Library Functions In C | Header Files, Uses & More (+ Examples)

Library functions in C programming language are pre-defined functions(or inbuilt functions) that are provided by the C Standard Library. These functions are stored in header files and offer a convenient way to implement common functionalities without rewriting code from scratch. From mathematical computations to string manipulation, library functions make programming more efficient and error-free.

In this article, we will explore the significance of library functions in C, their various types, and how they enhance productivity. We'll also discuss how to use these functions, the importance of including the correct header files, and some commonly used standard libraries.

What Are User-Defined Functions In C?

User-defined functions in C programming are functions created by the programmer to perform specific tasks or calculations within a program.

• These functions are not part of the language's standard library but are custom-written to modularize and organize code and promote code reusability.
• A user-defined function has the following elements: a name, a list of parameters, a return type (which can be void for functions that don't return a value), and a function body.
• By using user-defined functions, we can break down complex problems into smaller, manageable pieces, making the code more readable and maintainable.

These are in contrast to the library functions, which are the focus of this article.

What Are Library Functions In C?

The C Standard Library is a collection of header files and routines that provide a set of common functions and macros used in C programming. It is designed to provide a consistent and portable interface for performing various tasks, such as input/output operations, string manipulation, memory management, and mathematical calculations.

• The functions and macros in the standard library are defined by the C standard and are implemented by all conforming compilers in C, making them universally available across different platforms.
• Library functions in C are pre-written functions provided by this standard library or other libraries that we can use in our programs.

These functions are designed to perform common tasks, making programming easier and more efficient. Instead of writing these functions from scratch, we can simply include the appropriate library and call the functions as needed in our programs.

Here are some of the most commonly used standard library functions in C:

Header Files For Library Functions In C Programming

Header files in C programming are files that contain definition of functions, macros, and types, as well as definitions of constants. They serve as an interface between the code written by the programmer and the library functions provided by the C Standard Library or other external libraries.

By including the appropriate header files in a C program, we inform the compiler about the existence and details of the functions and other entities that will be used in the code.

Why Do We Use Header Files?

Header files are essential for several reasons, including:

1. Function Declaration: They provide the implementation of library functions, allowing the compiler to check for correct usage, such as the number and types of arguments and return types.
2. Encapsulation:Header files encapsulate the interface of a module or library. They provide a clear separation between what a module offers (the interface) and how it is implemented (the source code).
3. Maintainability: When you need to make changes to a module's interface, you only need to update the header file, not every source file that uses the module. This reduces the risk of introducing errors during code changes and simplifies maintenance.
4. Code Reusability: They allow for the separation of interface and implementation, making it possible to include common declarations in multiple source files without duplication.
5. Error Checking: Header files help catch errors in your code early in the development process. When a header file is included in a source file, the compiler ensures that the functions and data structures are used correctly based on the declarations provided in the header. This can prevent common programming mistakes.
6. Modularity: By using header files, we can organize code into separate modules, making it easier to manage and maintain.
7. Readability: Header files improve code readability by providing a concise summary of the functions and data structures available in a module. Seasoned programmers can quickly reference the header file to understand how to use the module's features without delving into the source code.
8. Consistency: They help ensure consistency in the declarations and definitions used across different parts of a program.

List Of Commonly Used Header Files In C

Here is a list of commonly used header files in C along with the purpose:

In practice, we include a header file using the #include preprocessor directive at the beginning of a C source file, like this:

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

This inclusion ensures that the compiler is aware of the library functions we intend to use, providing necessary declarations and definitions.

Implementation Of Library Functions In C

In this section, we’ll explore the implementation of several commonly used library functions in C, along with code examples to enhance our understanding.

The <stdio.h> Header File & Its Library Functions In C

The <stdio.h> header file provides functionalities for input and output operations in C. It includes functions for reading from and writing to standard input and output, file handling, and formatted input and output. This header is essential for performing basic and advanced I/O operations.

Rules and Regulations:

• Include this header when performing any form of input or output operations in C.
• Standard I/O functions typically interact with files or streams. The standard streams stdin, stdout, and stderr represent standard input, output, and error, respectively.
• When working with files, always check if the file operations are successful and close files using fclose() to avoid resource leaks.

Commonly Used Library Functions In C:

The printf() Library Function In C Example

This function is used to print formatted output to the standard output stream (stdout).

Syntax:

printf(format_string, arg1, arg2, ...);

Here,

• printf(): The function name, used for formatted output to the standard output stream.
• format_string: A string that specifies how subsequent arguments (arg1, arg2, etc.) should be formatted. It includes format specifiers (e.g., %d, %s) that define the type and format of the arguments.
• arg1, arg2, ...: The arguments to be formatted and printed according to the format_string.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgppbnQgbWFpbigpCgp7CgpwcmludGYoIlVuc3RvcCIpOwoKcmV0dXJuIDA7Cgp9

Output:

Unstop

Explanation:

In the above code example-

1. We start by including the <stdio.h> header file to use the printf() library function in C.
2. In the main() function, we print "Unstop" to the console using printf() and then return 0 to indicate that the program executed successfully.

