Why is my C++ array printing the same values? - c++

Question

I am working on a code where it will do Linux command piping. Basically in my code, it will parse the user input command, then run it using the execvp function.
However, to do this, I would need to know the command, as well as its parameters. I have been trying to get the parsing to work correctly, however, it seems that when I do a test case, the output from both of the arrays that store their respective programs is the same. The commands/parameters are stored in a char array called prgname1 and prgname2.
For instance, if I were to run my program with the parameter "ps aux | grep [username]", then the output of prgname1[0] and prgname2[0] are both [username]. They are supposed to be ps and grep, respectively.
Can anyone take a look at my code and see where I might be having an error which is causing this?
Thanks!
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <iostream>
#define MAX_PARA_NUM 5
#define MAX_COMMAND_LEN 1024
using namespace std;
int main(int argc, char *argv[]) {
char *prgname1[MAX_PARA_NUM], *prgname2[MAX_PARA_NUM];
char command[MAX_COMMAND_LEN];
int pfd[2];
pipe(pfd);
pid_t cid1, cid2;
char *full = argv[1];
char str[MAX_COMMAND_LEN];
int i = 0;
int j = 0;
int k = 0;
int ind = 0;
while (ind < strlen(full)) {
if (full[ind] == ' ') {
strncpy(command, str, i);
cout << command << endl;
prgname1[j] = command;
j++;
i = 0;
ind++;
}
else {
str[i] = full[ind];
i++;
ind++;
}
if(full[ind] == '|') {
i = 0;
j = 0;
ind+=2;
while (ind < strlen(full)) {
if (full[ind] == ' ') {
strncpy(command, str, i);
cout << command << endl;
prgname2[j] = command;
j++;
i = 0;
ind++;
}
else {
str[i] = full[ind];
i++;
ind++;
}
if (ind == strlen(full)) {
strncpy(command, str, i);
cout << command << endl;
prgname2[j] = command;
break;
}
}
}
}
// test output here not working correctly
cout << prgname1[0] << endl;
cout << prgname2[0] << endl;
// exits if no parameters passed
if (argc != 2) {
cout << "Usage:" << argv[0] << endl;
exit(EXIT_FAILURE);
}
// exits if there is a pipe error
if (pipe(pfd) == -1) {
cerr << "pipe" << endl;
exit(EXIT_FAILURE);
}
cid1 = fork(); // creates child process 1
// exits if there is a fork error
if (cid1 == -1 || cid2 == -1) {
cerr << "fork";
exit(EXIT_FAILURE);
}
// 1st child process executes and writes to the pipe
if (cid1 == 0) {
char **p = prgname1;
close(1); // closes stdout
dup(pfd[1]); // connects pipe output to stdout
close(pfd[0]); // closes pipe input as it is not needed
close(pfd[1]); // closes pipe output as pipe is connected
execvp(prgname1[0], p);
cerr << "execlp 1 failed" << endl;
cid2 = fork();
}
// 2nd child process reads from the pipe and executes
else if (cid2 == 0) {
char **p = prgname2;
close(0); // closes stdin
dup(pfd[0]); // connects pipe input to stdin
close(pfd[0]); // closes pipe input as pipe is connected
close(pfd[1]); // closes pipe output as it is not needed
execvp(prgname2[0], p);
cerr << "execlp 2 failed" << endl;
}
else {
sleep(1);
waitpid(cid1, NULL, 0);
waitpid(cid2, NULL, 0);
cout << "Program successfully completed" << endl;
exit(EXIT_SUCCESS);
}
return 0;
}

Answer 1

argv[1] gives you the first argument on the command line - not the entire command line. If you want the full list of command line arguments passed into the process, you will need to append argv[1], argv[2], ..., argv[argc - 1] together with a space between each.
Additionally, when you process it, you are setting the pointer for your prgname1[index] to command, so every time you set a given character pointer, they are all pointing to the same location (hence, they are all the same value). You need to allocate space for each element in prgname1 and copy command into it (using strncpy). Alternatively, using std::string and std::vector eliminates much of your current code.