition is True, expr1 is assigned to expr, otherwise expr2 is assigned to expr. We used it in the tutorial code to print a comma between elements in a list unless the element was the last one, in which case we printed a new line ‘:raw-latex:`\n`‘.

Note: There is a similar ternary construct in Python expr = expr1 if condition else epxr2.

Very similar to Python or R. The examples below should be self-explanatory.

if-else

// Interpretation of grades by Asian parent
if (grade == 'A') {
    printf("Acceptable\n");
} else if (grade == 'B') {
    printf("Bad\n");
} else if (grade == 'C') {
    printf("Catastrophe\n");
} else if (grade == 'D') {
    printf("Disowned\n");
} else {
    printf("Missing child report filed with local police\n")
}

for, while, do

// Looping variants

// the for loop in C consists of the keyword if followed
// (initializing statement; loop condition statement; loop update statement)
// followed by the body of the loop in curly braces
int arr[3] = {1, 2, 3};
for (int i=0; i<sizeof(arr)/sizeof(arr[0]); i++) {
    printf("%d\n", i);
}

int i = 3;
while (i > 0) {
    i--;
}

int i = 3;
do {
    i==;
} while (i > 0);

Automatic arrays

If you know the size of the arrays at initialization (i.e. when the program is first run), you can usually get away with the use of fixed size arrays for which C will automatically manage memory for you.

int len = 3;

// Giving an explicit size
double xs[len];
for (int i=0; i<len; i++) {
    xs[i] = 0.0;
}

// C can infer size if initializer is given
double ys[] = {1, 2, 3};

Pointers and dynamic memory management

Otherwise, we have to manage memory ourselves using pointers. Basically, memory in C can be automatic, static or dynamic. Variables in automatic memory are managed by the computer, when it goes out of scope, the variable disappears. Static variables essentially live forever. Dynamic memory is allocated in the stack, and you manage its lifetime.

Mini-glossary: * scope: Where a variable is visible - basically C variables have block scope - variables either live within a pair of curly braces (including variables in parentheses just before block such as function arguments and the counter in a for loop), or they are visible throughout the file. * stack: Computer memory is divided into a stack (small) and a heap (big). Automatic variables are put on the stack; dynamic variables are put in the heap. Hence if you have a very large array, you would use dynamic memory allocation even if you know its size at initialization.

Any variable in memory has an address represented as a 64-bit integer in most operating systems. A pointer is basically an integer containing the address of a block of memory. This is what is returned by functions such as malloc. In C, a pointer is denoted by ‘*’. However, the ‘*’ notation is confusing because its interpretation depends on whenever you are using it in a declaration or not. In a declaration

int *p = malloc(sizeof(int)); // p is a pointer to an integer
*p = 5; // *p is an integer

To get the actual address value, we can use the & address operator. This is often used so that a function can alter the value of an argument passed in (e.g. see address.c below).

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%%file pointers.c
#include <stdio.h>

int main()
{
    int i = 2;
    int j = 3;
    int *p;
    int *q;
    *p = i;
    q = &j;
    printf("p  = %p\n", p);
    printf("*p = %d\n", *p);
    printf("&p = %p\n", &p);
    printf("q  = %p\n", q);
    printf("*q = %d\n", *q);
    printf("&q = %p\n", &q);
}

Writing pointers.c

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%%bash

clang -Wall -Wno-uninitialized pointers.c -o pointers
./pointers

p  = 0x7fff55d34898
*p = 2
&p = 0x7fff55d34870
q  = 0x7fff55d34878
*q = 3
&q = 0x7fff55d34868

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%%file address.c
#include <stdio.h>

void change_arg(int *p) {
    *p *= 2;
}
int main()
{
    int x = 5;
    change_arg(&x);
    printf("%d\n", x);
}

Writing address.c

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%%bash

clang -Wall address.c -o address
./address

10

If we want to store a whole sequence of ints, we can do so by simply allocating more memory:

int *ps = malloc(5 * sizeof(int)); // ps is a pointer to an integer
for (int i=0; i<5; i++) {
    ps[i] = i;
}

The computer will find enough space in the heap to store 5 consecutive integers in a contiguous way. Since C arrays are all fo the same type, this allows us to do pointer arithmetic - i.e. the pointer ps is the same as &ps[0] and ps + 2 is the same as &ps[2]. An example at this point is helpful.