The <math.h> Header File & Its Library Functions In C

The <math.h> header file provides a set of mathematical functions for performing common arithematic operations such as trigonometric calculations, exponential and logarithmic functions, power and square root operations, and other mathematical utilities. These functions operate on float, double, and long double types.

Rules and Regulations:

• Include this header when performing mathematical computations that require functions beyond basic arithmetic operations.
• The functions in <math.h> are available in the standard library, and they generally operate on floating-point types.
• Be mindful of the domain and range of these functions to avoid invalid results or domain errors (e.g., passing a negative number to sqrt).

Commonly Used Library Functions In C:

The sqrt() Library Function In C Example

This function is primarily used to calculate the square root of a given number.

Syntax:

double sqrt(double x);

Here,

• double: The return type, indicating that the function returns a double-precision floating-point number.
• sqrt: The function name, which calculates the square root of a number.
• double x: The parameter, where x is the number for which the square root is to be calculated.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8bWF0aC5oPgoKaW50IG1haW4oKSB7Cgpkb3VibGUgbnVtID0gMTYuMDsKCmRvdWJsZSByZXN1bHQgPSBzcXJ0KG51bSk7CgpwcmludGYoIlNxdWFyZSByb290IG9mICUuMmYgaXMgJS4yZlxuIiwgbnVtLCByZXN1bHQpOwoKcmV0dXJuIDA7Cgp9

Output:

Square root of 16.00 is 4.00

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <math.h> for the sqrt() library functions in C.
2. We then calculate the square root of 16.0 using the sqrt() function, store it in the result, and print the result with a formatted message using the printf() function.

The <stdlib.h> Header File & Its Library Functions In C

The <stdlib.h> header file provides functions for performing various utility operations such as memory allocation, process control, conversions, and random number generation. It includes C library functions for managing dynamic memory, converting strings to numeric values, and handling program termination.

Rules and Regulations:

• Include this header when performing tasks related to dynamic memory management, process control, or conversions between strings and numeric values.
• Be cautious with memory allocation library functions in C to avoid memory leaks and ensure proper deallocation of allocated memory.
• Functions that allocate memory (e.g., malloc, calloc) should be paired with corresponding functions to free memory (e.g., free).

Commonly Used Library Functions In C:

The atoi() Library Function In C Example

This function is used to convert a string representing an integer into an actual integer value.

Syntax:

int atoi(const char* str);

Here,

• int: The return type, indicating that the function returns an integer.
• atoi: The function name, which converts a string to an integer.
• const char str*: The pointer to the string that will be converted to an integer. The string should represent a number in decimal format.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8c3RkbGliLmg+CgppbnQgbWFpbigpIHsKCmNvbnN0IGNoYXIqIHN0ciA9ICI0MiI7CgppbnQgbnVtID0gYXRvaShzdHIpOwoKcHJpbnRmKCJDb252ZXJ0ZWQgaW50ZWdlcjogJWRcbiIsIG51bSk7CgpyZXR1cm4gMDsKCn0=

Output:

Converted integer: 42

Explanation:

In the above code example-

1. We include <stdio.h> for printf() and <stdlib.h> for atoi() library functions in C repsectively.
2. We then convert the string "42" to an integer using atoi() function, store it in the integer variable num, and print the converted integer using the printf() function.

The <string.h> Header File & Its Library Functions In C

The <string.h> header file provides functions for manipulating strings and arrays of characters. It includes functions for copying, concatenating, comparing, and searching within strings. These functions operate on null-terminated strings, which are arrays of characters terminated by a null character ('\0').

Rules and Regulations:

• Include this header when performing operations on strings or character arrays.
• Functions in <string.h> expect null-terminated strings, so proper null termination is crucial for correct operation.
• Be cautious with buffer sizes and memory allocation to avoid buffer overflows and ensure that functions do not read or write outside the bounds of allocated memory.

Commonly Used Library Functions In C:

The strcpy() Library Function In C Example

The strcpy() function is used to copy the contents of one string to another.

Syntax:

char* strcpy(char* destination, const char* source);

Here,

• char*: The return type, which is a pointer to a character.
• strcpy: The function name, used to copy a string from one location to another.
• char destination*: The pointer to the destination buffer where the source string will be copied.
• const char source*: The pointer to the source string that will be copied to the destination buffer.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8c3RyaW5nLmg+CgppbnQgbWFpbigpIHsKCmNoYXIgc291cmNlW10gPSAiSGVsbG8sIFdvcmxkISI7CgpjaGFyIGRlc3RpbmF0aW9uWzIwXTsgLy8gRW5zdXJlIGVub3VnaCBzcGFjZSB0byBob2xkIHRoZSBjb3BpZWQgc3RyaW5nLgoKc3RyY3B5KGRlc3RpbmF0aW9uLCBzb3VyY2UpOwoKcHJpbnRmKCJTb3VyY2U6ICVzXG4iLCBzb3VyY2UpOwoKcHJpbnRmKCJEZXN0aW5hdGlvbjogJXNcbiIsIGRlc3RpbmF0aW9uKTsKCnJldHVybiAwOwoKfQ==

Output:

Source: Hello, World!
Destination: Hello, World!

