c++ compile (g++) fails

[pulmark]

Undefined references during linking and then some warnings. I don't understand why linkage fails, because header while has extern "C" linkage.

When I include the c file directly (#include fpc.c) in my project then it builds.

Below is the log of compile. I edited the make file to use C++ compiler and changed the std library option.

g++ -Wall -O2 -std=c++11 -DNDEBUG fpc.c cli.c -o fpc
fpc.c: In function ‘void byte_count(U8*, int, U32*, int)’:
fpc.c:412:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
412 | for(a = 0;a < (len & (~7));a += 8){
| ~~^~~~~~~~~~~~~~
fpc.c:431:9: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
431 | for(;a < len;)
| ~~^~~~~
fpc.c:433:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
433 | for(a = 0;a < sym_num;a++)
| ~~^~~~~~~~~
fpc.c: In function ‘void sort_inc(Fsym*, int)’:
fpc.c:442:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
442 | for(a = 0;a < num;a++){
| ~~^~~~~
fpc.c:460:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
460 | for(a = 0;a < num;a++){
| ~~^~~~~
fpc.c:466:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
466 | for(a = 0;a < num;a++){
| ~~^~~~~
fpc.c: In function ‘int construct_dec_table(U8*, Dnode*, int)’:
fpc.c:490:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
490 | for(a = 0;a < sym_num;a++){
| ~~^~~~~~~~~
fpc.c:505:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
505 | for(a = 0;a < sym_num;a++){
| ~~^~~~~~~~~
fpc.c: In function ‘void construct_enc_table(Enode*, Fsym*, int)’:
fpc.c:539:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
539 | for(a = 0;a < num;a++){
| ~~^~~~~
fpc.c:548:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
548 | for(a = 0;a < num;a++){
| ~~^~~~~
fpc.c: In function ‘U32 write_prefix_descr(Enode*, U8*, int)’:
fpc.c:676:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
676 | for(a = 0;a < sym_num;a++){
| ~~^~~~~~~~~
fpc.c:680:13: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
680 | while(a+1 < sym_num && previous == lookup[a+1].len)
| ~~~~^~~~~~~~~
fpc.c: In function ‘int FPC_compress_block(void*, const void*, int, int)’:
fpc.c:913:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
913 | for(a = 0;a < sym_num;a++)
| ~~^~~~~~~~~
fpc.c:924:14: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
924 | for(a = 0;a < sym_num && s[a].freq == 0;a++);
| ^~~~~~~~~
fpc.c:951:21: warning: comparison of integer expressions of different signedness: ‘U32’ {aka ‘unsigned int’} and ‘int’ [-Wsign-compare]
951 | if(compressed_size >= size){
| ~~~~~~~~~~~~~~~~^~~~~~~
fpc.c: In function ‘size_t comp_adaptive(void*, void*, size_t)’:
fpc.c:1038:41: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Wsign-compare]
1038 | for(int c = 1;c <= MBLOCK && (cSTEP) <= inlen - a;c++){
| ~~~~~~~~~^~~~~~~~~~~~
fpc.c: In function ‘size_t FPC_compress(void, void*, size_t, int)’:
fpc.c:77:23: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
77 | #define MIN(A,B) ((A) < (B)?(A):(B))
| ~~~~^~~~~
fpc.c:1095:14: note: in expansion of macro ‘MIN’
1095 | U32 step = MIN(inlen,bsize);
| ^
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: /tmp/ccwLzlL2.o: in function dec_file(_IO_FILE*, _IO_FILE*) [clone .part.0]': cli.c:(.text+0xae): undefined reference to FPC_decompress_block'
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: /tmp/ccwLzlL2.o: in function comp_file(_IO_FILE*, _IO_FILE*, int)': cli.c:(.text+0x2cc): undefined reference to FPC_compress'
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: /tmp/ccwLzlL2.o: in function bench_file(_IO_FILE*, unsigned int, int)': cli.c:(.text+0x528): undefined reference to FPC_compress'
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: cli.c:(.text+0x543): undefined reference to FPC_compress' /usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: cli.c:(.text+0x55e): undefined reference to FPC_compress'
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: cli.c:(.text+0x577): undefined reference to FPC_compress' /usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: cli.c:(.text+0x5d5): undefined reference to FPC_decompress'
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: cli.c:(.text+0x5f0): undefined reference to FPC_decompress' /usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: cli.c:(.text+0x60b): undefined reference to FPC_decompress'
/usr/lib64/gcc/x86_64-suse-linux/9/../../../../x86_64-suse-linux/bin/ld: cli.c:(.text+0x624): undefined reference to `FPC_decompress'
collect2: error: ld returned 1 exit status
make: *** [Makefile:5: fpc] Error 1

[kagiannis]

Hello

extern "C" is used when fpc.c is compiled as c but then included by a c++ application

To make it work you should either compile fpc.c seperately as c application

gcc -Wall -O2 -DNDEBUG -c fpc.c -o fpc.o 

and then link it with c++

g++ -Wall -O2 -DNDEBUG cli.c fpc.o -o fpc

or if you want to compile both with g++ you should remove extern "C"

[kagiannis]

Also you should notice that fpc.c uses variable length arrays that they are not present in standard c++. They are included only in standard c (C99 and above). So it can be compiled as c++ only with compilers that support variable length arrays as extension like g++ and clang++.

[pulmark]

Thanks for the info. I made a static lib Makefile for fpc.c by using g++ and it works. Probably will use it instead of just using the #include "fpc.c" trick.

[pulmark]

Makefile for libfpc.a:

CC = g++
TARGET = libfpc.a
CFLAGS= -pipe -fPIC -O3 -g -Wall -c -march=native -mtune=native

INC = -I./
OBJ_DIR=./obj
OUT_DIR=./
SRC_DIR=./

#Lib for fpc.c
FILES = $(SRC_DIR)/fpc.c
$(TARGET): $(patsubst $(SRC_DIR)/%.c,$(OBJ_DIR)/%.o,$(FILES))
ar -rcs -o $(OUT_DIR)/$@ $^

#Compiling every *.c to *.o
$(OBJ_DIR)/%.o: $(SRC_DIR)/%.c dirmake
$(CC) -c $(INC) $(CFLAGS) -o $@ $<

dirmake:
@mkdir -p $(OUT_DIR)
@mkdir -p $(OBJ_DIR)

clean:
rm -f $(OBJ_DIR)/*.o $(OUT_DIR)/$(TARGET)

rebuild: clean build