C system calls fails - c

Question

I'm trying to write a code which manipulates standard input and output and redirect them to files, and then use execvp (also tried other exec's) to run a program that simply uses printf and scanf , but the execvp fails..
Relevant code:
int pid2 = fork();
if (pid2 == 0) {
int fdInput = open("myinputfile", O_RDONLY);
close(0);
dup(fdInput);
int fdOutput = open("results.txt", O_WRONLY | O_CREAT | O_TRUNC);
close(1);
dup(fdOutput);
char* tmp[]={"...somepath/prog"};
execvp("...somepath/prog", tmp);
}
My prog:
int main(){
int x;
scanf("%d",&x);
printf("Hello World! %d",x);
return 0;
}
myinputfile contains only -> 4
I tried two main things:
copying the code from prog and hardcoding it into my code instead of the call to execvp, which works fine and I can see "Hello world! 4" in results.txt
running "mypath" in the terminal manually which also seems to work(with the standard I/O).
I can't understand why it's not working, I tried everything I could think of..

Answer 1

Your array of arguments passed to execvp() is not NULL-terminated.
Per the POSIX exec() documentation:
...
The argument argv is an array of character pointers to null-terminated strings. The application shall ensure that the last member of this array is a null pointer. These strings shall constitute the argument list available to the new process image. The value in argv[0] should point to a filename string that is associated with the process being started by one of the exec functions.
...
Your code should be
int pid2 = fork();
if (pid2 == 0) {
int fdInput = open("myinputfile", O_RDONLY);
close(0);
dup(fdInput);
int fdOutput = open("results.txt", O_WRONLY | O_CREAT | O_TRUNC);
close(1);
dup(fdOutput);
// note the NULL terminator
char* tmp[]={"...somepath/prog", NULL };
execvp("...somepath/prog", tmp);
}