How do I duplicate a file descriptor in C?

Answer: the dup system call. Here’s an example of duplicating the descriptor 1, which refers to the standard out pipe.

#include <unistd.h>

#define WRITE(F, S) write((F), (S), sizeof(S))

int main(void) {
  WRITE(1, "This is written to stdout via descriptor 1\n");
  int new_stdout = dup(1);
  WRITE(new_stdout, "This is written to stdout via new descriptor\n");
  return 0;

By duplicating the file descriptor, we get two references to the same underlying pipe.

Tagged #file-descriptors, #c, #system-calls, #posix, #programming.
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