Nexus

Libraries

2 min

Contents

<ctype.h> : Character Type Library

The <ctype.h> header is part of the C Standard Library. It provides functions for classifying and converting characters.

Common functions

Function Description
isalpha(c) Checks if a character is a letter.
isdigit(c) Checks if a character is a digit (0–9).
isalnum(c) Checks if a character is a letter or digit.
islower(c) Checks if a character is lowercase.
isupper(c) Checks if a character is uppercase.
isspace(c) Checks if a character is whitespace.
toupper(c) Converts a lowercase letter to uppercase.
tolower(c) Converts an uppercase letter to lowercase.
Important: Functions like toupper() and tolower() return the converted character rather than modifying the original variable directly.

<stdlib.h> : Standard Library

The <stdlib.h> header is part of the C Standard Library. It provides functions for memory management, string-to-number conversion, dynamic memory allocation, random number generation, and program control.

Common functions

Function Description
atoi(s) Converts a string to an int.
atof(s) Converts a string to a double.
strtol(s, end, base) Converts a string to a long.
strtoul(s, end, base) Converts a string to an unsigned long.
malloc(size) Allocates a block of memory dynamically.
calloc(n, size) Allocates memory and initializes it to zero.
realloc(ptr, size) Resizes a previously allocated memory block.
free(ptr) Releases dynamically allocated memory.
rand() Generates a pseudo-random integer.
srand(seed) Sets the seed for random number generation.
abs(n) Returns the absolute value of an integer.
exit(status) Terminates the program with a specified status.
qsort(...) Sorts elements in an array.
bsearch(...) Performs a binary search on a sorted array.

Common constants

Constant Description
EXIT_SUCCESS Indicates successful program termination.
EXIT_FAILURE Indicates unsuccessful program termination.
NULL Represents a null pointer constant.

Example: String-to-number conversion

Code
unsigned long val = strtoul(argv[1], NULL, 10);

  • argv[1] → First user-provided command-line argument (string).

  • NULL → Conversion end position is not needed.

  • 10 → Decimal number system.

Important:

  • malloc(), calloc(), and realloc() allocate memory that must eventually be released using free().

  • strtol() and strtoul() provide better error-checking capabilities than atoi().

  • EXIT_SUCCESS and EXIT_FAILURE are commonly used as return values from main() to indicate whether a program completed successfully.

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