Passing an argument using execvp - c

Question

I have this in my parent.c file
int main()
{
int n = 6;
int pid;
int status;
char* command = "./child";
for (i=1; i<=n; i++){
if((pid = fork()) == 0) {
execvp(command, NULL);
}
wait(&status);
}
My child.c file looks like this
int main(int argc, char *argv[]){
char *processnum = argv[0];
printf("This is the child %s\n", processnum);
return 0;
}
I basically just ran
gcc -o parent parent.c
gcc -o child child.c
./parent
This prints outs "This is the child (null)" 6 times, which is what I expect. But I want to be able to pass a parameter as I run child, in this case the process number.
So I changed my parent.c to look like this
for (i=1; i<=n; i++){
if(i == 1){
char* args = "1";
}
if(i == 2){
char* args = "2";
}
if(i == 3){
char* args = "3";
}
if(i == 4){
char* args = "4";
}
if(i == 5){
char* args = "5";
}
if(i == 6){
char* args = "6";
}
if((pid = fork()) == 0) {
execvp(command, args);
}
wait(&status);
}
What I thought would happen is that my program would print "This is the child 1", "This is the child 2" etc...
However, what actually happened is that the program seemed to run parent.c numerous times (I put a print statement at the start of parent.c and the output printed that statement like 20 times) instead of child.c
Can anyone explain why this is happening? Is there another way I can pass a parameter to child.c?
Thanks

Answer 1

here is the critical excerpt from the man page for execvp()
The execv(), execvp(), and execvpe() functions provide an array of
pointers to null-terminated strings that represent the argument list
available to the new program. The first argument, by convention,
should point to the filename associated with the file being executed.
The array of pointers must be terminated by a NULL pointer.

Answer 2

There are several issues with your program as it stands. First, as described by others, the second argument to execvp is a char ** type ... it ends up as the argv in the exec'ed program.
Second, the
if(i == 1){
char* args = "1";
}
...
code sets a variable args whose scope ends at the next line. You must declare the args in a scope in which you want to use it.
Third, converting a number to a string like this is very limited and tedious! You would do better using the standard C function sprintf (or even better snprintf).
Here is an updated parent.c:
#include <sys/wait.h>
#include <stdio.h>
#include <unistd.h>
int main()
{
int n = 6;
int pid;
int status;
char* command = "./child";
char *child_args[2];
char number_buff[32]; // more than big enough to hold any number!
child_args[1] = NULL;
for (int i=1; i<=n; i++){
sprintf(number_buff, "%d", i);
child_args[0] = number_buff;
if((pid = fork()) == 0) {
execvp(command, child_args);
}
wait(&status);
}
return 0;
}

Answer 3

Can anyone explain why this is happening? Is there another way I can pass a parameter to child.c?
You should be passing a char* array to execvp, not a char*. As mentioned in the comments, the last element of the array should be a null pointer. The following should work:
char* args[] = {"1\0", NULL};
execvp(command, args);
I'm guessing that execvp is failing since it can't dereference args as a char**, and thus the forked process is continuing the loop as a parent. You should be checking the return value of execvp to see that the function call worked.
Also, command and args should be null-terminated. I suggest using the function itoa to convert an int to a c string.