minimum is to create an os_log once in the parent, then, after fork, use an existing os_log instance or create a new one. Here's my reproduction sample. It looks like an OS bug, So what is the workaround till its solved and releases. #include <os/log.h> #include <stdio.h> #include <errno.h> #define USE_SWIFT_LOGGER 0 // set to 1 to use Logger() instead of os_log() #define USE_SHARED_LOG 0 // set to 1 to use a shared os_log instance instead of allocating one for each os_log() call #if USE_SWIFT_LOGGER // log messages using Swift's Logger class instead of calling os_log_... directly #include "forktest-Swift.h" #define LOG(msg) log_using_logger([NSString stringWithUTF8String:msg]) const char * logMethod = "Swift's Logger"; #elif USE_SHARED_LOG // use a shared os_log instance for all log messages static os_log_t log = os_log_create("com.ivanti.forktest", "foo"); #define LOG(msg) os_log_debug(log, msg) const char * logMethod = "shared log"; #else // create a new os_log instance for each log message #define LOG(msg) os_log_debug(os_log_create("com.ivanti.forktest", "foo"), msg) const char * logMethod = "new log instance for each message"; #endif static void forkTest() { int pid = fork(); if (pid < 0) { fprintf(stderr, "fork failed: %s\n", strerror(errno)); exit(1); } else if (pid == 0) { printf("in child process...\n"); LOG("in child process..."); printf("child process done\n"); } else { printf("child process pid: %d\n", pid); } } int main(int arc, const char ** argv) { printf("Log method: %s\n", logMethod); LOG("entering main..."); forkTest(); LOG("leaving main..."); return 0; }