Explanation:

In the above code example-

1. We include <stdio.h> and <string.h> for printf() and strcpy() library functions in C repsecitvely.
2. We have a source string "Hello, World!" and a destination character array destination.
3. The strcpy() function is used to copy the content of the source string to the destination array.
4. After the copy, both source and destination strings are printed using the printf() and format specifier %s, demonstrating that the content has been successfully copied from source to destination.

The <float.h> Header File & Its Library Functions In C

The <float.h> header file defines macros for the properties of floating-point types (float, double, and long double). It provides information about the limits and precision of these types, such as their minimum and maximum values, precision, and the smallest positive number.

Rules and Regulations:

• Include this header when you need to obtain information about the properties and limits of floating-point types.
• The constants defined in <float.h> are useful for writing portable and precise floating-point code.
• Use these constants to ensure that floating-point operations do not exceed the range or precision limits of the data types.

Commonly Used Library Functions In C:

The FLT_MAX Constant In C Example

This constant is used to represent the maximum finite positive value that can be represented using the float data type.

Syntax: No specific syntax is required, as it is a constant.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8ZmxvYXQuaD4KCmludCBtYWluKCkgewoKcHJpbnRmKCJNYXhpbXVtIGZpbml0ZSBwb3NpdGl2ZSB2YWx1ZSBmb3IgZmxvYXQ6ICVnXG4iLCBGTFRfTUFYKTsKCnJldHVybiAwOwoKfQo=

Output:

Maximum finite positive value for float: 3.40282e+38

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <float.h> for the FLT_MAX constant.
2. We then print the maximum finite positive value that a float can hold using printf() and FLT_MAX constant.

The <time.h> Header File & Its Library Functions In C

The <time.h> header file provides functions and types for manipulating and measuring time. It includes functions for getting the current time, measuring intervals, and formatting time values. This header is useful for time-related operations, such as timestamping events or implementing delays.

Rules and Regulations:

• Include this header when you need to work with time and date functions in C programs.
• Functions and types in <time.h> operate on time values and often use the time_t type, which is typically an alias for an integer type that represents the number of seconds since the Epoch (January 1, 1970).
• Functions for time formatting and manipulation require careful handling to ensure correct date and time representation.

Commonly Used Library Functions In C:

The time() Library Function In C Example

This function is used to retrieve the current time as a time_t value representing the number of seconds elapsed since the epoch.

Syntax:

time_t time(time_t* timer);

Here,

• time_t: The return type, which is a data type representing time (usually as the number of seconds since the Unix epoch, January 1, 1970).
• time: The function name, which returns the current calendar time.
• time_t timer*: An optional pointer to a time_t object where the function can store the current time. If this pointer is NULL, the function does not store the time.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8dGltZS5oPgoKaW50IG1haW4oKSB7Cgp0aW1lX3QgY3VycmVudF90aW1lOwoKdGltZSgmY3VycmVudF90aW1lKTsKCnByaW50ZigiQ3VycmVudCB0aW1lOiAlbGQgc2Vjb25kcyBzaW5jZSB0aGUgZXBvY2hcbiIsIGN1cnJlbnRfdGltZSk7CgpyZXR1cm4gMDsKCn0K

Output:

Current time: 1632284235 seconds since the epoch

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <time.h> for handling time() library functions in C.
2. We then use the time() function to get the current time in seconds since the epoch (January 1, 1970) and print the obtained value using the printf() function.

The <assert.h> Header File & Its Library Functions In C

The <assert.h> header file defines the assert macro, which is used to perform runtime checks of conditions in C programs. If the condition evaluated by assert is false, the program prints an error message and terminates. This is useful for debugging and validating assumptions made by the code.

Rules and Regulations:

• Include this header when you want to use assertions to enforce assumptions and invariants in your program.
• Assertions are primarily used for debugging and should not replace proper error handling in production code.
• If the macro NDEBUG is defined before including <assert.h>, the assert macro is disabled, and no code is generated for it.
• Use assertions to catch programming errors, not to handle run-time errors that can be anticipated and managed.

Commonly Used Library Functions In C:

The assert() Library Function In C Example

This macro is used to check if a condition is true during program execution. If the condition is false, it triggers a runtime error.

