FILE * streams with dup2()-ed descriptors — stdin not working - c

Question

After forking a child and dub2()-ing its stdin descriptor to the read-end of a pipe (its write-end is in the parent process) reading with read(0,...) (descriptor based) works fine. But reading with fgets(stdin,...) (stream based) does not work. Why?
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
int main(){
char string[]="MY TEST STRING";
pid_t pid;
int bufSize=80;
char rBuf[bufSize];
int downlink[2], wrlen=0, rdlen=0, status;
memset(rBuf,0,bufSize);
if (pipe (downlink) == -1){
printf("Error with pipe()\n");
exit(4);
}
pid=fork();
if (pid>0){ //parent
wrlen=wrlen+write(downlink[1], string, strlen(string)+1);
//dprintf(downlink[1],"%s", string);
sleep(6);
}
else if (pid == 0){ // child
dup2(downlink[0],STDIN_FILENO);
//rdlen=read(downlink[0], rBuf, bufSize); //works
//rdlen=read(STDIN_FILENO, rBuf, bufSize); //works
//scanf("%s", rBuf);fflush(stdin); //doesn't work, reads up to first blank
//scanf(stdin,"%s", rBuf);fflush(stdin); //doesn't work, reads up to first blank
fgets(rBuf, bufSize, stdin);fflush(stdin); //doesn't work
printf("c: %s", rBuf), fflush(stdout);
//status =execl("/usr/bin/octave","octave","--no-gui",NULL);
//status =execl("/usr/bin/man","man",NULL);
//printf("c: status%d", status), fflush(stdout);
}
else{ //error
printf("Error with fork()\n");
exit(4);
}
return 0;
}
In this code the fork()ed child is supposed to read from stdin (which is dub2()ed to downlink[0](=read-end of pipe from writing parent)) and printf() to stdout the received contets.
If the reading happens with read() (descriptor based) everything works fine. When reading with fgets() or scanf() (stream based) no data is printed.
What am I missing here?

Answer 1

fgets() reads a line, but your parent process never sends a line of text. So you need to add a newline to your string
char string[]="MY TEST STRING\n";
read() however just reads whatever is in the pipe when it becomes available - it does not try to read all the data it can up till a newline character, which is why you get data back when using read()
Even when you do not send a newline, fgets() would return when the write end of the pipe gets closed. However the pipe you create in your parent process gets copied into the child process.
That means that when the parent process exits, its write end of the pipe is closed - but not the write end of the pipe in the child process - leading to the pipe still being open when the parent exit.
So make sure you close() the write end of the pipe in your child process, as you don't need it:
else if (pid == 0){ // child
close(downlink[1]);
dup2(downlink[0],STDIN_FILENO);