A Complete Guide - C Programming Using the C Standard Library stdlib h, stdio h, math h, string h, time h

C Standard Library Overview

The C Standard Library provides a collection of functions and macros that perform various tasks such as input/output operations, memory management, program control, and mathematical calculations. Here, we'll explore the functionalities provided by five key header files: stdlib.h, stdio.h, math.h, string.h, and time.h.

1. stdlib.h

The stdlib.h header file is essential for essential standard functions, memory management, and type conversions.

• Memory Management

  • void* malloc(size_t size);: Allocates memory.
  • void* calloc(size_t num, size_t size);: Allocates and initializes memory.
  • void* realloc(void* ptr, size_t new_size);: Resizes allocated memory.
  • void free(void* ptr);: Frees allocated memory.

  Example:

  #include <stdlib.h>
  #include <stdio.h> int main() { int* arr = (int*)malloc(5 * sizeof(int)); if (!arr) { perror("Failed to allocate memory"); return EXIT_FAILURE; } for (int i = 0; i < 5; i++) { arr[i] = i + 1; } for (int i = 0; i < 5; i++) { printf("%d ", arr[i]); } free(arr); return EXIT_SUCCESS;
  }
  

• Process Control

  • int system(const char* command);: Executes a shell command.
  • void exit(int status);: Causes normal program termination.

  Example:

  #include <stdlib.h> int main() { system("ls -l"); exit(EXIT_SUCCESS);
  }
  

• Data Conversion

  • int atoi(const char* str);: Converts a string to an integer.
  • double atof(const char* str);: Converts a string to a double.
  • long atol(const char* str);: Converts a string to a long integer.

  Example:

  #include <stdlib.h>
  #include <stdio.h> int main() { const char* numstr = "12345"; int num = atoi(numstr); printf("The converted number is %d\n", num); return EXIT_SUCCESS;
  }
  

2. stdio.h

The stdio.h header file is critical for input and output operations.

• Input Functions

  • int scanf(const char* format, ...);: Reads formatted input.
  • char* gets(char* s);: Reads a line from stdin (unsafe; use fgets).

  Example:

  #include <stdio.h> int main() { int num; printf("Enter an integer: "); scanf("%d", &num); printf("You entered: %d\n", num); return 0;
  }
  

• Output Functions

  • int printf(const char* format, ...);: Prints formatted output.
  • int puts(const char* s);: Writes a string to stdout.

  Example:

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

• File Operations

  • FILE* fopen(const char* filename, const char* mode);: Opens a file.
  • int fclose(FILE* stream);: Closes a file.
  • size_t fread(void* ptr, size_t size, size_t nmemb, FILE* stream);: Reads data from a file.
  • size_t fwrite(const void* ptr, size_t size, size_t nmemb, FILE* stream);: Writes data to a file.

  Example:

  #include <stdio.h> int main() { FILE* file = fopen("example.txt", "w"); if (!file) { perror("Failed to open file"); return EXIT_FAILURE; } fprintf(file, "Hello, file!\n"); fclose(file); return EXIT_SUCCESS;
  }
  

3. math.h

The math.h header file provides mathematical functions and constants.

• Basic Functions

  • double sqrt(double x);: Computes the square root.
  • double pow(double base, double exponent);: Computes the power.
  • double sin(double x);: Computes the sine.
  • double cos(double x);: Computes the cosine.
  • double tan(double x);: Computes the tangent.

  Example:

  #include <math.h>
  #include <stdio.h> int main() { double num = 9.0; printf("Square root of %.2f is %.2f\n", num, sqrt(num)); double base = 2.0, exponent = 3.0; printf("%.2f raised to the power %.2f is %.2f\n", base, exponent, pow(base, exponent)); double angle = 30.0; double radian = angle * M_PI / 180.0; // Convert degrees to radians printf("Sin of %.2f degrees is %.2f\n", angle, sin(radian)); return 0;
  }
  

4. string.h

The string.h header file contains functions for string manipulation.

• Memory Functions

  • void* memcpy(void* dest, const void* src, size_t n);: Copies memory.
  • void* memmove(void* dest, const void* src, size_t n);: Copies memory, allows overlapping.
  • void* memset(void* s, int c, size_t n);: Sets a memory block.

  Example:

  #include <string.h>
  #include <stdio.h> int main() { char src[] = "Hello, World!"; char dest[20]; memcpy(dest, src, strlen(src) + 1); // Copy string with null terminator printf("Copied string: %s\n", dest); char buffer[15] = "Hello"; memset(buffer + 5, '!', 5); // Fill next 5 bytes with '!' printf("Buffer after memset: %s\n", buffer); return 0;
  }
  

• String Functions

  • size_t strlen(const char* str);: Computes string length.
  • char* strcpy(char* dest, const char* src);: Copies a string.
  • char* strcat(char* dest, const char* src);: Concatenates two strings.
  • int strcmp(const char* str1, const char* str2);: Compares two strings.
  • char* strchr(const char* str, int c);: Finds first occurrence of a character.
  • char* strstr(const char* haystack, const char* needle);: Finds the first occurrence of a substring.

  Example:

  #include <string.h>
  #include <stdio.h> int main() { char str1[] = "Hello"; char str2[] = ", World!"; printf("Length of str1: %lu\n", strlen(str1)); strcpy(str1, str2); printf("Copied string: %s\n", str1); strcat(str1, "!"); printf("Concatenated string: %s\n", str1); int cmp = strcmp(str1, ", World!!!"); printf("Comparison result: %d\n", cmp); char* found = strchr(str1, 'W'); if (found) { printf("Found 'W' at position %ld\n", found - str1); } char* substr = strstr(str1, "World"); if (substr) { printf("Found substring 'World' at position %ld\n", substr - str1); } return 0;
  }
  

5. time.h

The time.h header file provides functions for date and time manipulation.

• Time Functions

  • time_t time(time_t* t);: Returns the current time.
  • struct tm* localtime(const time_t* timep);: Converts time to local time.
  • struct tm* gmtime(const time_t* timep);: Converts time to UTC time.
  • char* asctime(const struct tm* tm);: Converts time to a string.

  Example: