#include <sys/param.h>

#include <sys/stat.h>
#include <sys/sysctl.h>

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

#include "scheme.h"

char *get_exename(char *buf, size_t size)
{
    int mib[4];
    pid_t pid;
    size_t len, plen;
    char **argv, **argv2;
    char *p, *path, *pathcpy, filename[PATH_MAX];
    struct stat sbuf;

    (void)size;

    pid = getpid();

    mib[0] = CTL_KERN;
    mib[1] = KERN_PROC_ARGS;
    mib[2] = pid;
    mib[3] = KERN_PROC_ARGV;

    buf = NULL;
    argv = NULL;
    len = 128;

    // Now, play The Guessing Game with sysctl(3) as to how big argv
    // is supposed to be. (It's loads of fun for the whole family!)

    while (len < SIZE_MAX / 2) {
        len *= 2;
        if ((argv2 = realloc(argv, len)) == NULL)
            break;
        argv = argv2;
        if (sysctl(mib, 4, argv, &len, NULL, 0) == -1) {
            if (errno == ENOMEM)
                continue;  // Go back and realloc more memory.
            break;         // Bail for some other error in sysctl(3).
        }
        // If you made it here, congrats! You guessed right!
        if (*argv != NULL)
            buf = strdup(*argv);
        break;
    }
    free(argv);

    // If no error occurred in the sysctl(3) KERN_PROC_ARGV call
    // above, then buf at this point contains some kind of pathname.

    if (buf != NULL) {
        if (strchr(buf, '/') == NULL) {
            // buf contains a `basename`-style pathname (i.e. "foo",
            // as opposed to "../foo" or "/usr/bin/foo"); search the
            // PATH for its location. (BTW the setgid(2), setuid(2)
            // calls are a pre-condition for the access(2) call
            // later.)

            if ((path = getenv("PATH")) != NULL && !setgid(getegid()) &&
                !setuid(geteuid())) {

                // The strdup(3) call below, if successful, will
                // allocate memory for the PATH string returned by
                // getenv(3) above.  This is necessary because the man
                // page of getenv(3) says that its return value
                // "should be considered read-only"; however, the
                // strsep(3) call below is going to be destructively
                // modifying that value. ("Hulk smash!")

                if ((path = strdup(path)) != NULL) {
                    pathcpy = path;
                    len = strlen(buf);
                    while ((p = strsep(&pathcpy, ":")) != NULL) {
                        if (*p == '\0')
                            p = ".";
                        plen = strlen(p);

                        // strip trailing '/'
                        while (p[plen - 1] == '/')
                            p[--plen] = '\0';

                        if (plen + 1 + len < sizeof(filename)) {
                            snprintf(filename, sizeof(filename), "%s/%s", p,
                                     buf);
                            if ((stat(filename, &sbuf) == 0) &&
                                S_ISREG(sbuf.st_mode) &&
                                access(filename, X_OK) == 0) {
                                buf = strdup(filename);
                                break;
                            }
                        }
                    }
                    free(path);  // free the strdup(3) memory allocation.
                }
            } else
                buf = NULL;  // call to getenv(3) or [sg]ete?[ug]id(2) failed.
        }
        if (buf != NULL && *buf != '/') {
            // buf contains a relative pathname (e.g. "../foo");
            // resolve this to an absolute pathname.
            if (strlcpy(filename, buf, sizeof(filename)) >=
                sizeof(filename) ||
                realpath(filename, buf) == NULL)
                buf = NULL;
        }
    }

    return buf;
}