Chapter 41 covers the fundamentals of shared libraries on Linux. Two types of object libraries exist on Linux:

The chapter explains how to build, install, version, and use shared libraries, and covers important advanced topics like the dynamic linker’s search algorithm and symbol resolution semantics.

§41.11 — Finding Shared Libraries at Run Time

• The dynamic linker (ld-linux.so) finds and loads shared libraries when a program starts.
• If a dependency string contains a slash → it’s used as a direct path (no searching).
• Otherwise: search order is DT_RPATH → LD_LIBRARY_PATH → DT_RUNPATH → /etc/ld.so.cache → /lib → /usr/lib.
• LD_LIBRARY_PATH is ignored for SUID/SGID programs (security).
• $ORIGIN in rpath expands to the directory of the executable at run time — enables portable “turn-key” applications.
• ldconfig rebuilds /etc/ld.so.cache — must be run after installing a new library.

§41.12 — Run-Time Symbol Resolution

• Global symbols in the main executable override same-named symbols in shared libraries (symbol interposition).
• Between multiple libraries, left-to-right order on the link command line determines priority.
• These semantics mirror static library behavior (for historical compatibility).
• -Bsymbolic makes a library bind its own internal symbol references to its own definitions.
• LD_PRELOAD enables intentional interposition for testing and debugging.
• Weak symbols (__attribute__((weak))) allow overridable default implementations.

§41.13 — Using Static Libraries

• When both .so and .a exist, the linker prefers .so by default.
• Three ways to force static: explicit .a path | -static flag | -Wl,-Bstatic/-Bdynamic toggles.
• -Wl,-Bstatic/-Wl,-Bdynamic allows mixing static and shared libraries selectively.
• Static linking is appropriate when the runtime environment lacks shared libraries or ABI stability is critical.

This end-to-end example builds a shared library, installs it, and shows all the concepts together.

/* ===== libstring.c — our shared library ===== */
#include <string.h>
#include <ctype.h>

/* Count vowels in a string */
int count_vowels(const char *s) {
    int count = 0;
    while (*s) {
        char c = tolower((unsigned char)*s);
        if (c=='a'||c=='e'||c=='i'||c=='o'||c=='u') count++;
        s++;
    }
    return count;
}

/* Reverse a string in-place */
void reverse_str(char *s) {
    int len = strlen(s);
    for (int i = 0, j = len-1; i < j; i++, j--) {
        char tmp = s[i]; s[i] = s[j]; s[j] = tmp;
    }
}

/* Weak symbol — can be overridden by application */
__attribute__((weak)) void on_error(const char *msg) {
    fprintf(stderr, "[libstring default error] %s\n", msg);
}

/* ===== app.c — the application ===== */
#include <stdio.h>
#include <string.h>

int count_vowels(const char *s);
void reverse_str(char *s);

/* Override the weak symbol from the library */
void on_error(const char *msg) {
    fprintf(stderr, "[APP] Error: %s\n", msg);
}

int main(void) {
    char buf[] = "Hello EmbeddedPathashala";
    printf("Vowels in '%s': %d\n", buf, count_vowels(buf));
    reverse_str(buf);
    printf("Reversed: %s\n", buf);
    return 0;
}

# ===== Build and deploy script =====

# 1. Compile shared library
gcc -fPIC -shared -o libstring.so.1 libstring.c
ln -sf libstring.so.1 libstring.so

# 2. Build app with $ORIGIN rpath (portable)
gcc -o app app.c -L. -lstring -Wl,-rpath,'$ORIGIN'

# 3. Check dependencies
ldd ./app
# libstring.so.1 => ./libstring.so.1

# 4. Run — works from current directory (rpath=$ORIGIN)
./app
# Vowels in 'Hello EmbeddedPathashala': 10
# Reversed: alahsahtaPdeddebmE olleH

# 5. System-wide installation alternative:
sudo cp libstring.so.1 /usr/local/lib/
sudo ldconfig
ldconfig -p | grep libstring
# libstring.so.1 => /usr/local/lib/libstring.so.1

# 6. Verify binary details
readelf -d app | grep -E "NEEDED|RPATH|RUNPATH"
file app