dynamic-linking

Name: dynamic-linking
Author: mohitmishra786

311installs19forks

How to install dynamic-linking?

mohitmishra786/low-level-dev-skills/dynamic-linking

$npx -y skills add mohitmishra786/low-level-dev-skills --skill dynamic-linking

Installs into the current project.

Summary

Dynamic linking skill for Linux/ELF shared libraries. Use when debugging library loading failures, configuring RPATH vs RUNPATH, understanding soname versioning, using dlopen/dlsym for plugin systems, LD_PRELOAD interposition, or controlling symbol visibility. Activates on querie

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SKILL.md

1	# Dynamic Linking
2
3	## Purpose
4
5	Guide agents through Linux dynamic linking: shared library creation, RPATH/RUNPATH configuration, soname versioning, `dlopen`/`dlsym` plugin patterns, `LD_PRELOAD` interposition, and symbol visibility control.
6
7	## Triggers
8
9	- "Cannot open shared object file: No such file or directory"
10	- "How do I set RPATH so my binary finds its shared library?"
11	- "How do I use dlopen/dlsym for a plugin system?"
12	- "What's the difference between RPATH and RUNPATH?"
13	- "How do I use LD_PRELOAD to intercept a function?"
14	- "How do I version my shared library with soname?"
15
16	## Workflow
17
18	### 1. Creating a shared library
19
20	```bash
21	# Compile with -fPIC (position-independent code)
22	gcc -fPIC -c src/mylib.c -o mylib.o
23
24	# Link shared library with soname
25	gcc -shared -Wl,-soname,libmylib.so.1 \
26	mylib.o -o libmylib.so.1.2.3
27
28	# Create symlinks (standard convention)
29	ln -s libmylib.so.1.2.3 libmylib.so.1 # soname link (used by ldconfig)
30	ln -s libmylib.so.1 libmylib.so # link link (used at compile time)
31
32	# Register with ldconfig (system-wide)
33	sudo cp libmylib.so.1.2.3 /usr/local/lib/
34	sudo ldconfig
35	```
36
37	### 2. Soname versioning convention
38
39	```text
40	libfoo.so.MAJOR.MINOR.PATCH
41	│
42	└── soname = libfoo.so.MAJOR
43	```
44
45	\| Version bump \| When \|
46	\|-------------\|------\|
47	\| PATCH \| Bug fix, ABI unchanged \|
48	\| MINOR \| New symbols added, backwards compatible \|
49	\| MAJOR \| ABI break — existing binaries will break \|
50
51	Inspect soname:
52
53	```bash
54	readelf -d libmylib.so.1.2.3 \| grep SONAME
55	objdump -p libmylib.so.1.2.3 \| grep SONAME
56	```
57
58	### 3. RPATH vs RUNPATH
59
60	```text
61	Both embed a library search path in the binary.
62
63	RPATH → searched BEFORE LD_LIBRARY_PATH
64	RUNPATH → searched AFTER LD_LIBRARY_PATH (controllable at runtime)
65
66	Recommendation: prefer RUNPATH (-Wl,--enable-new-dtags)
67	for deployment flexibility.
68	```
69
70	```bash
71	# Embed RPATH (old default)
72	gcc main.c -L./lib -lmylib \
73	-Wl,-rpath,'$ORIGIN/../lib' -o myapp
74
