strace-ltrace

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SKILL.md

---
name: strace-ltrace
description: strace and ltrace skill for system call and library call tracing. Use when a binary behaves incorrectly without crashing, diagnosing file-not-found errors, permission failures, network issues, or unexpected library calls by tracing syscalls and library function calls. Activates on queries about strace, ltrace, syscall tracing, library interception, ENOENT, EPERM, strace -e, or diagnosing binary behaviour without a debugger.
---

# strace / ltrace

## Purpose

Guide agents through tracing system calls with `strace` and library calls with `ltrace` — the most effective tools for diagnosing incorrect binary behaviour without a crash or debugger.

## Triggers

- "My program behaves incorrectly — how do I trace what it's doing?"
- "How do I find what files a binary is opening?"
- "strace shows ENOENT — how do I interpret it?"
- "How do I trace network calls with strace?"
- "What is ltrace and how does it differ from strace?"
- "How do I trace a running process?"

## Workflow

### 1. Basic strace usage

```bash
# Trace all syscalls of a command
strace ./myapp arg1 arg2

# Attach to running process
strace -p 12345

# Trace child processes too (-f = follow fork)
strace -f ./myapp

# Save to file (raw output — not stdout)
strace ./myapp 2> trace.txt

# Most useful: timestamps + summary
strace -t -f ./myapp 2>&1 | head -100
```

### 2. Filter by syscall category

```bash
# Trace file operations only
strace -e trace=file ./myapp

# Trace network syscalls
strace -e trace=network ./myapp

# Trace specific syscalls
strace -e trace=open,openat,read,write ./myapp

# Trace process management
strace -e trace=process ./myapp

# Trace memory operations
strace -e trace=memory ./myapp

# Trace signals
strace -e trace=signal ./myapp

# Multiple categories
strace -e trace=file,network ./myapp
```

| Category | Syscalls included |
|----------|------------------|
| `file` | open, openat, stat, access, unlink, rename, ... |
| `network` | socket, connect, bind, accept, send, recv, ... |
| `process` | fork, exec, wait, clone, exit, ... |
| `memory` | mmap, munmap, mprotect, brk, ... |
| `signal` | kill, sigaction, sigprocmask, ... |
| `ipc` | pipe, socket pair, shmget, ... |
| `desc` | close, dup, poll, select, epoll, ... |

### 3. Interpreting common errors

```bash
# See return values and errors
strace -e trace=file ./myapp 2>&1 | grep -E "ENOENT|EPERM|EACCES|ENOTSUP"
```

| Error | Meaning | Common cause |
|-------|---------|-------------|
| `ENOENT` | No such file or directory | Config file missing, wrong path |
| `EACCES` | Permission denied | File permissions, SELinux |
| `EPERM` | Operation not permitted | Missing capability, suid needed |
| `EADDRINUSE` | Address already in use | Port already bound |
| `ETIMEDOUT` | Connection timed out | Network unreachable, firewall |
| `ECONNREFUSED` | Connection refused | Server not listening |
| `EAGAIN` | Resource temporarily unavailable | Non-blocking I/O, try again |
| `ENOMEM` | Out of memory | Allocation failed |
| `EBADF` | Bad file descriptor | Using closed/invalid fd |
| `ENOEXEC` | Exec format error | Wrong binary format for arch |

```bash
# Find what file is not found
strace ./myapp 2>&1 | grep 'ENOENT'
# Example output:
# openat(AT_FDCWD, "/etc/myapp.conf", O_RDONLY) = -1 ENOENT (No such file or directory)
# → Config file expected at /etc/myapp.conf
```

### 4. Useful strace flags

```bash
# Show strings fully (default truncates at 32 chars)
strace -s 256 ./myapp

# Timestamps
strace -t ./myapp     # wall clock time
strace -T ./myapp     # time spent in each syscall
strace -r ./myapp     # relative timestamps

# System call count summary
strace -c ./myapp
# Shows count, time, errors per syscall — great for profiling

# Trace with PIDs in output (for -f)
strace -f -p ./myapp
# Output: [pid 12346] open("/etc/passwd", O_RDONLY) = 3

# Decode numerical arguments
strace -e verbose=all ./myapp

# Print instruction pointer at each syscall
strace -i ./myapp
```

### 5. ltrace — library call tracing

```bash
# Trace all library calls
ltrace ./myapp

# Trace specific library function
ltrace -e malloc,free,fopen ./myapp

# Trace nested calls (lib → lib)
ltrace -n 2 ./myapp   # indent nested calls

# Trace with syscalls too
ltrace -S ./myapp

# Attach to running process
ltrace -p 12345

# Summary statistics
ltrace -c ./myapp
```

Typical ltrace output:

```text
malloc(1024) = 0x55a1b2c3d000
fopen("/etc/myapp.conf", "r") = 0
free(0x55a1b2c3d000) = <void>
```

strace vs ltrace:

| | strace | ltrace |
|--|--------|--------|
| Traces | Kernel syscalls | User-space library calls |
| Overhead | Lower | Higher (PLT hooking) |
| Shows | `open()`, `read()`, `write()` | `fopen()`, `malloc()`, `printf()` |
| Use when | Binary interacts with OS/files/network | Binary calls library functions you can't see |

### 6. Practical diagnosis workflows

```bash
# Find missing config file
strace -e trace=openat,open ./myapp 2>&1 | grep ENOENT

# Find what network connections are made
strace -e trace=network -f ./myapp 2>&1 | grep connect

# Debug dynamic library loading failures
strace -e trace=openat ./myapp 2>&1 | grep "\.so"

# Find permission issues
strace -e trace=file ./myapp 2>&1 | grep -E "EACCES|EPERM"

# Debug slow startup (find where time is spent)
strace -c ./myapp 2>&1
# Look for high % time in unexpected syscalls

# Watch IPC/shared memory
strace -e trace=ipc,shm ./myapp

# Find what the binary exec's
strace -e trace=execve -f ./myapp
```

### 7. seccomp filter debugging

If a program is killed by a seccomp policy, strace reveals which syscall triggered it:

```bash
strace -e trace=all ./myapp 2>&1 | tail -5
# Often shows the last syscall before SIGSYS
```

For strace output patterns and ltrace filtering examples, see [references/strace-patterns.md](references/strace-patterns.md).

## Related skills

- Use `skills/debuggers/gdb` when strace shows the failing location and you need to inspect internals
- Use `skills/binaries/elf-inspection` to understand what libraries and symbols a binary uses
- Use `skills/binaries/dynamic-linking` for diagnosing `LD_*` and library loading issues
- Use `skills/profilers/linux-perf` for performance profiling (strace overhead is too high for perf)