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%%file pointers2.c
#include <stdio.h>
#include <stdlib.h>

int main()
{
    int *ps = malloc(5 * sizeof(int));
    for (int i =0; i < 5; i++) {
        ps[i] = i + 10;
    }

    printf("%d, %d\n", *ps, ps[0]); // remmeber that *ptr is just a regular variable outside of a declaration, in this case, an int
    printf("%d, %d\n", *(ps+2), ps[2]);
    printf("%d, %d\n", *(ps+4), *(&ps[4])); // * and & are inverses
}

Writing pointers2.c

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%%bash

clang -Wall pointers2.c -o pointers2
./pointers2

10, 10
12, 12
14, 14

Pointers and arrays

An array name is actually just a constant pointer to the address of the beginning of the array. Hence, we can dereference an array name just like a pointer. We can also do pointer arithmetic with array names - this leads to the following legal but weird syntax:

arr[i] = *(arr + i) = i[arr]

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%%file array_pointer.c
#include <stdio.h>

int main()
{
    int arr[] = {1, 2, 3};
    printf("%d\t%d\t%d\t%d\t%d\t%d\n", *arr, arr[0], 0[arr], *(arr + 2), arr[2], 2[arr]);
}

Writing array_pointer.c

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%%bash

clang -Wall array_pointer.c -o array_pointer
./array_pointer

1       1       1       3       3       3

More on pointers

Different kinds of nothing: There is a special null pointer indicated by the keyword NULL that points to nothing. It is typically used for pointer comparisons, since NULL pointers are guaranteed to compare as not equal to any other pointer (including another NULL). In particular, it is often used as a sentinel value to mark the end of a list. In contrast a void pointer (void *) points to a memory location whose type is not declared. It is used in C for generic operations - for example, malloc returns a void pointer. To totally confuse the beginning C student, there is also the NUL keyword, which refers to the '\0' character used to terminate C strings. NUL and NULL are totally different beasts.

Deciphering pointer idioms: A common C idiom that you should get used to is *q++ = *p++ where p and q are both pointers. In English, this says

  • *q = *p (copy the variable pointed to by p into the variable pointed to by q)
  • increment q
  • increment p
In [21]:

%%file pointers3.c
#include <stdio.h>
#include <stdlib.h>

int main()
{
    // example 1
    typedef char* string;
    char *s[] = {"mary ", "had ", "a ", "little ", "lamb", NULL};
    for (char **sp = s; *sp != NULL; sp++) {
        printf("%s", *sp);
    }
    printf("\n");

    // example 2
    char *src = "abcde";
    char *dest = malloc(5); // char is always 1 byte by C99 definition

    char *p = src + 4;
    char *q = dest;
    while ((*q++ = *p--)); // put the string in src into dest in reverse order

    for (int i = 0; i < 5; i++) {
        printf("i = %d, src[i] = %c, dest[i] = %c\n", i, src[i], dest[i]);
    }
}

Writing pointers3.c

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%%bash

clang -Wall pointers3.c -o pointers3
./pointers3

mary had a little lamb
i = 0, src[i] = a, dest[i] = e
i = 1, src[i] = b, dest[i] = d
i = 2, src[i] = c, dest[i] = c
i = 3, src[i] = d, dest[i] = b
i = 4, src[i] = e, dest[i] = a

pointers3.c:7:19: warning: unused typedef 'string' [-Wunused-local-typedef]
    typedef char* string;
                  ^
1 warning generated.

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%%file square.c
#include <stdio.h>

double square(double x)
{
    return x * x;
}

int main()
{
    double a = 3;
    printf("%f\n", square(a));
}

Writing square.c

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%%bash

clang -Wall square.c -o square
./square

9.000000

How to make a nice function pointer: Start with a regular function declaration func, for example, here func is a function that takes a pair of ints and returns an int

int func(int, int);

To turn it to a function pointer, just add a * and wrap in parenthesis like so

int (*func)(int, int);

Now func is a pointer to a function that takes a pair of ints and returns an int. Finally, add a typedef so that we can use func as a new type

typedef int (*func)(int, int);

which allows us to create arrays of function pointers, higher order functions etc as shown in the following example.

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%%file square2.c
#include <stdio.h>
#include <math.h>

// Create a function pointer type that takes a double and returns a double
typedef double (*func)(double x);

// A higher order function that takes just such a function pointer
double apply(func f, double x)
{
    return f(x);
}

double square(double x)
{
    return x * x;
}

double cube(double x)
{
    return pow(x, 3);
}

int main()
{
    double a = 3;
    func fs[] = {square, cube, NULL};

    for (func *f=fs; *f; f++) {
        printf("%.1f\n", apply(*f, a));
    }
}

Writing square2.c

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%%bash

clang -Wall -lm square2.c -o square2
./square2

9.0
27.0

As you have seen, the process of C program compilation can be quite messy, with all sorts of different compiler and linker flags to specify, libraries to add and so on. For this reason, most C programs are compiled using the make built tool that you are already familiar with. Here is a simple generic makefile that you can customize to compile your own programs adapted from the book 21st Century C by Ben Kelmens (O’Reilly Media).

  • TARGET: Typically the name of the executable
  • OBJECTS: The intermediate object files - typically there is one file.o for every file.c
  • CFLAGS: Compiler flags, e.g. -Wall (show all warnings), -g (add debug information), -O3 (use level 3 optimization). Also used to indicate paths to headers in non-standard locations, e.g. -I/opt/include
  • LDFLAGS: Linker flags, e.g. -lm (link against the libmath library). Also used to indicate path to libraries in non-standard locations, e.g. -L/opt/lib
  • CC: Compiler, e.g. gcc or clang or icc

In addition, there are traditional dummy flags * all: Builds all targets (for example, you may also have html and pdf targets that are optional) * clean: Remove intermediate and final products generated by the makefile

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%%file makefile
TARGET =
OBJECTS =
CFLAGS = -g -Wall -O3
LDLIBS =
CC = c99

all: TARGET

clean:
     rm $(TARGET) $(OBJECTS)

$(TARGET): $(OBJECTS)

Overwriting makefile

Just fill in the blanks with whatever is appropriate for your program. Here is a simple example where the main file test_main.c uses a function from stuff.c with declarations in stuff.h and also depends on the libm C math library.

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%%file stuff.h
#include <stdio.h>
#include <math.h>

void do_stuff();

Writing stuff.h

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%%file stuff.c
#include "stuff.h"

void do_stuff() {
    printf("The square root of 2 is %.2f\n", sqrt(2));
}

Writing stuff.c

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%%file test_make.c
#include "stuff.h"

int main()
{
    do_stuff();
}

Writing test_make.c

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%%file makefile
TARGET = test_make
OBJECTS = stuff.o
CFLAGS = -g -Wall -O3
LDLIBS = -lm
CC = clang

all: $(TARGET)

clean:
     rm $(TARGET) $(OBJECTS)

$(TARGET): $(OBJECTS)

Overwriting makefile

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! make

clang -g -Wall -O3    -c -o stuff.o stuff.c
clang -g -Wall -O3     test_make.c stuff.o  -lm -o test_make

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! ./test_make

The square root of 2 is 1.41

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# Make is clever enough to recompile only what has been changed since the last time it was called
! make

make: Nothing to be done for `all'.

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! make clean

rm test_make stuff.o

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! make

clang -g -Wall -O3    -c -o stuff.o stuff.c
clang -g -Wall -O3     test_make.c stuff.o  -lm -o test_make

Try to fix the following buggy program.

