file, strings, and strace

02What each view can hold

Read read-note.c as source evidence. #include <stdio.h> makes standard input/output declarations available. FILE *note declares a variable for a file handle. fopen("note.txt", "r") requests reading a file; the quotes delimit strings, not extra path characters. If the returned pointer is zero, the code returns 1; otherwise it closes the handle and returns 0.

The supplied note.txt is text, but no matched read-note executable is supplied. Therefore there is no actual classifier line, strings listing or syscall trace for this program. The separate strings card is a hand-written model. It cannot establish which branch any run took.

GNU strings normally uses a minimum of four printable characters, with scan coverage depending on its options/configuration. A listed word may be part of an error path or unused data. A real strace would record call names, arguments and results from one run. Neither source text nor a manual sample supplies that observation.

1/* Original teaching source. Not compiled. Not traced. */
2#include <stdio.h>
3 
4int main(void) {
5    FILE *note;
6 
7    note = fopen("note.txt", "r");
8    if (note == 0) {
9        return 1;
10    }
11    fclose(note);
12    return 0;
13}
14 

1Source calls fopen on note.txt.
2If the result is 0, main returns 1.
3Otherwise fclose runs and main returns 0.
4 
5Conceptual file view: missing. No binary.
6Conceptual strings view: missing. The path is in the source. A binary listing was not made.
7Conceptual syscall view: missing. fopen is the library call in the source. The system-call name was not recorded.

/* Original teaching source. Not compiled. Not traced. */
#include <stdio.h>

int main(void) {
    FILE *note;

    note = fopen("note.txt", "r");
    if (note == 0) {
        return 1;
    }
    fclose(note);
    return 0;
}

north desk note

Conceptual views of read-note. Not tool output.

Source view, from read-note.c, which does exist:
The text calls fopen on the path note.txt.
If that call's result is 0, main returns 1.
Otherwise the text calls fclose and main returns 0.
The characters note.txt appear in the source.

File-classifier view:
Missing. No binary is included, so there is no file(1) output to read.
A later line would be a classification of that binary.
This card does not invent one.

Strings view:
Missing. There is no strings(1) output on this card.
The source contains the path note.txt.
Whether those bytes would appear in a binary is unknown until a build exists.
GNU strings prints printable sequences at least 4 characters long by default.
How it was configured can limit it to loadable, initialized data sections.
A printed sequence is not a record of which branch ran.

Syscall view:
Missing. There is no strace output on this card.
The strace manual says each trace line has the system call name, its arguments, and its return value.
The manual's sample line is from cat /dev/null. That sample is not this program.
fopen is a C library call in the source. The system call name, the return value, and any process id are unknown here.

03GDB primer

A breakpoint is a location where a debugger pauses execution. break main requests a stop at main; run starts the selected program; start starts it with a temporary stop at main. next advances to another source line while stepping over called functions, and print evaluates an expression in the current program state. quit leaves the debugger.

The -g compiler flag makes source names, lines and values available for source-level debugging. GDB can also run or inspect a program without that information, though source-level views are limited and instruction-level commands may be needed. Optimization can further change what variables and lines are visible.

The command list below is an unrun primer for a future owned build. No GDB session exists here, so there is no address, paused value or process to report. On GNU/Linux, debugger launch settings can affect address randomization; a later transcript must record the environment rather than assuming addresses match an ordinary run.

GDB primer for a future owned executable. Not a session.
No matched executable is supplied for this activity.

1. Select the owned executable when starting GDB, or through its file command.
2. break main requests a breakpoint at main when that name can be resolved.
3. run starts the selected program. start starts it with a temporary stop at main.
4. next advances to another source line while stepping over called functions.
5. print evaluates an expression using the currently available program state.
6. quit leaves the debugger.

-g requests source-level debugging information. GDB can operate without it, but source names/lines and variables may be unavailable. Optimizations can also affect those views.
The exact binary, source mapping, commands, inputs and environment must accompany a future transcript. Addresses vary; GNU/Linux launch settings can affect address randomization.
Stop before starting GDB here. No breakpoint, address, value or process was observed.

1. Read the six command names. Write the job of break, run, and start in your notes.
2. Check: break sets a breakpoint, run starts the program, and start stops at the beginning of main.
3. Write the limit: no executable exists, so these commands are not run on this page.
4. Stop before any gdb command. Do not attach to a process.

04Read the strings card

The desk-note card is a different object from read-note.c. It is an author's list standing in for a strings listing. The binary this card names is not one you need to build.

Conceptual strings card. Not strings(1) output.
The binary this card describes is not one you need to build.

Name on the card: desk-note
Sequences written by the author of the card:
success
denied
note.txt

This list is a teaching stand-in for a strings listing.
It does not show which sequence a run would use.

1. List the three sequences on the card.
2. Write one sentence that says the card contains both success and denied.
3. Check: your sentence names both sequences and does not choose one as the path a run took.
4. Stop. Do not build desk-note to settle the question.

05Optional challenge

From the primer, name the command that would stop at the beginning of main, and write the reason this page cannot show the address. 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.

FILE *entry = fopen("ledger.txt", "r");
if (entry == 0) { return 3; }
fclose(entry);
return 0;
This snippet reads a library call; no syscall name or actual return was captured.

Stored-string model for an unrelated owned file includes ledger.txt.
Synthetic syscall-event model: an open request returned a failure.
Neither card is a captured run. No matched binary is supplied.

For a separately authorized owned program, plan a stop at main.
GDB commands taught: break main requests a breakpoint; run starts execution.
No debugger is started for this check.

07Explain your evidence

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

Check your explanation: cite the exact library-call path and branch; distinguish source, printable bytes and syscall events; label synthetic evidence and missing matched artifacts; describe the debugger sequence without claiming execution.