c - LD_PRELOAD problems with pthread_create -

i playing around concept of ld_preload. seems works fine until start use pthread library functions in code. when that, segmentation fault. ld_preload has kind of aversion pthread library or what?

shown here both program , ld_preloaded .so code.

program code

// main.c #include <pthread.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <sys/mman.h> #include <sys/types.h>  #define use_pthreads  #define num_threads     8  pthread_mutex_t m = pthread_mutex_initializer;  int sum; float total = 1; extern int __did_libc_start_main;  void *printhello(void *threadid) {    long tid;    tid = (long)threadid;    pthread_mutex_lock( &m );    sum++;    total *= total + tid * 0.097891313423578;    printf( "p%d, tid%d, total = %g, start = %d!\n", getpid(), tid,        total, 0 );    pthread_mutex_unlock( &m );    printf("hello world! it's me, thread #%ld!\n", tid);    pthread_exit(null); }  int main (int argc, char *argv[]) {    pthread_t threads[num_threads];    int rc;    long t;    char * p;    char * m;     fork();     p = (char*)mmap(0, 4096, prot_read|prot_write, map_private|map_anonymous,       -1, 0);    p[0] = 78;    printf( "p = %p, p[0] = %d, pid = %d!\n", p, p[0], getpid() );    m = (char*)malloc( 80 );    printf( "m = %p!\n", m ); #ifdef use_pthreads // if disable part of code, ld_preload works fine    for(t=0; t<num_threads; t++)    {       printf("in main: creating thread %ld\n", t);       rc = pthread_create(&threads[t], null, printhello, (void *)t);       if (rc){          printf("error; return code pthread_create() %d\n", rc);          exit(-1);       }    }    for(t=0; t<num_threads; t++)     pthread_join(threads[t], null);     printf( "\n\ntotal = %g\n\n", total );     /* last thing main() should */    pthread_exit(null); #endif    printf( "\n\n%d: done without major problems\n\n", getpid() );    return 0; } 

ld_preloaded .so code

// prelib.c #define _gnu_source  //#include <sys/syscall.h> //#include <sys/types.h> #include <unistd.h> #include <stdio.h> #include <dlfcn.h>  typedef pid_t (*getpidtype)(void); typedef void* (*mmaptype)(void *addr, size_t len, int prot, int flags,        int fildes, off_t off);  pid_t getpid(void) {     static int first_time = 1;     static getpidtype f;      printf("hello, getpid!\n");      if ( first_time )     {         f = (getpidtype)dlsym(rtld_next, "getpid");         first_time = 0;     }      return f(); } void *mmap(void *addr, size_t len, int prot, int flags,        int fildes, off_t off) {     static int first_time = 1;     static mmaptype f;      printf( "my mmap!\n" );     if ( first_time )     {         f = (mmaptype)dlsym(rtld_next, "mmap" );         first_time = 0;     }      return f( addr, len, prot, flags, fildes, off ); } 

i error when use_pthreads defined.

my mmap! << mutex_trylock >> << mutex_unlock >> hello, getpid! p = 0x2ab9bc969000, p[0] = 78, pid = 9763! m = 0x179d4010! in main: creating thread 0 mmap! << mutex_trylock >> mmap! << mutex_trylock >> << mutex_unlock >> hello, getpid! p = 0x2ab9bc969000, p[0] = 78, pid = 9762! m = 0x179d4010! in main: creating thread 0 mmap! << mutex_trylock >> segmentation fault 

i doubt problem, "first_time" hack not synchronized , not thread-safe. should using pthread_once function purpose, or roll own version mutex or semaphore.