一、问题描述

下载了nmealib库文件，默认工程进行编译，报错，提示如下：

gcc -I include  -c src/generate.c -o build/nmea_gcc/generate.o
gcc -I include  -c src/generator.c -o build/nmea_gcc/generator.o
ar rsc lib/libnmea.a build/nmea_gcc/generate.o build/nmea_gcc/generator.o build/nmea_gcc/parse.o build/nmea_gcc/parser.o build/nmea_gcc/tok.o build/nmea_gcc/context.o build/nmea_gcc/time.o build/nmea_gcc/info.o build/nmea_gcc/gmath.o build/nmea_gcc/sentence.o
ranlib lib/libnmea.a
gcc -I include  -c samples/generate/main.c -o samples/generate/main.o
gcc samples/generate/main.o -Llib -lnmea -o build/samples_generate
/usr/bin/ld: lib/libnmea.a(generate.o): in function `nmea_gsv_npack':
generate.c:(.text+0x729): undefined reference to `ceil'
collect2: error: ld returned 1 exit status
make: *** [Makefile:43: samples_generate] Error 1
rm samples/generate/main.o

网上搜索，因为使用了 ceil数学相关的库，说是链接需要添加 -lm 的参数。

二、解决办法

官方的 Makefile 文件

CC = gcc

BIN = lib/libnmea.a 
MODULES = generate generator parse parser tok context time info gmath sentence  
SAMPLES = generate generator parse parse_file math
 
OBJ = $(MODULES:%=build/nmea_gcc/%.o) 
LINKOBJ = $(OBJ) $(RES)

SMPLS = $(SAMPLES:%=samples_%)
SMPLOBJ = $(SAMPLES:%=samples/%/main.o)

INCS = -I include 
LIBS = -lm -Llib -lnmea

 
.PHONY: all all-before all-after clean clean-custom doc
 
all: all-before $(BIN) samples all-after 

all-before:
	mkdir -p build/nmea_gcc

clean: clean-custom 
	rm -f $(LINKOBJ) $(BIN) $(SMPLOBJ) $(SMPLS)

doc:
	$(MAKE) -C doc
	
remake: clean all

$(BIN): $(LINKOBJ)
	ar rsc $@ $^
	ranlib $@

build/nmea_gcc/%.o: src/%.c 
	$(CC) $(INCS) -c $< -o $@

samples: $(SMPLS)

samples_%: samples/%/main.o
	$(CC) $< $(LIBS) -o build/$@

samples/%/main.o: samples/%/main.c
	$(CC) $(INCS) -c $< -o $@

可以看到 LIBS 已经包含了 lm 。目前的报错该怎么解决呢？解决办法：修改 LIBS 中 lm 的位置，放在后面。 如下：

LIBS = -Llib -lnmea -lm

再次编译，官方工程编译成功。