Why this code with fork() generates Runtime Error in ejudge? - c

Question

Why this code may generate Runtime Error in ejudge? This program counts the number of words from stdin input. Words can be separated by any amount of ' ' and '\n'.
It seems like fork() can cause a problem but I am not sure why I don't get the same error on my computer.
ejudge uses gcc - Plain C, 64 bit, using -std=c11 or -std=gnu11
The task:
On the standard input stream a text string is given which consists of
words (a sequence of non-space characters), between which there can be
any number of whitespace characters, including line feeds.
You need to calculate the number of words if you know there are not
more than 255, and output this value to the standard output stream.
Use creating new processes so that each process reads not more than
one word, e.g. using scanf("%s", ...).
You can only output the result from the process which was started
first (i.e. from the original program).
The resulting program must return with return code 0.
The size of each word does not exceed 4096 bytes.
My code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
#define DBG(args...) fprintf(stderr, args)
//#define DBG(args...)
int main(int argc, char* argv[])
{
int status;
pid_t pid;
pid_t first_child;
for (int i = 0; i < 256; ++i) {
pid = fork();
if (pid == 0) { // child continue reading
char str[4097];
if (scanf("%s", str) != EOF)
continue;
exit(1);
} else {
if (i == 1) {
first_child = pid;
}
if (wait(&status) == first_child) {
break;
} else {
exit(WEXITSTATUS(status) + 1);
}
}
}
fprintf(stdout, "%i\n", WEXITSTATUS(status));
fflush(stdout);
fclose(stdout);
return 0;
}

Answer 1

Rewrote the algorithm and it worked!
In the first version, many unnecessary forks were made. For example, if 6 were intended it created 12.
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
// #define DBG(args...) fprintf(stderr, args)
#define DBG(args...)
int main(int argc, char* argv[])
{
int status;
pid_t first_pid;
pid_t pid = fork();
if (pid != 0) {
wait(&status);
printf("%i\n", WEXITSTATUS(status));
return 0;
}
for (int i = 0; i < 256; ++i) {
char str[4097];
if (scanf("%s", str) == EOF) {
DBG("PID %i\n", pid);
exit(0);
}
pid = fork();
if (pid != 0)
break;
}
DBG("PID %i waiting\n", pid);
wait(&status);
exit(WEXITSTATUS(status) + 1);
}