This is a strange error to get on Debian sid amd64 : /usr/bin/gcc-10 -I. -Iinclude -fPIC -pthread -std=iso9899:1999 -m64 -g -O0 -march=k8 -mtune=k8 -Wl,-rpath=/opt/bw/lib,--enable-new-dtags -fno-builtin -malign-double -mpc80 -std=iso9899:1999 -m64 -g -O0 -march=k8 -mtune=k8 -Wl,-rpath=/opt/bw/lib,--enable-new-dtags -fno-builtin -malign-double -mpc80 -DOPENSSL_USE_NODELETE -DL_ENDIAN -DOPENSSL_PIC -DOPENSSLDIR="\"/opt/bw/ssl\"" -DENGINESDIR="\"/opt/bw/lib/engines-1.1\"" -I/opt/bw/include -MMD -MF crypto/threads_pthread.d.tmp -MT crypto/threads_pthread.o -c -o crypto/threads_pthread.o crypto/threads_pthread.c crypto/threads_pthread.c: In function ‘CRYPTO_THREAD_lock_new’: crypto/threads_pthread.c:48:5: warning: implicit declaration of function ‘pthread_mutexattr_settype’; did you mean ‘pthread_mutexattr_destroy’? [-Wimplicit-function-declaration] 48 | pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE); | ^~~~~~~~~~~~~~~~~~~~~~~~~ | pthread_mutexattr_destroy crypto/threads_pthread.c:48:38: error: ‘PTHREAD_MUTEX_RECURSIVE’ undeclared (first use in this function); did you mean ‘PTHREAD_MUTEX_RECURSIVE_NP’? 48 | pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE); | ^~~~~~~~~~~~~~~~~~~~~~~ | PTHREAD_MUTEX_RECURSIVE_NP crypto/threads_pthread.c:48:38: note: each undeclared identifier is reported only once for each function it appears in make[1]: *** [Makefile:5104: crypto/threads_pthread.o] Error 1 make[1]: Leaving directory '/opt/bw/build/openssl-1.1.1h_debian_sid_5.8.0-2-amd64.004' make: *** [Makefile:174: all] Error 2 Command exited with non-zero status 2 Why should the include of pthread.h be absent here ? -- Dennis Clarke RISC-V/SPARC/PPC/ARM/CISC UNIX and Linux spoken GreyBeard and suspenders optional