75	# Embed RUNPATH (new default with --enable-new-dtags)
76	gcc main.c -L./lib -lmylib \
77	-Wl,-rpath,'$ORIGIN/../lib' \
78	-Wl,--enable-new-dtags -o myapp
79
80	# Inspect
81	readelf -d myapp \| grep -E 'RPATH\|RUNPATH'
82	chrpath -l myapp # show
83	chrpath -r '/new/path' myapp # modify existing
84	```
85
86	`$ORIGIN` resolves to the directory of the binary at runtime — use it for relocatable installations.
87
88	### 4. Library search order
89
90	```text
91	1. DT_RPATH (if no DT_RUNPATH present)
92	2. LD_LIBRARY_PATH (env var, ignored for suid binaries)
93	3. DT_RUNPATH
94	4. /etc/ld.so.cache (populated by ldconfig from /etc/ld.so.conf)
95	5. /lib, /usr/lib
96	```
97
98	Debug with:
99
100	```bash
101	LD_DEBUG=libs ./myapp # trace library loading decisions
102	ldd myapp # show resolved libraries
103	ldd -v myapp # verbose with version requirements
104	```
105
106	### 5. dlopen / dlsym plugin pattern
107
108	```c
109	#include <dlfcn.h>
110
111	typedef int (plugin_fn_t)(const char input);
112
113	void load_plugin(const char *path) {
114	// RTLD_NOW: resolve all symbols immediately (fail fast)
115	// RTLD_LAZY: resolve on first call (default)
116	// RTLD_LOCAL: symbols not visible to other loaded libs
117	// RTLD_GLOBAL: symbols visible globally
118	void *handle = dlopen(path, RTLD_NOW \| RTLD_LOCAL);
119	if (!handle) {
120	fprintf(stderr, "dlopen: %s\n", dlerror());
121	return;
122	}
123
124	// Clear previous errors
125	dlerror();
126
127	plugin_fn_t fn = (plugin_fn_t)dlsym(handle, "plugin_run");
128	const char *err = dlerror();
129	if (err) {
130	fprintf(stderr, "dlsym: %s\n", err);
131	dlclose(handle);
132	return;
133	}
134
135	fn("hello");
136	dlclose(handle);
137	}
138	```
139
140	Link with `-ldl`:
141
142	```bash
143	gcc main.c -ldl -o myapp
144	```
145
146	### 6. LD_PRELOAD interposition
147
148	`LD_PRELOAD` loads a library before all others — its symbols override the application's.
149
150	```c
151	// myinterpose.c — intercept malloc
152	#define _GNU_SOURCE
153	#include <stdio.h>
154	#include <dlfcn.h>
155
156	void *malloc(size_t size) {
157	static void (real_malloc)(size_t) = NULL;
158	if (!real_malloc)
159	real_malloc = dlsym(RTLD_NEXT, "malloc"); // find next malloc in chain
160
161	void *ptr = real_malloc(size);
162	fprintf(stderr, "malloc(%zu) = %p\n", size, ptr);
163	return ptr;
164	}
165	```
166
167	```bash
168	gcc -shared -fPIC -o myinterpose.so myinterpose.c -ldl
169
170	# Apply to any binary
171	LD_PRELOAD=./myinterpose.so ./myapp
172	LD_PRELOAD=/path/to/libfaketime.so ./myapp # time manipulation
173	```
174
175	### 7. Symbol visibility control
176
177	Limit exported symbols to reduce binary size and avoid clashes:
178
179	```c
180	// Mark default: visible to linker
181	__attribute__((visibility("defaul

