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: