22 Dec 2023

Notes on C

Brian's blog on C

C is not a big language, and it is not well served by a big book. – Brian W. Kernighan, Dennis M. Ritchie

/* Hello World Program */
#include <stdio.h>

int main(void) {
    printf("Hello World!\n");
}

#include is a preprocessor directive.

How to know what to include? man 3 printf

main() is the first function to be executed in a C program.

Compile with gcc -o hello hello.c

Variables in C

Placeholders for values. Restrictions on names:

Names can't start with numbers.
Same for two underscores.
Same for single underscore and capital A-Z.

Variable types

int: integer types.
float: floating point types.
char: character type.
string: array of characters.

Booleans in C

Traditionally, 0 is false, and any other value is true. #include <stdbool.h> to include a bool type.

Arithmetic

Standard operators. Also, ternary operator.

Ternary operator is NOT flow control. It is an expression that evaluates to something.

Also, there is pre and post decrement. Stir clear of these unless you know what you are doing.

Weird Ass comma operators

int x = (1, 2, 3);
/* x is 3 in this case */

Conditional operators

Standard. == means both should be equal for true. Else, it will be false. != is the exact opposite.

<, <= and >, >= carry the same meaning as math.
Boolean ops

&& only of both are true. || if atleast one is true. ! takes the current value and inverts it. They operate on stuff meant to be boolean kinda.

The first two have something called short circuiting. If the first one is false, the second one isn't even evaluated in case of &&. Similarly if the first one is true, second one isn't even evaluated in case of ||.

Special functions

printf: Well, prints stuff. Look up manpage for more info.

sizeof: returns the size of anything. It's return type is an unsigned int called size_t.

NOTE: It is compile time to use sizeof

Control flow

Always remember braces!

if-else

No surprises here, should work the way you expect it to.

while

Yeah, same. Can't declare variables in the brackets, so there's that. Else no surprises.

while (do while this thing is true)

for

The below template works pretty much always.

for (initialize things; loop if this is true; do this after each loop)

Switch case

Always specify when you need a fallthrough.

#include <stdio.h>
int x = 0;

int main() {
  switch (x) {
  case 1:
    printf("1\n");
    break;
  case 2:
    printf("2\n");
    break;
  default:
    printf("any other value\n");
  }
}

If break isn't there all the other cases are evaluated unless a break is encountered.

Functions in C

If the parentheses in a C function are empty, it means it can take in any number of arguments. To specify no arguments, use void.

Arguments are copied. To modify the original thing pass a pointer.

A prototype is the signature that tells the compiler what the function takes in and spits out. Ends with a semicolon.

Pointers in C

Hold memory locations. Really, that's all there is to it.

#include <stdio.h>

int main(void)
{
    int i = 10;

    printf("The value of i is %d\n", i);
    printf("And its address is %p\n", (void *)&i);

    // %p expects the argument to be a pointer to void
    // so we cast it to make the compiler happy.
}

The address of anything can be obtained with & in front of it. To get the value from the address, use * in front of it.

Note on pointer declaration: int *p, q; over here only p is a pointer, q is a regular int.

NULL pointer

This means that the pointer does not point to anything. Dereferencing it causes memory error at best and random behaviour at worst.

Pointer arithmetic

Integers can be added to pointers and the pointers move forward or backward by those many units. C makes sure that the pointer is incremented by sizeof(type) if the pointer is type *.

void pointer

Can point to anything.
Cannot be dereferenced.
No pointer arithmetic.
sizeof(void *) will most likely crash.

Arrays in C

No surprises here either. You cannot have arrays with variable length, (you technically can), and you need to store the value of the length separately.

If you declared an array in the same scope you can check its size using sizeof(arr)/size(arr[0]).

Stuff like this also works:

int a[10] = {0, 11, 22, [5]=55, 66, 77};

Intermediate values and others are set to be 0. We can leave the size to be blank if we specify all values in the constructor initializer.

Arrays also act as pointers.

int main() {
  int a[10] = {0};
  int *p = a;
  p = &a[0];
}

Always pass the size of the array as a separate variable.

For multidimensional arrays, you have to pass all the dimensions except for the first one.

Array and pointer equivalence

E1[E2] = (*((E1) + (E2)))=

Strings in C

Arrays of characters terminated by the null character.

int main() {
  char *s = "Hello world\n";
  char t[] = "Loss pro max\n";
}

In the above example, s is immutable because it points to a hardcoded place in memory. On the other hand, the array copies the individual bytes from the hardcoded location and is therefore mutable.

strlen function returns the length of a null-terminated string and its return type is size_t.

strcpy makes a copy of the string byte by byte. Notice that doing t = s does not exactly copy the string as it only changes t to point to the same hardocded string and is not two different memory locations.

