Problems with using execvp on a constructed string

Question

I'm trying to write a shell, and part of its construction is executing code from a user-inputted string (buffer). However, when I attempt to execvp the string with additional inputs (ae. echo a), it always screws the pooch and returns -1. I'm at a loss as to why. Here's the relevant pieces:

char * buffer = calloc(100, sizeof(char));
...
fgets(buffer, 100, stdin);
buffer[strlen(buffer) - 1] = 0; // necessary because of a newline inserted by fgets
...
cmd = strsep(&buffer, " ");
char * str = malloc(50 * sizeof(char));
strcat(str, "./");
strcat(str, cmd);
strcat(str, ".out");
...
i = execvp(str, (char * *) buffer);

Answer 1

The argument buffer is wrong. The second argument of execvp is an array of pointers. With this cast, you are hidding a compiler warning, but it does not work.

Answer 2

I can see a couple of potential problems here.

First, you're allocating space with malloc (meaning the contents aren't initialized), but immediately using strcat to write to it. Unless (by whatever change) the first character is a '\0', that's going leave you with a string starting with garbage, followed by data you're trying to put there. It would also (very easily) lead to writing past the end of the buffer, giving undefined behavior.

If it were up to me, I think I'd use sprintf instead of strcat. At least what you've shown would work out to: sprintf(str, "./%43s.out", cmd);

Answer 3

Two problems:

1. You are passing an uninitialized string to the first strcat.
2. execvp expects an array of strings, not a single string with null-separated fields.