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Type

dynamic-linking

311installs19forks

View on GitHub

How to install dynamic-linking?

mohitmishra786/low-level-dev-skills/dynamic-linking

$npx -y skills add mohitmishra786/low-level-dev-skills --skill dynamic-linking

Installs into the current project.

Summary

Files · 1

View on GitHub

SKILL.md

1	# Dynamic Linking
2
3	## Purpose
4
5	Guide agents through Linux dynamic linking: shared library creation, RPATH/RUNPATH configuration, soname versioning, `dlopen`/`dlsym` plugin patterns, `LD_PRELOAD` interposition, and symbol visibility control.
6
7	## Triggers
8
9	- "Cannot open shared object file: No such file or directory"
10	- "How do I set RPATH so my binary finds its shared library?"
11	- "How do I use dlopen/dlsym for a plugin system?"
12	- "What's the difference between RPATH and RUNPATH?"
13	- "How do I use LD_PRELOAD to intercept a function?"
14	- "How do I version my shared library with soname?"
15
16	## Workflow
17
18	### 1. Creating a shared library
19
20	```bash
21	# Compile with -fPIC (position-independent code)
22	gcc -fPIC -c src/mylib.c -o mylib.o
23
24	# Link shared library with soname
25	gcc -shared -Wl,-soname,libmylib.so.1 \
26	mylib.o -o libmylib.so.1.2.3
27
28	# Create symlinks (standard convention)
29	ln -s libmylib.so.1.2.3 libmylib.so.1 # soname link (used by ldconfig)
30	ln -s libmylib.so.1 libmylib.so # link link (used at compile time)
31
32	# Register with ldconfig (system-wide)
33	sudo cp libmylib.so.1.2.3 /usr/local/lib/
34	sudo ldconfig
35	```
36
37	### 2. Soname versioning convention
38
39	```text
40	libfoo.so.MAJOR.MINOR.PATCH
41	│
42	└── soname = libfoo.so.MAJOR
43	```
44
45	\| Version bump \| When \|
46	\|-------------\|------\|
47	\| PATCH \| Bug fix, ABI unchanged \|
48	\| MINOR \| New symbols added, backwards compatible \|
49	\| MAJOR \| ABI break — existing binaries will break \|
50
51	Inspect soname:
52
53	```bash
54	readelf -d libmylib.so.1.2.3 \| grep SONAME
55	objdump -p libmylib.so.1.2.3 \| grep SONAME
56	```
57
58	### 3. RPATH vs RUNPATH
59
60	```text
61	Both embed a library search path in the binary.
62
63	RPATH → searched BEFORE LD_LIBRARY_PATH
64	RUNPATH → searched AFTER LD_LIBRARY_PATH (controllable at runtime)
65
66	Recommendation: prefer RUNPATH (-Wl,--enable-new-dtags)
67	for deployment flexibility.
68	```
69
70	```bash
71	# Embed RPATH (old default)
72	gcc main.c -L./lib -lmylib \
73	-Wl,-rpath,'$ORIGIN/../lib' -o myapp
74
75	# Embed RUNPATH (new default with --enable-new-dtags)
76	gcc main.c -L./lib -lmylib \
77	-Wl,-rpath,'$ORIGIN/../lib' \
78	-Wl,--enable-new-dtags -o myapp
79
80	# Inspect
81	readelf -d myapp \| grep -E 'RPATH\|RUNPATH'
82	chrpath -l myapp # show
83	chrpath -r '/new/path' myapp # modify existing
84	```
85
86	`$ORIGIN` resolves to the directory of the binary at runtime — use it for relocatable installations.
87
88	### 4. Library search order
89
90	```text
91	1. DT_RPATH (if no DT_RUNPATH present)
92	2. LD_LIBRARY_PATH (env var, ignored for suid binaries)
93	3. DT_RUNPATH
94	4. /etc/ld.so.cache (populated by ldconfig from /etc/ld.so.conf)
95	5. /lib, /usr/lib
96	```
97
98	Debug with:
99
100	```bash
101	LD_DEBUG=libs ./myapp # trace library loading decisions
102	ldd myapp # show resolved libraries
103	ldd -v myapp # verbose with version requirements
104	```
105
106	### 5. dlopen / dlsym plugin pattern
107
108	```c
109	#include <dlfcn.h>
110
111	typedef int (plugin_fn_t)(const char input);
112
113	void load_plugin(const char *path) {
114	// RTLD_NOW: resolve all symbols immediately (fail fast)
115	// RTLD_LAZY: resolve on first call (default)
116	// RTLD_LOCAL: symbols not visible to other loaded libs
117	// RTLD_GLOBAL: symbols visible globally
118	void *handle = dlopen(path, RTLD_NOW \| RTLD_LOCAL);
119	if (!handle) {
120	fprintf(stderr, "dlopen: %s\n", dlerror());
121	return;
122	}
123
124	// Clear previous errors
125	dlerror();
126
127	plugin_fn_t fn = (plugin_fn_t)dlsym(handle, "plugin_run");
128	const char *err = dlerror();
129	if (err) {
130	fprintf(stderr, "dlsym: %s\n", err);
131	dlclose(handle);
132	return;
133	}
134
135	fn("hello");
136	dlclose(handle);
137	}
138	```
139
140	Link with `-ldl`:
141
142	```bash
143	gcc main.c -ldl -o myapp
144	```
145
146	### 6. LD_PRELOAD interposition
147
148	`LD_PRELOAD` loads a library before all others — its symbols override the application's.
149
150	```c
151	// myinterpose.c — intercept malloc
152	#define _GNU_SOURCE
153	#include <stdio.h>
154	#include <dlfcn.h>
155
156	void *malloc(size_t size) {
157	static void (real_malloc)(size_t) = NULL;
158	if (!real_malloc)
159	real_malloc = dlsym(RTLD_NEXT, "malloc"); // find next malloc in chain
160
161	void *ptr = real_malloc(size);
162	fprintf(stderr, "malloc(%zu) = %p\n", size, ptr);
163	return ptr;
164	}
165	```
166
167	```bash
168	gcc -shared -fPIC -o myinterpose.so myinterpose.c -ldl
169
170	# Apply to any binary
171	LD_PRELOAD=./myinterpose.so ./myapp
172	LD_PRELOAD=/path/to/libfaketime.so ./myapp # time manipulation
173	```
174
175	### 7. Symbol visibility control
176
177	Limit exported symbols to reduce binary size and avoid clashes:
178
179	```c
180	// Mark default: visible to linker
181	__attribute__((visibility("defaul

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