A better implementation of the `execvp` call: the code implements `execvp`

I'm trying to implement execvp () using execv (). I don't know if the code is the best implementation.

my code:

#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <stdio.h>

extern char **environ; 

int my_execvp(char *file, char *argv[])
{
  char *p, *colon, *pathseq = getenv("PATH");
  size_t len;

  if(strchr (file, '/'))
    return execv(file, argv);

  if (pathseq == NULL) {
        len = confstr(_CS_PATH, (char *) NULL, 0);
        pathseq = (char *) alloca(1 + len);
        pathseq[0] = ':';
        (void) confstr (_CS_PATH, pathseq + 1, len);
  }
  len = strlen(file) + strlen(pathseq) + 2;

  for(p = pathseq; p && *p; p = colon)
  {
     char b[len];
     colon = strchr(p, ':');
     if(colon)
     {
        memcpy(b, p, (size_t) (colon - p));
        b[colon++-p] = 0;
     }
     else
        strcpy(b, p);
     strcat (b, "/");
     strcat (b, file);
     if(!access(b, F_OK)) {
            execv(b, argv);
            fprintf(stderr, "an error occurred in execv\n");
            abort();
     }
   }
   errno = ENOENT;
     return -1;
}

int main()
{
    /* "ls / -l" */
    char *arg[] = {"ls", "/", "-l", NULL};
    my_execvp (arg[0], arg);

    return 0;
} 

And the question I have, is it a good implementation? How could it be improved or write a better implementation?