Error safeguarding the input and output file descriptors

Question

Alright, I'm doing a pipe to connect with the children of my process. First of nothing I tried to do a safeguard of my fds so I can access them later for some stuff, but somehow it just gets stuck when duplicating the fds.

int pipeFd [2];
int pid;
pipe (pipeFd);

//Safeguard of the Original FDs
int fdSG [2];
perror ("fdsg create");
dup2 (1, fdSG [1]);
perror ("dup2 sfg1");
dup2 (0, fdSG [0]);
perror ("dup2 sfg2");
dup2 (pipeFd [1], 1);

The program gets stuck in the last instruction showed here.

The terminal output is the following:

fdsg create: Success
dup2 sfg1: Bad file descriptor
dup2 sfg2: Bad file descriptor
dup2: Bad file descriptor

Does any of you have any clue why this is happening?

Answer 1

From the code you've shown you haven't initalised fdSG. That's not correct, the arguments of dup2 both need to be valid file descriptors.

Since you seem to want to copy a fd rather than replace an existing one you should use dup for those backup copies instead, it picks a free fd and uses that. (Alternatively you could initalise fdSG to be valid fds too).

From the manpage:

dup() uses the lowest-numbered unused descriptor for the new descriptor.