Predefined Functions in C

Predefined functions (also called library functions or built-in functions) are functions that are already written, compiled, and provided by C’s standard libraries. You use them by including the appropriate header file with #include.

Syntax

#include <headerFile.h>

int main() {
    functionName(parameters);
    return 0;
}

Common Library Categories

1. Input / Output — `<stdio.h>`

Function	Description
`printf()`	Prints formatted output to screen
`scanf()`	Reads formatted input from user
`getchar()`	Reads a single character
`putchar()`	Prints a single character
`fgets()`	Reads a string (safe, handles spaces)

#include <stdio.h>
int main() {
    printf("Hello, World!");
    return 0;
}

2. String Handling — `<string.h>`

Function	Description
`strlen(s)`	Returns length of string (excluding `\0`)
`strcpy(d, s)`	Copies string `s` into `d`
`strcat(d, s)`	Appends `s` to end of `d`
`strcmp(s1, s2)`	Compares two strings (returns 0 if equal)
`strrev(s)`	Reverses a string (Turbo C)

3. Math Functions — `<math.h>`

Function	Description
`sqrt(x)`	Square root of x
`pow(x, y)`	x raised to power y
`abs(x)`	Absolute value of integer x
`fabs(x)`	Absolute value of float x
`ceil(x)`	Rounds up to nearest integer
`floor(x)`	Rounds down to nearest integer

#include <stdio.h>
#include <math.h>
int main() {
    printf("sqrt(25) = %.0f\n", sqrt(25));   // 5
    printf("pow(2,10)= %.0f\n", pow(2, 10)); // 1024
    printf("abs(-7)  = %d\n",   abs(-7));    // 7
    return 0;
}

4. Standard Library — `<stdlib.h>`

Function	Description
`malloc(size)`	Allocates memory at runtime
`calloc(n, size)`	Allocates and zero-initializes memory
`realloc(ptr, size)`	Resizes allocated memory
`free(ptr)`	Frees allocated memory
`rand()`	Generates a pseudo-random number
`exit(code)`	Terminates the program immediately
`atoi(s)`	Converts string to integer

5. Character Handling — `<ctype.h>`

Function	Description
`isalpha(c)`	Is the character a letter?
`isdigit(c)`	Is the character a digit?
`isupper(c)`	Is it uppercase?
`islower(c)`	Is it lowercase?
`toupper(c)`	Convert to uppercase
`tolower(c)`	Convert to lowercase

6. Time Functions — `<time.h>`

Function	Description
`time(NULL)`	Returns current time as seconds since epoch
`clock()`	Returns processor time used by the program

Example: Using Multiple Libraries

#include <stdio.h>
#include <math.h>
#include <string.h>

int main() {
    char name[] = "Vishal";
    printf("Name length: %d\n", strlen(name));   // 6
    printf("sqrt(49)  : %.0f\n", sqrt(49));      // 7
    return 0;
}

Fundamentals

Functions

Array

String

Structure

Union

Pointer

File Handling

Preprocessor

Predefined Functions

Predefined Functions in C

Syntax

Common Library Categories

1. Input / Output — `<stdio.h>`

2. String Handling — `<string.h>`

3. Math Functions — `<math.h>`

4. Standard Library — `<stdlib.h>`

5. Character Handling — `<ctype.h>`

6. Time Functions — `<time.h>`

Example: Using Multiple Libraries

​Predefined Functions in C

​Syntax

​Common Library Categories

​1. Input / Output — <stdio.h>

​2. String Handling — <string.h>

​3. Math Functions — <math.h>

​4. Standard Library — <stdlib.h>

​5. Character Handling — <ctype.h>

​6. Time Functions — <time.h>

​Example: Using Multiple Libraries

Predefined Functions in C

Syntax

Common Library Categories

1. Input / Output — `<stdio.h>`

2. String Handling — `<string.h>`

3. Math Functions — `<math.h>`

4. Standard Library — `<stdlib.h>`

5. Character Handling — `<ctype.h>`

6. Time Functions — `<time.h>`

Example: Using Multiple Libraries