A beginner license check

02The comparison

const char *text declares a pointer used here to read characters. A pointer identifies a memory location; const indicates the characters are not modified through this declaration. text[count] selects a character at an offset, starting at 0. The while loop repeats until it reaches '\0', the zero byte ending a C string. The count excludes that terminator. For the supplied ASCII inputs, each character occupies one byte. This function counts bytes, not Unicode characters in general.

The helper returns -1 for a null pointer, the special pointer value 0 meaning no valid location. For the supplied ordinary text strings, it counts characters before the terminator. This small function assumes a valid terminated string within readable memory; it is not a general safe input-validation routine. A null pointer is different from a string terminator: one means no text location, while the other marks the end inside a valid text sequence.

desk_code compares the count with 4 using ==, returns 1 on equality and 0 otherwise. These are chosen function results, not observed process exits. A strings list cannot prove that branch was taken, and a conceptual branch or decompiler sketch cannot replace a matched binary trace.

The hand-drawn listing uses x86-style notation as a teaching model. eax names the result register in this model. call invokes the named function; cmp compares values; jne means jump if not equal. mov places a value in the named register, ret returns, and fail: labels a location the jump can select. These names explain the drawing; they do not claim a compiler emitted it.

1desk_code:
2  call length_of      ; result in eax
3  cmp  eax, 4
4  jne  fail           ; jump if not equal
5  mov  eax, 1
6  ret
7fail:
8  mov  eax, 0
9  ret

1desk_code:
2  call length_of      ; result in eax
3  cmp  eax, 4
4  jne  fail           ; jump if not equal
5  mov  eax, 1
6  ret
7fail:
8  mov  eax, 0
9  ret

/* Desk code check. Original teaching program. Not a product license. Not compiled. */

int length_of(const char *text) {
    int count;

    if (text == 0) {
        return -1;
    }
    count = 0;
    while (text[count] != '\0') {
        count = count + 1;
    }
    return count;
}

int desk_code(const char *text) {
    if (length_of(text) == 4) {
        return 1;
    }
    return 0;
}

03Trace a different input

Use the fresh activity text notes and trace its five characters until the terminator. Keep the count, equality result and function return separate. The later understanding question supplies a different string.

Source trace of owned-check.c. Not a program run.

Rule visible in desk_code:
length_of(text) is compared with 4.
Equal: return 1.
Otherwise: return 0.
A text pointer that is 0 makes length_of return -1, which is not 4, so desk_code returns 0.
The end of a text value in this source is the '\0' marker. The count stops before that marker.

Worked input "desk":
Characters d, e, s, k, then the end marker.
Count is 4.
4 equals 4, so the source returns 1.
Predicted from the source. Not observed.

Activity input "notes":
Characters n, o, t, e, s, then the end marker.
Count is 5.
5 equals 4 is false, so the source returns 0.
Predicted from the source. Not observed.

A matched run would be confirmation. No binary was built, so nothing was confirmed by execution.

1. Count the characters in notes before the end marker. Check: the count is 5.
2. Compare 5 with 4 using the condition in desk_code.
3. Check: the source trace returns 0.
4. Stop. Do not compile the check and do not feed it an input.

04Stage evidence note

This is the module note and the stage note. It is ungraded. Use one line for each blank on the card. The limit line has to say that an executable, a disassembly, a decompiler listing, and a syscall trace are still missing. A source prediction for the desk code is not a confirmed run.

Recommended reading order for the note: Compiled and interpreted, then Enough C and assembly, then file, strings, and strace, then the Ghidra lesson, then this check. The catalog list is unchanged.

Stage evidence note. Ungraded. Fill the blanks from the lessons. No binary was run.

1. hello.c role: ____
   Planned hello ELF role: ____
   Was the ELF file produced? ____

2. The compare/jump card is a: conceptual model / captured disassembly. Circle one.
   What is still missing? ____

3. A strings list can show a stored sequence. It cannot by itself show: ____

4. The decompiler sketch compared with compare.c differs in: ____
   Is the sketch the original source file? ____

5. desk_code returns 1 when length_of(text) equals: ____
   Source prediction is not the same thing as a run. Was a binary run? ____

Rubric: one filled line for each item, plus a limit.
A limit names what the page still does not have: an owned executable, a disassembly, a decompiler listing, or a syscall trace.
Stop. Do not compile or run anything to fill a blank.

1. Fill blank 1 from the hello card: hello.c is source, and the ELF row is an unproduced compiler-output role.
2. Fill blank 2 with the words conceptual model. Name the missing disassembly.
3. Fill blank 3 with what a strings list cannot show: which branch ran.
4. Fill blank 4 with a name that differs between compare.c and the sketch, and say the sketch is not the source file.
5. Fill blank 5 with 4, and write that no binary was run. Stop without compiling.

05Optional challenge

From the source, predict the return of desk_code for the two-character input ab. Write the count, the comparison, and the sentence that a run is still missing. About ten minutes.

06Fresh cases for the checks

Use these fictional cases for the checks below. They have different inputs from the worked examples. Read them here or make notes; no installation or external service is needed.

Fresh source rule: desk_code returns 1 exactly when length_of(text) equals 4; otherwise 0.
Input for this case: cord
All four letters count; there is no space or end-marker character in this input.
This is a source prediction only, not a run.

07Explain your evidence

Optional written self-check: explain one decision using the supplied case. No tool run is required.

Check your explanation: count spaces when present and exclude the end marker; connect the length comparison to its branch; distinguish source/model predictions from disassembly and an observed run.