Syntax:

assert(expression);

Here,

• assert: The macro name used for debugging purposes.
• expression: The condition to be tested, which should be an expression that evaluates to true or false.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8YXNzZXJ0Lmg+CgppbnQgbWFpbigpIHsKCmludCB4ID0gMTA7CgppbnQgeSA9IDU7CgovLyBFbnN1cmUgeSBpcyBub3QgemVybyBiZWZvcmUgZGl2aWRpbmcuCgphc3NlcnQoeSAhPSAwKTsKCmludCByZXN1bHQgPSB4IC8geTsKCnByaW50ZigiUmVzdWx0OiAlZFxuIiwgcmVzdWx0KTsKCnJldHVybiAwOwoKfQ==

Output:

Result: 2

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <assert.h> for the assert() library function in C.
2. Next, we check that y is not zero using assert() before performing the division to prevent a divide-by-zero error.
3. We then compute the result of x divided by y and print it using the printf() function.

The <complex.h> Header File & Its Library Functions In C

The <complex.h> header file provides functions and macros for complex number arithmetic. It defines a complex number type and a set of functions for performing standard mathematical operations on complex numbers, such as addition, subtraction, multiplication, division, and various complex-specific operations like obtaining the magnitude or phase.

Rules and Regulations:

• Include this header when working with complex numbers in C programs.
• The complex numbers are represented using the complex keyword, and there are several macros and functions available for operations on these numbers.
• Complex numbers are stored as pairs of double values representing the real and imaginary parts.
• Functions in <complex.h> typically return results of type double complex.

Commonly Used Library Functions In C:

The cabs() Library Function In C Example

This function is used to calculate the magnitude (absolute value) of a complex number.

Syntax:

double cabs(double complex z);

Here,

• double: The return type, indicating that the function returns a double-precision floating-point number.
• cabs: The function name, which computes the absolute value (or magnitude) of a complex number.
• double complex z: The parameter, where z is a complex number of type double complex.

Code:

I2luY2x1ZGUgPGNvbXBsZXguaD4KCiNpbmNsdWRlIDxzdGRpby5oPgoKaW50IG1haW4oKSB7Cgpkb3VibGUgY29tcGxleCB6ID0gMy4wICsgNC4wICogSTsKCmRvdWJsZSBhYnNfdmFsID0gY2Ficyh6KTsKCnByaW50ZigiQWJzb2x1dGUgdmFsdWUgb2YgeiA9ICVsZlxuIiwgYWJzX3ZhbCk7CgpyZXR1cm4gMDsKCn0=

Output:

Absolute value of z = 5.000000

Explanation:

In the above code example-

1. We start by including <complex.h> for complex number function like cabs() and <stdio.h> for printf() library function in C.
2. Next we create a complex number z with real part as 3.0 and imaginary part as 4.0.
3. We then compute the absolute value of z using cabs() function and print the result to the console.

The <setjmp.h> Header File & Its Library Functions In C

The <setjmp.h> header file is used to work with non-local jumps within a program. It provides functions and macros that enable you to perform longjmp and setjmp operations, and non-local control transfers.

Rules and Regulations:

• Include this header when using library functions in C related to non-local jumps such as setjmp and longjmp.
• The setjmp should be called before longjmp to save the program's state.
• Ensure that longjmp is used cautiously to avoid undefined behavior, such as jumping over variable declarations or into code where stack frames may be altered.

Commonly Used Library Functions In C:

The setjmp() Library Function In C Example

This library function saves the current environment for later use by longjmp and returns zero initially.

Syntax:

int setjmp(jmp_buf env);

Here,

• int: The return type, indicating that the function returns an integer.
• setjmp: The function name, used to save the current execution context.
• jmp_buf env: A buffer where the function saves the current context, allowing longjmp to later restore this context.

Code:

I2luY2x1ZGUgPHNldGptcC5oPgojaW5jbHVkZSA8c3RkaW8uaD4KCmptcF9idWYgZW52OwoKdm9pZCBmdW5jdGlvbigpIHsKcHJpbnRmKCJJbiBmdW5jdGlvblxuIik7CmxvbmdqbXAoZW52LCAxKTsgLy8gSnVtcCBiYWNrIHRvIHRoZSBwb2ludCB3aGVyZSBzZXRqbXAgd2FzIGNhbGxlZAp9CgppbnQgbWFpbigpIHsKaWYgKHNldGptcChlbnYpICE9IDApIHsKcHJpbnRmKCJSZXR1cm5lZCBmcm9tIGxvbmdqbXBcbiIpOwp9IGVsc2UgewpwcmludGYoIkNhbGxpbmcgZnVuY3Rpb25cbiIpOwpmdW5jdGlvbigpOwp9CnJldHVybiAwOwp9

Output:

Calling function
In function
Returned from longjmp

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <setjmp.h> for setjmp() and longjmp() library function in C.
2. Next, we use setjmp() function to save the current execution state in env.
3. When longjmp() is called in function, it jumps back to the setjmp point, returning a non-zero value and causing the program to print "Returned from longjmp."
4. If longjmp is not executed, the program prints "Calling function" and then calls function.