Structs

Ordered data-type containing various kinds of data fields.

struct car {
    char *name;
    float price;
    int speed;
};

// Now with an initializer! Same field order as in the struct declaration:
struct car saturn = {"Saturn SL/2", 16000.99, 175};

printf("Name:      %s\n", saturn.name);
printf("Price:     %f\n", saturn.price);
printf("Top Speed: %d km\n", saturn.speed);

struct car saturn = {.speed=175, .name"Saturn SL/2"};= something like this can also be done.

Whatever isn't initialized explicitly is initialised to 0 in memory.

Dot to access fields, arrow to access if it is a pointer to a struct.

Note: Do NOT compare structs directly.

File handling

FILE * is a pointer to a file in C. fprintf and fscanf take the first arguments as the file pointer and the rest is the same as printf and scanf.

To open a file, use fopen("file_path", "mode"). Mode can be r or w (for read or write).

Note: fgetc returns an int. This is because EOF doesn't fit in char.

fscanf and fprintf take the file pointer as the first argument. fputc, fputs, fgetc and fgets take them as the last argument.

Binary files

Use fread and fwrite to read and write from files. While writing structs and stuff, serialize your data because of endianness. Append b after the mode to indicate binary data.

fread returns the number of bytes read so useful to check if something has ben read or not.

typedef

Basically creates an alias for an existing type. Scoped. Useful for structs and arrays and pointers.

//  Anonymous struct! It has no name!
//         |
//         v
//      |----|
typedef struct {
    char *name;
    int leg_count, speed;
} animal;                         // <-- new name

//struct animal y;  // ERROR: this no longer works--no such struct!
animal z;           // This works because "animal" is an alias

typedef int *intptr;

int a = 10;
intptr x = &a, y = &a;  // "intptr" is type "int*"

// Make type five_ints an array of 5 ints
typedef int five_ints[5];

five_ints x = {11, 22, 33, 44, 55};

Manual Memory management

Allocate on heap manually, free manually.

malloc()

int *p = malloc(sizeof(*p)) is a common method to allocate memory. It returns NULL if memory can't be allocated so it is a good safety check.

int *x;

if ((x = malloc(sizeof(int) * 10)) == NULL)
    printf("Error allocating 10 ints\n");
    // do something here to handle it
}

Array allocation

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    // Allocate space for 10 ints
    int *p = malloc(sizeof(int) * 10);

    // Assign them values 0-45:
    for (int i = 0; i < 10; i++)
        p[i] = i * 5;

    // Print all values 0, 5, 10, 15, ..., 40, 45
    for (int i = 0; i < 10; i++)
        printf("%d\n", p[i]);

    // Free the space
    free(p);
}

calloc()

Similar to malloc, though it has slightly higher overhead than malloc(). Also returns NULL when nothing can be returned. First argument takes the number of elements to store in memory, second one takes the size of elements.

realloc()

Extend or shorten the existing ptr. Returns the new pointer.

Tries to extend the same pointer, if it can't be done, it finds some new place.
Again returns NULL if reallocation fails for some reason.
realloc(NULL, size) is the same as malloc(size).

#include <stdio.h>
#include <stdlib.h>

int main(void) {
  // Allocate space for 20 floats
  float *p = malloc(sizeof *p * 20); // sizeof *p same as sizeof(float)

  // Assign them fractional values 0.0-1.0:
  for (int i = 0; i < 20; i++)
    p[i] = i / 20.0;

  {
    // But wait! Let's actually make this an array of 40 elements
    float *new_p = realloc(p, sizeof *p * 40);

    // Check to see if we successfully reallocated
    if (new_p == NULL) {
      printf("Error reallocing\n");
      return 1;
    }

    // If we did, we can just reassign p
    p = new_p;
  }
  // And assign the new elements values in the range 1.0-2.0
  for (int i = 20; i < 40; i++)
    p[i] = 1.0 + (i - 20) / 20.0;

  // Print all values 0.0-2.0 in the 40 elements:
  for (int i = 0; i < 40; i++)
    printf("%f\n", p[i]);

  // Free the space
  free(p);
}

Here is a really good example to read a line of arbitrary length with realloc()

#include <stdio.h>
#include <stdlib.h>

// Read a line of arbitrary size from a file
//
// Returns a pointer to the line.
// Returns NULL on EOF or error.
//
// It's up to the caller to free() this pointer when done with it.
//
// Note that this strips the newline from the result. If you need
// it in there, probably best to switch this to a do-while.

char *readline(FILE *fp)
{
    int offset = 0;   // Index next char goes in the buffer
    int bufsize = 4;  // Preferably power of 2 initial size
    char *buf;        // The buffer
    int c;            // The character we've read in

    buf = malloc(bufsize);  // Allocate initial buffer

    if (buf == NULL)   // Error check
        return NULL;

