C - undefined reference to - c

Question

I am currently working on a project in C. I searched through multiple similar questions, but the answers are too unspecific for me, because I haven't really worked with C in the past.
My problem is the following:
The project consists of multiple different files. I am editing the file "pm3/armsrc/epa.c". Now i need to use a function from another file. This function is located in "pm3/common/mbedtls/sha1.c.
I tried to include this file in my epa.c like this:
#include "../common/mbedtls/sha1.h"
It seems to work and I could access the function. But after starting the Makefile in order to compile the project (which worked fine until now), i get an error message in epa.c:
undefined reference to `mbedtls_sha1_ret'
In other answers it seems to be a problem with the linker and the gcc call, but the Makefile, which calls multiple other Makefiles in order to build everything are all pre-made. As you can see here in the "make all" output, the file with the needed function is successfully builded:
Compiling mbedtls
cd ../common/mbedtls && make all
make[2]: Entering directory '/pm3/common/mbedtls'
ar rcs libmbedtls.a aes.o asn1parse.o asn1write.o base64.o bignum.o ctr_drbg.o entropy_poll.o entropy.o error.o timing.o ecp.o ecp_curves.o certs.o camellia.o blowfish.o cipher_wrap.o cipher.o cmac.o des.o ecdsa.o md.o md_wrap.o md5.o oid.o pem.o arc4.o pk.o pk_wrap.o pkwrite.o pkcs5.o pkcs12.o pkparse.o platform.o platform_util.o rsa.o rsa_internal.o sha1.o sha256.o sha512.o threading.o x509.o x509_crl.o x509_crt.o
ranlib libmbedtls.a
make[2]: Leaving directory '/pm3/common/mbedtls'
If someone could maybe take a look at the Makefiles or if someone has another possibility in order to get the compilation working, I would be very happy. Here is the link to the git-repo: https://github.com/Proxmark/proxmark3. My knowledge of Makefiles is very low, so I don't see any problems in the pre-made ones.

Answer 1

Without diving into the makefile, you did use the include directive correctly,
but probably the executable's make command (link) did not include the
library you now used (and refernced).
Look for the place in the makefile where there is a call to the linker
(gcc or maybe g++ or ld), and see if the library (probably mbedtls) appears on the
list of files being linked.