Reading the number of characters in a string in C - c

Question

I want to use pipes to read a string in child process and send it to the parent, then count the number of characters in the string and display the number. The program runs only until I input a string. Can you please tell me what I'm doing wrong?
#include <stdlib.h>
#include <stdio.h>
#include<string.h>
#define SIZE 1024
int main()
{
int i,n,x;
char msg[SIZE];
char str[SIZE];
int p3[2];
pipe(p3);
int pid=fork();
if(pid==0){
close(p3[0]); //closing read end
printf("Enter a string\n");
fgets(msg,SIZE,stdin);
write(p3[1],msg,sizeof(msg)); //writting into pipe
close(p3[1]); //closing write end
}
else {
close(p3[1]); //close write end
read(p3[0], str, sizeof(str));
int c = 0;
while (str[c] != "Done")
{
for(i=0; i<sizeof(str);i++){
if (str[c] >= 'a' && str[c] <= 'z') {
c++;
}
}
}
printf("%d letters\n", c);
}
return 0;
}

Answer 1

Use strlen to determine the length of the string then use for loop to iterate upto the length, something like below:
if (strncmp(str, "Done", 4) == 0)
exit(0);
int len = strlen(str);
for (l = 0; l < len; l++)
{
if (str[l] >= 'a' && str[l] <= 'z')
{
c++;
}
}
printf("%d letters\n", c);
close(p3[0]); //close write end

Answer 2

You can use strlen() to determine the length of any null-terminated string;
In while (str[c] != "Done") you're comparing a char to a string literal. You should use strcmp() or strncmp() to compare strings;
The inner-loop (for(i=0; i<sizeof(str);i++) {...}) is using str[c], but I believe you want str[i], besides, the loop is not counting (c++) when the char is not in the interval [a,z], therefore your outer-loop may never end;
You really don't need nested loops. Try something like:
const char *ptr = str;
size_t index = 0; // An index to access str - just to help you.
// Iterate over ptr, one character at a time, until we find the null terminator.
while (*ptr != '\0') {
// Read 4 chars from ptr and check if it's "Done"
if (strncmp(ptr, "Done", 4) == 0)
break;
// Do something here. For example:
// if (str[index] >= 'a' && str[index] <= 'z')
// lowercase_counter++;
// if (str[index] >= 'A' && str[index] <= 'Z')
// uppercase_counter++;
// if (str[index] >= '0' && str[index] <= '9')
// number_counter++;
// NOTE: str[index] is equivalent to *ptr, thus you can use one or another.
ptr++;
index++;
}
Be sure to not increment ptr elsewhere - the idea is for each loop iteration to check a single character - you can still read ahead using strcmp or whatever, like we did for "Done". If you do that, also take care to not access ptr beyond the str boundaries.