The <locale.h> Header File & Its Library Functions In C

The <locale.h> header file is used for handling localization and internationalization in programs. It provides functions and macros for dealing with various aspects of local settings, such as character encoding, date and time formats, and message translations.

Rules and Regulations:

• Include this header when using locale-related library functions such as setlocale, localeconv, and lc_collate.
• Locale settings affect various aspects of program behavior, including string comparison and numeric formatting.
• Ensure to use setlocale to set the desired locale before performing locale-specific operations.

Commonly Used Library Functions In C:

The setlocale() Library Function In C Example

This library function is used to set or query the program's locale. It allows you to configure how various locale categories behave, such as character encoding or date and time formatting.

Syntax:

char *setlocale(int category, const char *locale);

Here,

• char *: The return type, which is a pointer to a character string.
• setlocale: The function name, used to set or query the program's locale.
• int category: The category of locale settings to be changed or queried, such as LC_ALL, LC_TIME, etc.
• const char *locale: A string representing the locale name. If set to NULL(null pointer), the current locale setting for the specified category is returned.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8bG9jYWxlLmg+CgppbnQgbWFpbigpIHsKCi8vIFNldCB0aGUgbG9jYWxlIGZvciBkYXRlIGFuZCB0aW1lIGZvcm1hdHRpbmcgdG8gVS5TLiBFbmdsaXNoCgpzZXRsb2NhbGUoTENfVElNRSwgImVuX1VTLlVURi04Iik7CgovLyBRdWVyeSBhbmQgcHJpbnQgdGhlIGN1cnJlbnQgbG9jYWxlIHNldHRpbmcgZm9yIExDX1RJTUUKCmNoYXIgKmN1cnJlbnRMb2NhbGUgPSBzZXRsb2NhbGUoTENfVElNRSwgTlVMTCk7CgpwcmludGYoIkN1cnJlbnQgbG9jYWxlIGZvciBMQ19USU1FOiAlc1xuIiwgY3VycmVudExvY2FsZSk7CgpyZXR1cm4gMDsKCn0=

Output:

Current locale for LC_TIME: C

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <locale.h> for setlocale() library functions in C.
2. Next, we set the locale for date and time formatting to U.S. English using setlocale() function.
3. We then query and print the current locale setting for LC_TIME using printf() to confirm the change.

The <stdarg.h> Header File & Its Library Functions In C

The <stdarg.h> header file is used for working with functions that accept a variable number of arguments. It provides a mechanism for functions to receive a variable number of arguments, making it possible to write more flexible and versatile functions.

Rules and Regulations:

• Include this header when you need to create functions that accept a variable number of arguments.
• Use the va_start, va_arg, and va_end macros to handle variable arguments safely and correctly.
• Ensure that the va_end macro is called before the function returns to clean up the argument list.

Commonly Used Library Functions In C:

The va_arg() Library Function In C Example

This macro retrieves the next argument from the va_list. It requires two arguments: the va_list and the type of the argument to retrieve.

Syntax:

type va_arg(va_list ap, type);

Here,

• type: It specifies the type of the next argument to be retrieved from the variable argument list.
• va_arg: The macro used to access the next argument in a variable argument list.
• va_list ap: A variable of type va_list that holds the information needed to retrieve additional arguments. This is typically initialized using va_start.

Code:

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

Output:

Sum = 100

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <stdarg.h> for handling variable arguments like va_arg, va_list etc.
2. Next, we define a sum() function that calculates the sum of a variable number of integers.
3. Using va_start, we initialize va_list to process the arguments, then iterate through them with va_arg to add each integer to total.
4. After processing, we use va_end to clean up. In main() function, we call sum() with four integers, print the result using printf(), and return 0 to indicate successful execution.

The <errno.h> Header File & Its Library Functions In C

The <errno.h> header file is used for error handling and provides a standard mechanism to access and manage error codes in C programs. It defines the errno variable, which stores error codes for various library and system functions.

Rules and Regulations:

• Include this header when handling errors from system or library functions that set the errno variable.
• The errno variable is an external integer variable that is set to a specific error code when a function fails.
• Always check errno after a function fails and ensure it is not used before setting it to a valid value.
• After handling an error, you can clear errno by setting it to 0.

Commonly Used Library Functions In C:

The perror() Library Function In C Example

This library function is used to print an error message corresponding to the value of errno. It helps in diagnosing errors by providing a human-readable description of the error.

