Using additional file descriptors for input/output

bash_fd_poc.c

/**
 * Compile: gcc fd.c -o fd_copy
 * Usage:   fd_copy 3< input_file 4> output_file
 * Result:  Copies contents of input_file to output_file
 */

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	struct stat st;
	if (!fstat(3, &st) && !fstat(4, &st)) {
		char b;
		FILE *input = fdopen(3, "r");
		FILE *output = fdopen(4, "w");
		
		while (!feof(input)) {
			fread(&b, 1, 1, input);
			fwrite(&b, 1, 1, output);
		}
	}
	else
		fprintf(stderr, "oh no, please give me fds!\n");
}