Update c.html.markdown

Fixes #2846, #2849, #2851, #2852 (Thanks to @noncombatant)

c.html.markdown

@@ -16,6 +16,15 @@ C is the lowest-level language most programmers will ever use, but
 it more than makes up for it with raw speed. Just be aware of its manual
 memory management and C will take you as far as you need to go.
 
+> **About compiler flags**
+> 
+> By default, gcc and clang are pretty quiet about compilation warnings and
+> errors, which can be very useful information. Using some
+> stricter compiler flags is recommended. Here is an example you can
+> tweak to your liking:
+>
+> `-Wall -Wextra -Werror -O0 -ansi -pedantic -std=c11`
+
 ```c
 // Single-line comments start with // - only available in C99 and later.
 
@@ -302,7 +311,7 @@ int main (int argc, char** argv)
 
   // branching with multiple choices: switch()
   switch (a) {
-  case 0: // labels need to be integral *constant* expressions
+  case 0: // labels need to be integral *constant* expressions (such as enums)
     printf("Hey, 'a' equals 0!\n");
     break; // if you don't break, control flow falls over labels
   case 1:
@@ -438,17 +447,25 @@ int main (int argc, char** argv)
   for (xx = 0; xx < 20; xx++) {
     *(my_ptr + xx) = 20 - xx; // my_ptr[xx] = 20-xx
   } // Initialize memory to 20, 19, 18, 17... 2, 1 (as ints)
+  
+  // Be careful passing user-provided values to malloc! If you want
+  // to be safe, you can use calloc instead (which, unlike malloc, also zeros out the memory)
+  int* my_other_ptr = calloc(20, sizeof(int));
 
   // Note that there is no standard way to get the length of a
   // dynamically allocated array in C. Because of this, if your arrays are
   // going to be passed around your program a lot, you need another variable
   // to keep track of the number of elements (size) of an array. See the
   // functions section for more info.
-  int size = 10;
+  size_t size = 10;
-  int *my_arr = malloc(sizeof(int) * size);
+  int *my_arr = calloc(size, sizeof(int));
   // Add an element to the array
   size++;
   my_arr = realloc(my_arr, sizeof(int) * size);
+  if (my_arr == NULL) {
+    //Remember to check for realloc failure!
+    return
+  }
   my_arr[10] = 5;
 
   // Dereferencing memory that you haven't allocated gives
@@ -546,7 +563,7 @@ array in C.
 */
 // Size must be passed!
 // Otherwise, this function has no way of knowing how big the array is.
-void printIntArray(int *arr, int size) {
+void printIntArray(int *arr, size_t size) {
     int i;
     for (i = 0; i < size; i++) {
         printf("arr[%d] is: %d\n", i, arr[i]);
@@ -559,7 +576,7 @@ printIntArray(my_arr, size);
 // will print "arr[0] is: 1" etc
 */
 
-// if referring to external variables outside function, must use extern keyword.
+// if referring to external variables outside function, you should use the extern keyword.
 int i = 0;
 void testFunc() {
   extern int i; //i here is now using external variable i
@@ -656,6 +673,7 @@ typedef void (*my_fnp_type)(char *);
 // ...
 // my_fnp_type f;
 
+
 //Special characters:
 /*
 '\a'; // alert (bell) character
@@ -742,10 +760,10 @@ as the C file.
 #define ADD(a, b) (a + b)
 
 /* Structs and typedefs can be used for consistency between files. */
-typedef struct node
+typedef struct Node
 {
     int val;
-    struct node *next;
+    struct Node *next;
 } Node;
 
 /* So can enumerations. */