c multiple file descriptors - c

Question

I am trying to make a C program that concatenates the contents of two files at a target file. To do this I must have at least 2 files open at the same time but I haven't figured out why I can't. The problem can be sufficiently described by the two pieces of code below:
Why does this work:
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
int main(int argc, char* argv[]){
size_t fdes = open(argv[1], O_RDONLY);
void * buf;
int bytes;
while ((bytes = (int)read(fdes, buf, 3)) > 0) {
printf("%s", (char*)buf);
}
close(fdes);
return 0;
}
and this doesn't?
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
int main(int argc, char* argv[]){
size_t fdes = open(argv[1], O_RDONLY);
size_t fdes2 = open(argv[2], O_RDONLY);
void * buf;
int bytes;
while ((bytes = (int)read(fdes, buf, 3)) > 0) {
printf("%s", (char*)buf);
}
close(fdes);
close(fdes2);
return 0;
}
can't i have multiple files open?

Answer 1

I make some changes, and this works:
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
int main(int argc, char* argv[]){
size_t fdes = open(argv[1], O_RDONLY);
size_t fdes2 = open(argv[2], O_RDONLY);
size_t length = 0;
char * buf = (char *)malloc(sizeof(char) * length);
int bytes;
while ((bytes = (int)read(fdes, buf, sizeof(buf) - 1)) > 0) {
printf("%s", buf);
}
while ((bytes = (int)read(fdes2, buf, sizeof(buf) - 1)) > 0) {
printf("%s", buf);
}
close(fdes);
close(fdes2);
return 0;
}
First, buf should be char *
Second, you need to point buf to somewhere (malloc or a char array)
Reference: https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.bpxbd00/rtrea.htm