Reverse a string without the <string.h> header

I have a few comments.

1. main takes argc/argv normally. Also start the function with the opening brace in colum 0

2. Define one variable per line.

3. Your request for input might be better printed to stderr so that it can be separated from program output.

4. Your initial loop should test for EOF. Currently it doesn't You can test this in the *NIX shell with (program is your executable):

   /bin/echo -n "hello world" | ./program
   

5. terminate seems redundant

6. You can and perhaps should define loop variables in the loop (as long as you don't need the value outside the loop):

   for (int i = 0; i < n; ++i) {
   

7. The second for-loop is a little messy. For a start it has a nested loop - my advice is to avoid nested loops and extract the inner loop to a function. This is not always practical, but more often than not it is useful. In this case, you can write a little function (eg. print_chars) that prints a specified number of characters.

8. Your terminating loop can be replaced by a call to the print_chars function.

9. putchar is preferable to printf("%c", ...)

10. while, for, if etc should be followed by a space.

11. Testing for ' ' might be better done using isspace, which will test for other space-like characters too.

Here is my version, for what it is worth:

#include <stdio.h>
#include <ctype.h>

#define N 100

static inline void print_chars(const char *s, long n)
{
    for (int i = 0; i < n; ++i) {
        putchar(s[i]);
    }
}

int main(int argc, char **argv)
{
    char line[N];
    int ch; /* note that ch is an int to allow it to hold 'EOF' */
    int len = 0;

    fprintf(stderr, "Enter a sentence: ");

    while ((ch = getchar()) != EOF) {
        if (ch == '?' || ch == '.' || ch == '!' || ch == '\n' || len >= N-1) {
            break;
        }
        line[len++] = (char) ch;
    }
    line[len] = '\0';

    char *end = line + len;
    char *s = end;
    while (--s >= line) {
        if (isspace(*s)) {
            print_chars(s + 1, end - s - 1);
            putchar(*s);
            end = s;
        }
    }
    print_chars(line, end - line);
    if (ch != EOF) {
        putchar(ch);
    }
    return 0;
}

The others have pointed out a lot so I will not repeat them.

But part of programming is spotting code that can be split out into other functions for readability. You have a few places were functions would make the code more readable:

#include <stdio.h>

#define N 100

int endOfSentence(int x) {return x == '?' || x == '.' || x == '!' || x == '\n';}
char* findWord(char* ch, int len)
{
    if (len == 0)
    {return NULL;}

    while(len > 0 && ch[len-1] != ' ')
    {--len;}

    return ch+len;
}
void printWord(char* beg, char* end)
{
    while(beg != end) {putchar(*beg);++beg;}
}

int main(void) {
  char ch[N];
  char terminate;
  int i = 0;
  int j, k;

  printf("Enter a sentence: ");
  while(i < N && !endOfSentence(ch[i] = getchar())) { ++i;}

  int len = i;        //Determine the length of the string
  terminate = ch[i];

  char* word;
  while(len >= 0 && (word = findWord(ch, len)))
  {
       printWord(word, ch+len);
       len = word - ch - 1;
       if (len > 0)
       {    putchar(' ');}
  }

  putchar(terminate);
  putchar('\n');

  return 0;
}

Fixed:

> gcc 22914.c
> ./a.out 
Enter a sentence: you can cage a swallow can't you?
you can't swallow a cage can you?