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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.

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Syntax

#include <headerFile.h>

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

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Common Library Categories

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1. Input / Output — <stdio.h>

FunctionDescription
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;
}

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2. String Handling — <string.h>

FunctionDescription
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)

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3. Math Functions — <math.h>

FunctionDescription
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;
}

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4. Standard Library — <stdlib.h>

FunctionDescription
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

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5. Character Handling — <ctype.h>

FunctionDescription
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

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6. Time Functions — <time.h>

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

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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;
}