Syntax:

void perror(const char *s);

Here,

• void: The return type, indicating that the function does not return a value.
• perror(): The function name, which prints a description of the last error that occurred.
• const char *s: A string that is printed before the error message. It is a constant pointer to a character string.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8ZXJybm8uaD4KCiNpbmNsdWRlIDxzdHJpbmcuaD4KCmludCBtYWluKCkgewoKRklMRSAqZmlsZSA9IGZvcGVuKCJub25leGlzdGVudC50eHQiLCAiciIpOwoKaWYgKGZpbGUgPT0gTlVMTCkgewoKcGVycm9yKCJFcnJvciBvcGVuaW5nIGZpbGUiKTsKCnByaW50ZigiRXJyb3IgY29kZTogJWRcbiIsIGVycm5vKTsKCn0KCnJldHVybiAwOwoKfQ==

Output:

Error opening file: No such file or directory
Error code: 2

Explanation:

In the above code example-

1. We start by including <stdio.h> for file operations, <errno.h> for perror() library function, and <string.h> for error message handling.
2. We then attempt to open a file named "nonexistent.txt" in read mode.
3. The fopen() function returns a file pointer (file), or NULL if the file can't be opened.
4. If the file cannot be opened (file == NULL), we use perror() function to print an error message and display the error code stored in errno.

The <graphics.h> Library Header File & Its Library Functions In C

The <graphics.h> header file provides functions for graphical operations, particularly for creating graphics-based applications in a DOS environment. It includes library functions for drawing shapes, setting colors, and managing graphics modes.

Rules and Regulations:

• Include this header when using graphics-related library functions in C programs.
• The <graphics.h> library is typically available in Turbo C/C++ compilers and is not part of the standard C library.
• It is primarily used in older, DOS-based systems and is not supported in modern compilers.
• To use <graphics.h>, the initgraph function must be called to initialize the graphics mode.

Commonly Used Library Functions In C:

The line() Library Function In C Example

This library function is used to draw a straight line between two specified points on the graphics screen.

Syntax:

line(int x1, int y1, int x2, int y2);

Here,

• line(): The function name, which is used to draw a line on a graphical screen.
• int x1, int y1: The coordinates of the starting point of the line.
• int x2, int y2: The coordinates of the ending point of the line.

Code:

I2luY2x1ZGUgPGdyYXBoaWNzLmg+CgppbnQgbWFpbigpIHsKCmludCBnZCA9IERFVEVDVCwgZ207Cgppbml0Z3JhcGgoJmdkLCAmZ20sIE5VTEwpOwoKLy8gRHJhdyBhIGxpbmUgZnJvbSAoMTAwLCAxMDApIHRvICgzMDAsIDIwMCkKCmxpbmUoMTAwLCAxMDAsIDMwMCwgMjAwKTsKCmdldGNoKCk7IC8vIFdhaXQgZm9yIGEga2V5IHByZXNzCgpjbG9zZWdyYXBoKCk7IC8vIENsb3NlIHRoZSBncmFwaGljcyBzeXN0ZW0KCnJldHVybiAwOwoKfQo=

Output:

A graphics window will open, displaying a line from (100, 100) to (300, 200).

Explanation:

In the above code example-

1. We start by including <graphics.h> for graphics functions like line().
2. Next, we initialize the graphics system using initgraph() with gd and gm to detect and set the graphics mode.
3. We then draw a line from coordinates (100, 100) to (300, 200) using the line() function.
4. We next wait for a key press from the user with getch() function and close the graphics system with closegraph() function.

The <wchar.h> Library Header File & Its Library Functions In C

The <wchar.h> header file provides functions and types for handling wide characters and wide character strings. Wide characters (wchar_t) are used to represent extended character sets that cannot be accommodated by regular character types, such as international character sets.

Rules and Regulations:

• Include this header when working with wide characters or wide character strings in C programs.
• Functions in <wchar.h> are analogous to those in <string.h>, but they operate on wide character types.
• Wide characters are typically used for internationalization, allowing for the representation of a broader range of characters.
• Properly manage memory when working with wide character strings, similar to regular strings.

Commonly Used Library Functions In C:

The wprintf() Library Function In C Example

This library function prints wide character data to the console or a stream. It is similar to printf(), but it deals with wide character strings and formats.

Syntax:

int wprintf(const wchar_t *format, ...);

Here,

• int: The return type, indicating that the function returns an integer.
• wprintf(): The function name, which is used for formatted wide-character output.
• const wchar_t *format: The format string, specifying how to format the output. It is a constant pointer to a wide character string.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8d2NoYXIuaD4KCmludCBtYWluKCkgewoKd2NoYXJfdCBuYW1lW10gPSBMIkphaSI7CgppbnQgYWdlID0gMzA7Cgp3cHJpbnRmKEwiTXkgbmFtZSBpcyAlbHMsIGFuZCBJIGFtICVkIHllYXJzIG9sZC5cbiIsIG5hbWUsIGFnZSk7CgpyZXR1cm4gMDsKCn0K

Output:

My name is Jai, and I am 30 years old.