    // Main loop--read until newline or EOF
    while (c = fgetc(fp), c != '\n' && c != EOF) {

        // Check if we're out of room in the buffer accounting
        // for the extra byte for the NUL terminator
        if (offset == bufsize - 1) {  // -1 for the NUL terminator
            bufsize *= 2;  // 2x the space

            char *new_buf = realloc(buf, bufsize);

            if (new_buf == NULL) {
                free(buf);   // On error, free and bail
                return NULL;
            }

            buf = new_buf;  // Successful realloc
        }

        buf[offset++] = c;  // Add the byte onto the buffer
    }

    // We hit newline or EOF...

    // If at EOF and we read no bytes, free the buffer and
    // return NULL to indicate we're at EOF:
    if (c == EOF && offset == 0) {
        free(buf);
        return NULL;
    }

    // Shrink to fit
    if (offset < bufsize - 1) {  // If we're short of the end
        char *new_buf = realloc(buf, offset + 1); // +1 for NUL terminator

        // If successful, point buf to new_buf;
        // otherwise we'll just leave buf where it is
        if (new_buf != NULL)
            buf = new_buf;
    }

    // Add the NUL terminator
    buf[offset] = '\0';

    return buf;
}

int main(void)
{
    FILE *fp = fopen("foo.txt", "r");

    char *line;

    while ((line = readline(fp)) != NULL) {
        printf("%s\n", line);
        free(line);
    }

    fclose(fp);
}

Variadic functions

#include <stdio.h>
#include <stdarg.h>

int add(int count, ...)
{
    int total = 0;
    va_list va;

    va_start(va, count);   // Start with arguments after "count"

    for (int i = 0; i < count; i++) {
        int n = va_arg(va, int);   // Get the next int

        total += n;
    }

    va_end(va);  // All done

    return total;
}

int main(void)
{
    printf("%d\n", add(4, 6, 2, -4, 17));  // 6 + 2 - 4 + 17 = 21
    printf("%d\n", add(2, 22, 44));        // 22 + 44 = 66
}

Deep C Secrets

Arrays are not pointers

// in file 1
int arr[10];

// in file 2
extern int *arr;

This is semantically incorrect and leads to an access violation. It thinks that arr holds the contents to the pointer. Making a reference to arr[i] is equivalent to saying that start at wherever arr points, move over i places using the contents of arr[0..8] as the pointer value.

Instead just declare it with extern int arr[].

Pointers to string literals

This is READ-ONLY memory as it is hardcoded to the .text section of the binary! Trying to deref it at runtime is a page-fault in most cases since that section of the binary is marked read-only by the OS.

On the other hand, if they are initialized by an array, then you can see in the disassembly that they are actually loaded on the stack and can use it.

Shared libraries

They need to have the names libname.so where name is what you link with -lname. LD_LIBRARY_PATH and LD_RUN_PATH are used for

Some small things

Const and volatile

Unlike with references in C++, you can have top level cv qualifiers for a pointer. That is to say,

void f(int * const p);

is valid. However, the above example is meaningless as the pointer is copied into the function. It makes sense for int const * p as that implies that the underlying data is constant.

Volatile is for raw hardware and basically means a value should be looked up from

Restrict keyword

Used for function optimization. If a pointer is an argument to a function, then the function is not aliased at all, atleast never written into. Mostly used as a hint to the compiler. However, it better not be used unless you're sure what you're doing.

Structs in C

Padding bytes

You can optionally specify padding bytes in struct fields and also limit their size to a fixed number of bytes. Once again, useful in embedded.

Offsets

If you have a pointer to a struct that is NULL, but you use the -> operator to get a member and then take its address, it will give the size of the member. Basically acting as if the member is stored at memory 0.

Another thing to note would be the fact that the pointer to the first member of the struct is also the pointer to the struct.

Unions

Just like structs, if the composing members of the union all have the same type, say int, then the pointer to the union is also a pointer to the int, no matter which one of the variants was set last.

Trigraph

Some ancient feature you shouldn't be bothered about. Basically ??! getting converted to pipe symbol and other shenanigans.

Passing Multidimensional VLAs to functions

#include <stdio.h>

void print_matrix(int h, int w, int m[h][w])
{
    for (int row = 0; row < h; row++) {
        for (int col = 0; col < w; col++)
            printf("%2d ", m[row][col]);
        printf("\n");
    }
}

int main(void)
{
    int rows = 4;
    int cols = 7;

    int matrix[rows][cols];

    for (int row = 0; row < rows; row++)
        for (int col = 0; col < cols; col++)
            matrix[row][col] = row * col;

    print_matrix(rows, cols, matrix);
}

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