Using strtol() follow up
#include
You didn't supply the contents of &quot;stdafx.h&quot;, but I was able to compile simply by removing that include, so you probably don't need it.
You do seem to be missing a necessary include of &lt;string.h&gt;.
malloc()
In main() you allocate a fixed length buffer on the heap.  There's no advantage over stack allocation (unless you're compiling for a system with a tiny stack), and no need to multiply by 1: sizeof (char) is necessarily 1, by definition:
char str[MAXLENGTH];

Also, I note that there aren't many platforms that support 3400-bit longs, so you could probably make the input buffer a bit smaller.
Having done this, your call to readline() can be
readline(&amp;str, sizeof str)

readline()
You take a pointer to pointer, but don't ever change the pointed-to value, so you could introduce a const between the stars, or (better) remove a level of indirection:
bool readline(char *buf, const size_t buf_len) {
    if (fgets(buf, buf_len, stdin)) {
        str_cmp(buf);
        return true;
    }
    return false;
}

Also, it's not clear whether you want to return true for a partial-line read; it's probably worth a comment to clarify this.
str_cmp()
You should remember the value returned by strchr, rather than recomputing it:
void str_cmp(char* str) {
    char *nl = strchr(str, '\n');
    if (nl)
        *nl = '\0';
}

I also prefer to write '\0' in preference to 0 to show we're dealing with string characters here.
Since it's only used from one place, we can inline this function.  It's a judgement call, but I don't see enough benefit from having this separate.
back to main():
This code doesn't do what it claims:
char* err = str_to_long(str, &amp;cnvt_numb);
/* ... */
fprintf(stderr, &quot;That number '%s' was not convertible...\n&quot;, err);

The value you get back as err isn't the string - it's only the portion after the parseable digits.  So for input such as 10x, you'll just print x; it's not obvious that's what you want.  I'd show the whole string to the user:
fprintf(stderr, &quot;That number '%s' was not convertible...\n&quot;, str);

You probably don't want to return EXIT_SUCCESS in that case, either.
You test for output of LONG_MAX, but not LONG_MIN.  Actually, both of those are legitimate outputs, so by not testing errno, you are unnecessarily reducing the range you accept.
There's a call to getch() but that's not declared in any of the headers.  Perhaps you meant getchar()?  Either way, asking for extra input just to exit the program is user-hostile, and I'd just excise that part.

Putting it all together
Here's what I ended up with:
#include &lt;errno.h&gt;
#include &lt;stdio.h&gt;
#include &lt;stdbool.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;

#define MAXLENGTH 40            /* supports 128-bit long */


bool readline(char* buf, const size_t buf_len) {
    if (!fgets(buf, buf_len, stdin))
        return false;
    char *nl = strchr(buf, '\n');
    if (nl)
        *nl = '\0';
    return true;
}

// returns NULL on success, or the unparseable portion of input on failure
char* str_to_long(const char* const str, long* out) {
    char* end;
    errno = 0;
    *out = strtol(str, &amp;end, 10);
    if (!*end &amp;&amp; end != str)
        end = NULL;
    return end;
}


int main() {
    char str[MAXLENGTH];
    long cnvt_numb = 0;

    printf(&quot;Enter a number: &quot;);
    while (!readline(str, sizeof str)) {
        printf(&quot;Try again...\n&quot;);
    }

    char* err = str_to_long(str, &amp;cnvt_numb);
    if (err) {
        fprintf(stderr, &quot;That number '%s' was not convertible...\n&quot;, str);
        return EXIT_FAILURE;
    } else if (errno == ERANGE) {
        fprintf(stderr, &quot;INPUT OUT OF RANGE...\n&quot;);
        return EXIT_FAILURE;
    } else {
        printf(&quot;Your number was: %ld\n&quot;, cnvt_numb);
        return EXIT_SUCCESS;
    }
}

It accepts input from -9223372036854775808 to 9223372036854775807 (on this system, where long is 64 bits), and correctly reports a range error for -9223372036854775809 and for 9223372036854775808.
12345aoeu is rejected with

That number '12345aoeu' was not convertible...