printf

Prints formatted output to the console.

printf("Hello, %s!\n", "World");

scanf

Reads formatted input from the console.

scanf("%d", &num);

getchar

Reads a single character from the console.

char ch = getchar();

putchar

Writes a single character to the console.

putchar('A');

fabs

Returns the absolute value of a floating-point number.

double result = fabs(-3.14);

ceil

Rounds up a floating-point number to the nearest integer.

double rounded = ceil(3.14);

floor

Rounds down a floating-point number to the nearest integer.

double rounded = floor(3.14);

sqrt

Calculates the square root of a number.

double squareRoot = sqrt(25.0);

pow

Calculates the power of a number.

double power = pow(2, 3);

strlen

Returns the length of a string.

int length = strlen("Hello");

strcpy

Copies one string to another.

char dest[20]; strcpy(dest, "Hello");

strcat

Concatenates two strings.

char str1[20] = "Hello, "; strcat(str1, "World");

strcmp

Compares two strings.

int result = strcmp("abc", "abd");

strchr

Finds the first occurrence of a character in a string.

char *position = strchr("Hello", 'e');

malloc

Allocates a block of memory on the heap.

int *arr = (int*)malloc(5 * sizeof(int));

calloc

Allocates a block of memory on the heap and initializes it to zero.

int *arr = (int*)calloc(5, sizeof(int));

realloc

Changes the size of a previously allocated block of memory.

arr = (int*)realloc(arr, 10 * sizeof(int));

free

Deallocates a block of memory on the heap.

free(arr);

isalpha

Checks if a character is an alphabetic character.

int result = isalpha('A');

isdigit

Checks if a character is a digit.

int result = isdigit('5');

toupper

Converts a character to uppercase.

char upper = toupper('a');

tolower

Converts a character to lowercase.

char lower = tolower('A');

fclose

Opens a file.

FILE *file = fopen("example.txt", "r");

fclose

Closes a file.

fclose(file);

fread

Reads data from a file.

fread(buffer, sizeof(char), 100, file);

fwrite

Writes data to a file.

fwrite(data, sizeof(int), 5, file);

rand

Generates a pseudo-random number.

int randomNum = rand();

srand

Seeds the random number generator.

srand(time(NULL));

time

Returns the current calendar time.

time_t t = time(NULL);

ctime

Converts a time value to a string.

char *str = ctime(&t);

localtime

Converts a time value to a structure representing local time.

struct tm *local = localtime(&t);

memcpy

Copies a block of memory.

int source[5] = {1, 2, 3, 4, 5}; memcpy(dest, source, 5 * sizeof(int));

memmove

Moves a block of memory, even if the source and destination overlap.

memmove(dest, source, 5 * sizeof(int));

memset

Sets a block of memory to a specific value.

int arr[10]; memset(arr, 0, 10 * sizeof(int));

strncpy

Copies a certain number of characters from one string to another.

char dest[10]; strncpy(dest, "Hello", 5);

strncat

Concatenates a certain number of characters from one string to another.

char str1[10] = "Hello, "; strncat(str1, "World", 3);

strncmp

Compares a certain number of characters in two strings.

int result = strncmp("abc", "abd", 2);

getenv

Retrieves the value of an environment variable.

char *value = getenv("PATH");

system

Executes a shell command.

system("ls -l");