Question

I am using Linux.
I am trying to write a program in c that will print a string backward.
Here is my code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main (){
    char string[100];
    printf ("Enter string:\n");
    gets (string);
    int length = strlen (string)-1;
    for (length = length; length>=0; length--){
        puts (string[length]);
    }
}

And here is the error:

a.c:10: warning: passing argument 1 of ‘puts’ makes pointer from integer without a cast
/usr/include/stdio.h:668: note: expected ‘const char *’ but argument is of type ‘char’
/tmp/cc5rpeG7.o: In function `main':
a.c:(.text+0x29): warning: the `gets' function is dangerous and should not be used.

What should I do?

Answer 1

First:

NEVER NEVER NEVER NEVER NEVER use gets(); it will introduce a point of failure in your code. There’s no way to tell gets() how big the target buffer is, so if you pass a buffer sized to hold 10 characters and there’s 100 characters in the input stream, gets() will happily store those extra 90 characters in the memory beyond the end of your buffer, potentially clobbering something important. Buffer overruns are an easy malware exploit; the Morris worm specifically exploited a gets() call in sendmail.

Use fgets() instead; it allows you to specify the maximum number of characters to read from the input stream. However, unlike gets(), fgets() will save the terminating newline character to the buffer if there’s room for it, so you have to account for that:

char string[100]; 
char *newline;
printf("Enter a string: ");
fflush(stdout);
fgets(string, sizeof string, stdin);
newline = strchr(buffer, '\n');      // search for the newline character
if (newline)                         // if it's present
  *newline = 0;                      // set it to zero

Now that’s out of the way…

Your error is coming from the fact that puts() expects an argument of type char *, but you’re passing an argument of type char, hence the “pointer from integer without cast” message (char is an integral type). To write a single character to stdout, use putchar() or fputc().