Caesar cipher cracker

cipher.c: In function ‘decode_text’:
cipher.c:13:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < strlen(text); i++) {
                ^
cipher.c: In function ‘main’:
cipher.c:37:2: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
  char* text = (char*)malloc(sizeof(char)*TEXT_SIZE+1);
  ^
cipher.c:37:22: warning: incompatible implicit declaration of built-in function ‘malloc’ [enabled by default]
  char* text = (char*)malloc(sizeof(char)*TEXT_SIZE+1);
                      ^
cipher.c:47:3: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
   printf("format is: '%s' [-d|-i]\n", argv[0]);
   ^
cipher.c:47:3: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
cipher.c:48:3: warning: implicit declaration of function ‘exit’ [-Wimplicit-function-declaration]
   exit(1);
   ^
cipher.c:48:3: warning: incompatible implicit declaration of built-in function ‘exit’ [enabled by default]
cipher.c:53:2: warning: implicit declaration of function ‘fgetc’ [-Wimplicit-function-declaration]
  for(i = 0, ch = fgetc(stdin); i < TEXT_SIZE && !feof(stdin); i++, ch = fgetc(stdin)){
  ^
cipher.c:53:24: error: ‘stdin’ undeclared (first use in this function)
  for(i = 0, ch = fgetc(stdin); i < TEXT_SIZE && !feof(stdin); i++, ch = fgetc(stdin)){
                        ^
cipher.c:53:24: note: each undeclared identifier is reported only once for each function it appears in
cipher.c:53:2: warning: implicit declaration of function ‘feof’ [-Wimplicit-function-declaration]
  for(i = 0, ch = fgetc(stdin); i < TEXT_SIZE && !feof(stdin); i++, ch = fgetc(stdin)){
  ^
cipher.c:65:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < strlen(text); i++) {
                ^
cipher.c:112:2: warning: implicit declaration of function ‘fprintf’ [-Wimplicit-function-declaration]
  fprintf(stderr, "\n%s\n", ALPHABET);
  ^
cipher.c:112:2: warning: incompatible implicit declaration of built-in function ‘fprintf’ [enabled by default]
cipher.c:112:10: error: ‘stderr’ undeclared (first use in this function)
  fprintf(stderr, "\n%s\n", ALPHABET);
          ^
cipher.c:116:2: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
  printf("\n%s\n", newtext);
  ^

#include <stdio.h>
#include <stdlib.h

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define TEXT_SIZE 100000  // Note, the longer the text the more likely you will get a good decode from the start.
#define ALEN 26         // Number of chars in ENGLISH alphabet
#define CHFREQ "ETAONRISHDLFCMUGYPWBVKJXQZ" // Characters in order of appearance in English documents.
#define ALPHABET "ABCDEFGHIJKLMNOPQRSTUVWXYZ"


/* Decode the given text using the given map and store the result in newtext */
void decode_text(char* text, char* newtext, char map[][2]) {
    for (unsigned int i = 0; i < strlen(text); i++) {
        // If the current character in text is not a letter, copy it to newtext as punctuation and digits should be maintained.
        if (!isalpha(text[i])) {
            newtext[i] = text[i];
            continue;
        }
        for (unsigned int j = 0; j < ALEN; j++) {
            if (text[i] == map[j][1]) {
                newtext[i] = map [j][0];
            }
        }
    }
}

char upcase(char ch){
    if(islower(ch))
        ch -= 'a' - 'A';
    return ch;
}

int main(int argc, char **argv){
    // first allocate some space for our input text (we will read from stdin).
    char* text = (char*)malloc(sizeof(char)*TEXT_SIZE+1);
    char opt;
    int tmpi;

    // Check the CLI arguments and extract the mode: interactive or dump and store in opt.

    if(!(argc == 2 && isalpha(opt = argv[1][1]) && (opt == 'i' || opt == 'd'))){
        printf("format is: '%s' [-d|-i]\n", argv[0]);
        exit(1);
    }

    // Now read TEXT_SIZE or feof worth of characters (whichever is smaller) and convert to uppercase as we do it.
    {
        unsigned int i;
        for(char ch = fgetc(stdin); i < TEXT_SIZE && !feof(stdin); i++, ch = fgetc(stdin)){
            text[i] = (isalpha(ch)?upcase(ch):ch);
        }
        text[i] = '\0'; // terminate the string properly.
    }

    // Assign alphabet to one dimension of text frequency array and a counter to the other dimension
    char textfreq[ALEN][2];
    for (unsigned int i = 0; i < ALEN; i++) {
        textfreq[i][0] = ALPHABET[i];
        textfreq[i][1] = 0;
    }

    // Count frequency of characters in the given text
    for (unsigned int i = 0; i < strlen(text); i++) {
        for (unsigned int j = 0; j < ALEN; j++) {
            if (text[i] == textfreq[j][0]) textfreq[j][1]+=1;
        }
    }

    //Sort the character frequency array in descending order
    for (unsigned i = 0; i < ALEN-1; i++) {
        for (unsigned j= 0; j < ALEN-i-1; j++) {
            if (textfreq[j][1] < textfreq[j+1][1]) {
                tmpi = textfreq[j][1];
                char tmpc = textfreq[j][0];
                textfreq[j][1] = textfreq[j+1][1];
                textfreq[j][0] = textfreq[j+1][0];
                textfreq[j+1][1] = tmpi;
                textfreq[j+1][0] = tmpc;
            }
        }
    }

    //Map characters to most occurring English characters
    char map[ALEN][2];
    for (unsigned int i = 0; i < ALEN;