C_notes
Must read: Brian Jorgensen Hall’s blog
C is not a big language, and it is not well served by a big book.
/* 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
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
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
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
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
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
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 asmalloc(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()`
```c
#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);
}