Redirect stdin from exec to reading pipe - c

Question

Basically I'm programming my own xargs implementation for practicing. The main difference with the original xargs is that what I do is to buffer the first 4 lines I get from stdin in the parent process and write it in the pipe I created, so it processes 4 lines at a time instead of each line. Then, in the child process I redirect stdin to the reading pipe so when I call exec it should receive the arguments. After that, when child ends, the parent should do this again until all stdin was read.
So let's say I do cat directories.txt | ./my_xargs ls where directories is a file that has:
/var/
/opt/
/dev/
I should get the result of running ls /var/ /opt/ /dev/. But instead I'm getting as if I've run just ls without parameters.
I tried several things:
Writing a file and redirecting that file descriptor to stdin. Didn't work.
I already checked the pipe is written correctly debugging and also printing the result of reading the pipe instead of calling exec.
closing stdin and opening a new file and write what I read in the child to that file. When I debug I can even see that that file descriptor is 0.
removing the line close(pd[0]); didn't work either.
#ifndef NARGS
#define NARGS 4
#endif
#define LINE_SIZE 1024
#define PATH_MAX 1024
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
typedef enum { false, true } bool;
int
main(int argc, char *argv[])
{
bool eof = false;
int pd[2];
size_t len = 0;
while (!eof) {
if (pipe(pd) == -1) {
fprintf(stderr, "Error creating pipe\n");
}
pid_t pid;
pid = fork();
if (pid == -1) {
fprintf(stderr, "Fork error\n");
}
if (pid == 0) { // child
close(pd[1]); // child doesn't write
dup2(pd[0], 0); // changing reading pipe por stdin
close(pd[0]);
execvp(argv[1], argv + 1);
perror("Exec failed.\n");
} else {
close(pd[0]); // parent doesn't read
char total_params[PATH_MAX] = "";
char* newLine = NULL;
int i = 0;
while (i < NARGS && !eof) {
// Reading stdin
eof = getline(&newLine, &len, stdin) < 1;
// Removing '\n'
if ( i+1 < NARGS) {
newLine[strcspn(newLine, "\n")] = ' ';
} else {
newLine[strcspn(newLine, "\n")] = '\0';
}
strcat(total_params, newLine);
i++;
}
free(newLine);
write(pd[1], &total_params, strlen(total_params));
close(pd[1]);
wait(NULL);
}
}
return 0;
}