Explanation:

In the above code example-

1. We start by including <stdio.h> for standard I/O functions and <wchar.h> for wide character support like wprint() library function in C.
2. Next , we define a wide character array name with the value "Jai" and an integer age with the value 30.
3. We then use wprintf() function to print the name and age in a wide-character string format.

The <ctype.h > Header File & Its Library Functions In C

The <ctype.h> header file provides functions for testing and mapping characters. It includes functions to classify and transform individual characters, such as checking if a character is a letter, digit, or whitespace, and converting characters to uppercase or lowercase.

Rules and Regulations:

• Include this header when working with character classification or conversion in C programs.
• The functions typically take and return int values, which represent unsigned char values or EOF.
• Properly handle input values to avoid undefined behavior, especially with signed characters that may have negative values.

Commonly Used Library Functions In C:

The isalpha() Library Function In C Example

This library function is used to check if a character is an alphabetic character (either uppercase or lowercase).

Syntax:

int isalpha(int c);

Here,

• int: It is the return type, indicating that the function returns an integer.
• isalpha(): It represents the function name, which checks if the given character is an alphabetic letter.
• int c: The parameter, where c is the character (as an integer value) to be checked.

Code:

I2luY2x1ZGUgPHN0ZGlvLmg+CgojaW5jbHVkZSA8Y3R5cGUuaD4KCmludCBtYWluKCkgewoKY2hhciBjaGFyYWN0ZXIgPSAnQSc7CgppZiAoaXNhbHBoYShjaGFyYWN0ZXIpKSB7CgpwcmludGYoIiVjIGlzIGFuIGFscGhhYmV0aWMgY2hhcmFjdGVyLlxuIiwgY2hhcmFjdGVyKTsKCn0gZWxzZSB7CgpwcmludGYoIiVjIGlzIG5vdCBhbiBhbHBoYWJldGljIGNoYXJhY3Rlci5cbiIsIGNoYXJhY3Rlcik7Cgp9CgpyZXR1cm4gMDsKCn0=

Output:

A is an alphabetic character.

Explanation:

In the above code example-

1. We start by including <stdio.h> for printf() and <ctype.h> for isalpha() library functions in C.
2. Next, we define a character with the value 'A'.
3. We then use isalpha() function to check if character is an alphabetic character. If it is, we print that it is an alphabetic character using printf(); otherwise, we print that it is not.

Advantages Of Using Library Functions In C

The C standard library provides a set of built-in functions and utilities that can significantly simplify and enhance the development of C programs. Here are some advantages of using these library functions in C:

• Portability: Library functions in C are part of the C standard, which means they are available on virtually all C compilers and platforms. This ensures that code using these functions can be easily ported and run on different systems without modification.
• Code Reusability: Library functions save time and effort by providing pre-written, well-tested code for common tasks. Developers can reuse these functions instead of writing the same code from scratch.

#include <math.h>
int main() {
double x = 4.0;
double result = sqrt(x);
return 0;
}

• Efficiency: Library functions in C are typically highly optimized and written in C or assembly language, making them efficient and fast. Using library functions often leads to more efficient code than custom implementations.

#include <stdlib.h>
int main() {
int* arr = (int*)malloc(10 * sizeof(int));
// Memory allocation done efficiently by malloc.
free(arr); // Efficient memory deallocation.
return 0;
}

• Standardization: C library functions adhere to the C standard, which ensures consistency and compatibility across different systems and compilers. This standardization reduces the likelihood of subtle bugs caused by platform-specific behavior.
• Safety and Reliability: Many C library functions perform boundary checks and error handling, making them safer to use compared to custom implementations. This reduces the risk of memory leaks, buffer overflows, and other common programming errors.

#include <stdio.h>
int main() {
FILE* file = fopen("example.txt", "r");
if (file != NULL) {
// Read from the file safely with error handling.
fclose(file); // Properly close the file.
}
return 0;
}

• Consistency: Using library functions in C programs ensures that code follows established conventions and coding styles. This makes the codebase more consistent and easier to maintain, especially in collaborative projects.
• Documentation: C library functions come with comprehensive documentation, making it easier for developers to understand how to use them. Standardized documentation helps in quickly learning and using these functions effectively.

#include <stdlib.h>
int main() {
// Refer to official documentation for details on functions like malloc and free.
return 0;
}

List Of Library Functions In C For All Header Files

In this section, we have created comprehensive lists of library functions in C, grouped as per the respective header files. The prototype and short description provided along will help you quickly grasp the meaning and purpose of these functions.

The <stdio.h> Library Functions In C

The <math.h> Library Functions In C

The <stdlib.h> Library Functions In C

The <time.h> Library Functions In C

The <string.h> Library Functions In C

The <wchar.h> Library Functions In C

The <complex.h> Library Functions In C

The <setjmp.h> Library Functions In C

The <locale.h> Library Function In C

The <errno.h> Library Functions In C

The <ctype.h> Library Functions In C

Conclusion

In this article, we’ve explored the core library functions in C through a detailed examination of key header files such as <stdio.h>, <stdlib.h>, <math.h>, <string.h>, and others. Each header file serves a distinct purpose, providing essential functions that simplify programming tasks ranging from basic input and output to complex mathematical operations and memory management.

