In the C programming language, library functions are pre-defined functions that are available for programmers to use directly without writing their own implementations. These functions are stored in header files, which must be included in a program using the #include directive.
Library functions help save time, reduce errors, and provide reliable, optimized operations for common programming tasks.
Why Use Library Functions?
- They are pre-tested and reliable
- Help you write code faster
- Reduce the need to reinvent common functionalities
- Improve program readability
- Ensure portability across different systems
Common Library Functions in C (With Categories)
Below are the most frequently used categories of library functions along with examples:
1. Input/Output Functions
Header File: stdio.h
| Function | Description |
|---|---|
printf() | Prints formatted output to the screen |
scanf() | Reads formatted input from the keyboard |
gets() | Reads a string (deprecated) |
puts() | Prints a string |
getchar() | Reads a single character |
putchar() | Prints a single character |
2. String Handling Functions
Header File: string.h
| Function | Description |
|---|---|
strlen() | Returns the length of a string |
strcpy() | Copies one string to another |
strcat() | Concatenates two strings |
strcmp() | Compares two strings |
strrev() | Reverses a string (not standard everywhere) |
3. Mathematical Functions
Header File: math.h
| Function | Description |
|---|---|
sqrt() | Calculates square root |
pow() | Calculates a number raised to a power |
abs() | Returns absolute value (int) |
fabs() | Returns absolute value (float/double) |
sin(), cos(), tan() | Trigonometric functions |
log(), log10() | Logarithmic functions |
4. Character Handling Functions
Header File: ctype.h
| Function | Description |
|---|---|
isalnum() | Checks if alphanumeric |
isalpha() | Checks if alphabetic |
isdigit() | Checks if a digit |
tolower() | Converts to lowercase |
toupper() | Converts to uppercase |
5. Memory Management Functions
Header File: stdlib.h
| Function | Description |
|---|---|
malloc() | Allocates memory dynamically |
calloc() | Allocates multiple memory blocks |
realloc() | Resizes previously allocated memory |
free() | Frees dynamically allocated memory |
6. General Utility Functions
Header File: stdlib.h
| Function | Description |
|---|---|
system() | Executes system commands |
exit() | Terminates the program |
rand() | Generates a random number |
atoi() | Converts string to integer |
atof() | Converts string to float |
7. Time and Date Functions
Header File: time.h
| Function | Description |
|---|---|
time() | Returns current system time |
ctime() | Converts time to string format |
difftime() | Finds difference between two times |
Using a Library Function
Example: Using sqrt() function from <math.h>:
#include <stdio.h>
#include <math.h>
int main() {
double num = 25, result;
result = sqrt(num);
printf("Square root of %.2f is %.2f", num, result);
return 0;
}
Conclusion
Library functions form the backbone of C programming. They reduce complexity, enhance performance, and allow programmers to focus on core logic rather than low-level details. From input/output to string handling, mathematics, and memory management — library functions make C more powerful and efficient.
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