How to use execvp() - c

Question

The user will read a line and i will retain the first word as a command for execvp.
Lets say he will type "cat file.txt" ... command will be cat . But i am not sure how to use this execvp(), i read some tutorials but still didn't get it.
#include <stdio.h>
#include <stdlib.h>
int main()
{
char *buf;
char command[32];
char name[32];
char *pointer;
char line[80];
printf(">");
while((buf = readline(""))!=NULL){
if (strcmp(buf,"exit")==0)
break;
if(buf[0]!=NULL)
add_history(buf);
pointer = strtok(buf, " ");
if(pointer != NULL){
strcpy(command, pointer);
}
pid_t pid;
int status;
if ((pid = fork()) < 0) {
printf("*** ERROR: forking child process failed\n");
exit(1);
}
else if (pid == 0) {
if (execvp(command, buf) < 0) {
printf("*** ERROR: exec failed\n");
exit(1);
}
}
else
while (wait(&status) != pid)
;
free(buf);
printf(">");
}///end While
return 0;
}

Answer 1

The first argument is the file you wish to execute, and the second argument is an array of null-terminated strings that represent the appropriate arguments to the file as specified in the man page.
For example:
char *cmd = "ls";
char *argv[3];
argv[0] = "ls";
argv[1] = "-la";
argv[2] = NULL;
execvp(cmd, argv); //This will run "ls -la" as if it were a command

Answer 2

In cpp, you need to pay special attention to string types when using execvp:
#include <iostream>
#include <string>
#include <cstring>
#include <stdio.h>
#include <unistd.h>
using namespace std;
const size_t MAX_ARGC = 15; // 1 command + # of arguments
char* argv[MAX_ARGC + 1]; // Needs +1 because of the null terminator at the end
// c_str() converts string to const char*, strdup converts const char* to char*
argv[0] = strdup(command.c_str());
// start filling up the arguments after the first command
size_t arg_i = 1;
while (cin && arg_i < MAX_ARGC) {
string arg;
cin >> arg;
if (arg.empty()) {
argv[arg_i] = nullptr;
break;
} else {
argv[arg_i] = strdup(arg.c_str());
}
++arg_i;
}
// Run the command with arguments
if (execvp(command.c_str(), argv) == -1) {
// Print error if command not found
cerr << "command '" << command << "' not found\n";
}
Reference:
execlp、execvp用法與範例