Understanding these library functions and their proper usage is crucial for writing efficient and effective C programs. The collection of functions provided by these respective header files not only enhances productivity by offering pre-written, reliable routines but also contributes to writing portable and maintainable code. From handling strings and performing mathematical calculations to managing files and dynamic memory, these library functions in C form the backbone of this programming language.

Frequently Asked Questions

Q. What are the different categories of library functions in C language?

Functions in the C language library can be categorized into several types based on their functionality and purpose.

1. Input and Output Functions: These library functions are used for reading input from the user and writing output to the screen or files. Examples include printf(), scanf(), and file-related functions like fopen() and fclose.
2. String Handling Functions: String functions are used for tasks such as copying, concatenating, comparing, and searching within strings. Functions like strlen(), strcpy(), and strtok() fall into this category.
3. Memory Management Functions: These C library functions handle dynamic memory allocation and deallocation, such as malloc(), free(), and memory manipulation functions like memcpy().
4. Mathematical Functions: Mathematical operations and calculations are performed using functions like sqrt, sin, pow, and others in this category.
5. Date and Time Functions: Date and time-related operations are facilitated by functions like time, mktime(), and strftime().
6. Character Handling Functions: These C library functions handle character-based operations, including checks for alphanumeric characters, case conversions, and more. Examples include isalpha(), tolower(), and isdigit().
7. File Handling Functions: File input and output operations are supported by C library functions like fread(), fwrite(), fseek(), and file manipulation functions like remove and rename.
8. Error Handling Functions: Functions like perror() and errno() are used for handling errors and generating error messages.
9. Locale and Multibyte Functions: These functions deal with internationalization and multibyte character encodings, including setlocale() and conversion functions like mbstowcs().
10. Miscellaneous Functions: This category encompasses a wide range of utility functions, including abort, exit, and random number generation functions.

Q. How to create a library function in C?

Creating a library function in C involves writing a function or set of functions in one or more source files and then compiling them into a library file that can be linked with other C programs. Here are the steps to create a library function in C:

1. Write Your C Code: First, write the C code for the functions you want to include in your library. You can create a .c source file (e.g., mylib.c) and define your functions in it.
2. Create a Header File: Create a header file (e.g., mylib.h) that declares the function prototypes you want to include in your library. This output header file should be included in both your library source file and any other C files that will use the library.
3. Compile the Library: Compile the source file(s) into a library file using a C compiler. You can create a static library (.a file) or a dynamic/shared library (.so or .dll file) depending on your needs.
4. Use the Library: To use your library functions in another C program, include the library header file and link your program with the library file during the compilation process.
5. Run the Program: Now you can run your program, which uses the functions from your library.

Q. What is the syntax of function in C?

The basic syntax of a function in C is as follows:

return_type function_name(parameter_list) {
// Function body (statements and logic)
// ...
return return_value; // Optional return statement
}

Here:

• return_type: The data type of the value that the function returns. If the function does not return a value, this is void.
• function_name: The name of the function, used to call it in the code.
• parameter_list: A list of parameters (if any) enclosed in parentheses. Each parameter is specified by its data type and name, separated by commas. Parameters provide input to the function.
• *{ / Function body */ return return_value; }**: The function body enclosed in curly braces. It contains statements and logic to perform the function's task. The return statement (if present) provides the value to be returned to the caller.

Q. What is the purpose of including header files in C?

Header files, such as <stdio.h>, <stdlib.h>, and others, contain definitions of functions, macros, and types that are essential for various programming tasks. Including these headers allows a program to use the C library functions and constants defined in them, ensuring that the compiler knows about the functions' signatures and how to link them correctly during compilation.

Q. What are the common pitfalls to avoid when using library functions in C?

The common pitfalls that we need to avoid when using library functions in C are as follows:

• Buffer Overflows: Ensure that you do not write more data than the buffer can hold.
• Memory Leaks: Always free dynamically allocated memory using the free() function.
• Invalid Input: Validate inputs to functions like scanf() to prevent undefined behavior.
• Error Handling: Check for errors after performing file operations or memory allocations using malloc(), calloc(), etc.

You might also be interested in reading the following:

1. Compilation In C | Detail Explanation Using Diagrams & Examples
2. The Sizeof() Operator In C | A Detailed Explanation (+Examples)
3. Pointers In C | Ultimate Guide With Easy Explanations (+Examples)
4. Comma Operator In C | Code Examples For Both Separator & Operator
5. Function Pointers In C | A Comprehensive Guide With Code Examples