piping the result of execl command, which is directory list, to parent process? - c++

Question

I'm doing some practice on process management in Linux and how to use system calls and communication between child and parent processes. I need to implement a pipe to get the string provided by child process, which is the directory list as string and pass it to the parent process to count the number of lines in that string and find the number of files in that directory by doing that. The problem i faced is here:
error: initializer fails to determine size of ‘dirFileList’
char dirFileList[] = read(tunnel[0],buf,MAX_BUF)
Also my code is down below:
#define die(e) do { fprintf(stderr, "%s\n", e); exit(EXIT_FAILURE); } while (0);
#define MAX_BUF 2024
int main()
{
const char *path = (char *)"/"; /* Root path */
const char *childCommand = (char *)"ls |"; /* Command to be executed by the child process */
const char *parentCommand = (char *)"wc -l"; /* Command to be executed by the parent process */
int i = 0; /* A simple loop counter :) */
int counter = 0; /* Counts the number of lines in the string provided in the child process */
int dirFileNum; /* Keeps the list of files in the directory */
int tunnel[2]; /* Defining an array of integer to let the child process store a number and parent process to pick that number */
pid_t pID = fork();
char buf[MAX_BUF]; /* Fork from the main process */
if (pipe(tunnel) == -1) /* Pipe from the parent to the child */
die("pipe died.");
if(pID == -1) /* Check if the fork result is valid */
{
die("fork died.");
}
else if(pID == 0) /* Check if we are in the child process */
{
dup2 (tunnel[1], STDOUT_FILENO); /* Redirect standard output */
close(tunnel[0]);
close(tunnel[1]);
execl(childCommand, path); /* Execute the child command */
die("execl died.");
}
else /* When we are still in the main process */
{
close(tunnel[1]);
char dirFileList[] = read(tunnel[0],buf,MAX_BUF); /* Read the list of directories provided by the child process */
for(i;i<strlen(dirFileList);i++) /* Find the number of lines in the list provided by the child process */
if(dirFileList[i] == '\n')
counter++;
printf("Root contains %d files.", counter); /* Print the result */
wait(NULL); /* Wait until the job is done by the child process */
}
return 0;
}

Answer 1

If you'd shown us the whole error message, we'd see it's referring to this line:
char dirFileList[] = read(tunnel[0],buf,MAX_BUF);
You can't declare an indeterminate array like that. And if you read the man page of read(2), you'll see that the return value is
On success, the number of bytes read ...
On error, -1 ...
So you want something like
int bytes_read = read(...);
if (bytes_read < 0) {
perror("read");
exit(1);
}
Some additional review (which you didn't ask for, but may be instructive):
Don't cast string literals to char*, especially when you're then assigning to const char* variables.
Instead of just printing a fixed message on error, you can be more informative after a call that has set errno if you use perror() - see my sample above.
die() could be implemented as a function, which will make it easier to debug and to use correctly than a macro.