In [37]:

%%file buggy.c

# Create a function pointer type that takes a double and returns a double
double *func(double x);

# A higher order function that takes just such a function pointer
double apply(func f, double x)
{
    return f(x);
}

double square(double x)
{
    return x * x;
}

double cube(double x)
{
    return pow(3, x);
}

double mystery(double x)
{
    double y = 10;
    if (x < 10)
        x = square(x);
    else
        x += y;
        x = cube(x);
    return x;
}

int main()
{
    double a = 3;
    func fs[] = {square, cube, mystery, NULL}

    for (func *f=fs, f != NULL, f++) {
        printf("%d\n", apply(f, a));
    }
}

Writing buggy.c

In [38]:

! clang -g -Wall buggy.c -o buggy

buggy.c:2:3: error: invalid preprocessing directive
# Create a function pointer type that takes a double and returns a double
  ^
buggy.c:5:3: error: invalid preprocessing directive
# A higher order function that takes just such a function pointer
  ^
buggy.c:6:14: error: unknown type name 'func'
double apply(func f, double x)
             ^
buggy.c:18:12: warning: implicitly declaring library function 'pow' with type
      'double (double, double)'
    return pow(3, x);
           ^
buggy.c:18:12: note: include the header <math.h> or explicitly provide a
      declaration for 'pow'
buggy.c:35:9: error: expected ';' after expression
    func fs[] = {square, cube, mystery, NULL}
        ^
        ;
buggy.c:35:10: error: use of undeclared identifier 'fs'
    func fs[] = {square, cube, mystery, NULL}
         ^
buggy.c:35:13: error: expected expression
    func fs[] = {square, cube, mystery, NULL}
            ^
buggy.c:35:17: error: expected expression
    func fs[] = {square, cube, mystery, NULL}
                ^
buggy.c:35:5: warning: expression result unused [-Wunused-value]
    func fs[] = {square, cube, mystery, NULL}
    ^~~~
2 warnings and 7 errors generated.

What other language has an annual Obfuscated Code Contest http://www.ioccc.org/? In particular, the following features of C are very conducive to writing unreadable code:

  • lax rules for identifiers (e.g. _o, _0, _O, O are all valid identifiers)
  • chars are bytes and pointers are integers
  • pointer arithmetic means that array[index] is the same as *(array+index) which is the same as index[array]!
  • lax formatting rules especially with respect to whitespace (or lack of it)
  • Use of the comma operator to combine multiple expressions together with the ?: operator
  • Recursive function calls - e.g. main calling main repeatedly is legal C

Here is one winning entry from the 2013 IOCCC entry that should warm the heart of statisticians - it displays sparklines (invented by Tufte).

main(a,b)char**b;{int c=1,d=c,e=a-d;for(;e;e--)_(e)<_(c)?c=e:_(e)>_(d)?d=e:7;
while(++e<a)printf("\xe2\x96%c",129+(**b=8*(_(e)-_(c))/(_(d)-_(c))));}

In [39]:

%%file sparkl.c
main(a,b)char**b;{int c=1,d=c,e=a-d;for(;e;e--)_(e)<_(c)?c=e:_(e)>_(d)?d=e:7;
while(++e<a)printf("\xe2\x96%c",129+(**b=8*(_(e)-_(c))/(_(d)-_(c))));}

Writing sparkl.c

In [40]:

! gcc -Wno-implicit-int -include stdio.h -include stdlib.h -D'_(x)=strtof(b[x],0)' sparkl.c -o sparkl

In [42]:

import numpy as np
np.set_printoptions(linewidth=np.infty)
print(' '.join(map(str, (100*np.sin(np.linspace(0, 8*np.pi, 30))).astype('int'))))

0 76 98 51 -31 -92 -88 -21 60 99 68 -10 -82 -96 -41 41 96 82 10 -68 -99 -60 21 88 92 31 -51 -98 -76 0

In [43]:

%%bash

./sparkl 0 76 98 51 -31 -92 -88 -21 60 99 68 -10 -82 -96 -41 41 96 82 10 -68 -99 -60 21 88 92 31 -51 -98 -76 0

▅██▇▃▁▁▄▇▉▇▄▁▁▃▆██▅▂▁▂▅██▆▂▁▁▅

If you have too much time on your hands and really want to know how not to write C code (unless you are crafting an entry for the IOCCC), I recommend this tutorial http://www.dreamincode.net/forums/topic/38102-obfuscated-code-a-simple-introduction/

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