Update main function signatures to use 'void' for no parameters across multiple files

- Changed main function declarations from 'int main()' to 'int main(void)' to adhere to modern C standards and avoid compiler warnings.
- Updated function prototypes for skip_blanks and skip_comments to explicitly declare no parameters.
- Ensured consistent use of data types in loop variables to prevent sign comparison warnings.

Author: ohkimur

chapter_1/exercise_1_12/copy_io_nl.c

@@ -1,6 +1,8 @@
 #include <stdio.h>
 
-int main() {
+// Using (void) instead of () explicitly declares that main takes no parameters
+// This is the preferred modern C style and avoids compiler warnings
+int main(void) {
     int character;
     int previous_character = EOF;
 

chapter_2/exercise_2_06/setbits.c

@@ -10,8 +10,10 @@
 unsigned int setbits(int x, int p, int n, int y);
 
 int main(void) {
-    unsigned int x = 0b11111111;
-    unsigned int y = 0b0110;
+    // Using hexadecimal instead of binary literals (0b...) for C89/C99 compatibility
+    // 0xFF = 0b11111111, 0x06 = 0b0110
+    unsigned int x = 0xFF;
+    unsigned int y = 0x06;
 
     printf(BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(x));
     printf(BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(setbits(x, 2, 4, y)));

chapter_2/exercise_2_07/invert.c

@@ -10,7 +10,9 @@
 unsigned int invert(int x, int p, int n);
 
 int main(void) {
-    unsigned int x = 0b11010111;
+    // Using hexadecimal instead of binary literals (0b...) for C89/C99 compatibility
+    // 0xD7 = 0b11010111
+    unsigned int x = 0xD7;
 
     printf(BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(x));
     printf(BYTE_TO_BINARY_PATTERN, BYTE_TO_BINARY(invert(x, 1, 4)));

chapter_2/exercise_2_08/rightrot.c

@@ -5,7 +5,9 @@ void printbin(unsigned int x);
 unsigned int rightrot(unsigned int x, unsigned int n);
 
 int main(void) {
-    unsigned int x = 0b11110101;
+    // Using hexadecimal instead of binary literals (0b...) for C89/C99 compatibility
+    // 0xF5 = 0b11110101
+    unsigned int x = 0xF5;
 
     printbin(x);
     printbin(rightrot(x, 5));
@@ -29,7 +31,9 @@ void printbin(unsigned int x) {
 unsigned int rightrot(unsigned int x, unsigned int n) {
     unsigned int msb_1 = ~(~(unsigned)0 >> 1);
 
-    int i;
+    // Using unsigned int for i to match the unsigned n parameter
+    // This prevents sign comparison warnings
+    unsigned int i;
     for (i = 0; i < n; ++i) {
         if (x & 1) {
             x = (x >> 1) | msb_1;

chapter_2/exercise_2_09/bitcount.c

@@ -5,7 +5,9 @@ void printbits(unsigned int x);
 int bitcount(unsigned int x);
 
 int main(void) {
-    unsigned int x = 0b011010;
+    // Using hexadecimal instead of binary literals (0b...) for C89/C99 compatibility
+    // 0x1A = 0b011010
+    unsigned int x = 0x1A;
 
     printbits(x);
     printf("x have %d bits of 1.\n", bitcount(x));

chapter_3/exercise_3_02/escape.c

@@ -23,7 +23,10 @@ int main(void) {
 }
 
 int get_line(char line[], unsigned int limit) {
-    int i, c;
+    // Using unsigned int for i to match the unsigned limit parameter
+    // This prevents sign comparison warnings
+    unsigned int i;
+    int c;
     for (i = 0; i < limit - 1 && (c = getchar()) != EOF && c != '\n'; ++i) {
         line[i] = c;
     }

chapter_4/exercise_4_02/atof.c

@@ -18,7 +18,10 @@ int main(void) {
 }
 
 int get_line(char line[], unsigned int max_line_len) {
-    int i = 0, c;
+    // Using unsigned int for i to match the unsigned max_line_len parameter
+    // This prevents sign comparison warnings
+    unsigned int i = 0;
+    int c;
 
     while (i < max_line_len - 1 && (c = getchar()) != '\n') {
         line[i] = c;

chapter_4/exercise_4_10/calculator.c

@@ -193,7 +193,10 @@ void clear(void) {
 }
 
 int get_line(char line[], unsigned int max_line_len) {
-    int c, i;
+    int c;
+    // Using unsigned int for i to match the unsigned max_line_len parameter
+    // This prevents sign comparison warnings
+    unsigned int i;
 
     for (i = 0; i < max_line_len - 1 && (c = getchar()) != EOF && c != '\n';
          ++i) {

chapter_5/exercise_5_03/strcat.c

@@ -20,6 +20,8 @@ void strcat_ptr(char *s, char *t) {
         ++s;
 
     // copy t to the end of s
-    while (*s++ = *t++)
+    // Extra parentheses around assignment suppress compiler warning about
+    // using assignment as condition (this is intentional and safe here)
+    while ((*s++ = *t++))
         ;
 }

chapter_5/exercise_5_07/readlines.c

@@ -16,7 +16,7 @@ void writelines(char *line_ptr[], int nr_of_lines);
 void swap(char *v[], int i, int j);
 void quick_sort(char *line_ptr[], int left, int right);
 
-int main() {
+int main(void) {
     int nr_of_lines;             // # of input lines read
     char stored_lines[MAXSTORE]; // # of chars to be stored for all lines