From 3e403451c7dd24c2ffb4f510a3876a152dea0a14 Mon Sep 17 00:00:00 2001
From: Luca Boccassi <luca.boccassi@microsoft.com>
Date: Fri, 29 Mar 2019 16:29:49 +0000
Subject: [PATCH 1/2] Build shared lib by default, add options to turn it off

The vast majority of use cases want a shared library, so to be more
user and packager friendly invert the makefile logic and always build
both static and shared libraries by default.
Add BUILD_STATIC_ONLY variable for the corner cases where only a static
library is needed

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
---
 README       | 7 +++----
 src/Makefile | 4 ++--
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/README b/README
index a48fdde..d894e7e 100644
--- a/README
+++ b/README
@@ -19,14 +19,13 @@ successful.
 
 Build
 =====
-
-To build static library libbpf.a:
+To build both static libbpf.a and shared libbpf.so:
   cd src
   make
 
-To build both static libbpf.a and shared libbpf.so libraries in directory
+To build only static libbpf.a library in directory
 build/ and install them together with libbpf headers in a staging directory
 root/:
   cd src
   mkdir build root
-  BUILD_SHARED=y OBJDIR=build DESTDIR=root make install
+  BUILD_STATIC_ONLY=y OBJDIR=build DESTDIR=root make install
diff --git a/src/Makefile b/src/Makefile
index 51b52e8..4846ab8 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -10,7 +10,7 @@ ifneq ($(FEATURE_REALLOCARRAY),)
 	ALL_CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
 endif
 
-ifdef BUILD_SHARED
+ifndef BUILD_STATIC_ONLY
 	ALL_CFLAGS += -fPIC -fvisibility=hidden
 endif
 
@@ -23,7 +23,7 @@ OBJS := $(addprefix $(OBJDIR)/,bpf.o btf.o libbpf.o libbpf_errno.o netlink.o \
 	nlattr.o str_error.o libbpf_probes.o bpf_prog_linfo.o xsk.o)
 
 LIBS := $(OBJDIR)/libbpf.a
-ifdef BUILD_SHARED
+ifndef BUILD_STATIC_ONLY
 	LIBS += $(OBJDIR)/libbpf.so
 endif
 

From 438584bfc211ec43a59879bc76f8f09af5ea0ee1 Mon Sep 17 00:00:00 2001
From: Luca Boccassi <luca.boccassi@microsoft.com>
Date: Thu, 28 Mar 2019 19:31:46 +0000
Subject: [PATCH 2/2] Makefile: use pkg-config to get libelf flags

libbpf uses libelf symbols internally, so it depends on it and needs to
link against it.

Upstream this was fixed by linking directory to libelf:

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=89dedaef49d36adc2bb5e7e4c38b52fa3013c7c8

For this project, intended to be used as a general library, try first
to use pkg-config instead, and only fallback to just passing -lelf.
This will be useful for cross compiling, among other cases.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
---
 README       | 14 ++++++++++++++
 src/Makefile | 10 +++++++++-
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/README b/README
index d894e7e..9adc1fc 100644
--- a/README
+++ b/README
@@ -19,6 +19,13 @@ successful.
 
 Build
 =====
+libelf is an internal dependency of libbpf and thus it is required to link
+against and must be installed on the system for applications to work.
+pkg-config is used by default to find libelf, and the program called can be
+overridden with PKG_CONFIG.
+If using pkg-config at build time is not desired, it can be disabled by setting
+NO_PKG_CONFIG=1 when calling make.
+
 To build both static libbpf.a and shared libbpf.so:
   cd src
   make
@@ -29,3 +36,10 @@ root/:
   cd src
   mkdir build root
   BUILD_STATIC_ONLY=y OBJDIR=build DESTDIR=root make install
+
+To build both static libbpf.a and shared libbpf.so against a custom libelf
+dependency installed in /build/root/ and install them together with libbpf
+headers in a build directory /build/root/:
+
+  cd src
+  PKG_CONFIG_PATH=/build/root/lib64/pkgconfig DESTDIR=/build/root make install
diff --git a/src/Makefile b/src/Makefile
index 4846ab8..b825956 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -16,6 +16,14 @@ endif
 
 CFLAGS ?= -g -O2 -Werror -Wall
 ALL_CFLAGS += $(CFLAGS)
+ALL_LDFLAGS += $(LDFLAGS)
+ifdef NO_PKG_CONFIG
+	ALL_LDFLAGS += -lelf
+else
+	PKG_CONFIG ?= pkg-config
+	ALL_CFLAGS += $(shell $(PKG_CONFIG) --cflags libelf)
+	ALL_LDFLAGS += $(shell $(PKG_CONFIG) --libs libelf)
+endif
 
 OBJDIR ?= .
 
@@ -54,7 +62,7 @@ $(OBJDIR)/libbpf.a: $(OBJS)
 	$(AR) rcs $@ $^
 
 $(OBJDIR)/libbpf.so: $(OBJS)
-	$(CC) -shared $(LDFLAGS) $^ -o $@
+	$(CC) -shared $(ALL_LDFLAGS) $^ -o $@
 
 $(OBJDIR)/libbpf.pc:
 	sed -e "s|@PREFIX@|$(PREFIX)|" \