How to print pthread_t

This will print out a hexadecimal representation of a pthread_t, no matter what that actually is:

void fprintPt(FILE *f, pthread_t pt) {  unsigned char *ptc = (unsigned char*)(void*)(&pt);  fprintf(f, "0x");  for (size_t i=0; i<sizeof(pt); i++) {    fprintf(f, "%02x", (unsigned)(ptc[i]));  }}

To just print a small id for a each pthread_t something like this could be used (this time using iostreams):

void printPt(std::ostream &strm, pthread_t pt) {  static int nextindex = 0;  static std::map<pthread_t, int> ids;  if (ids.find(pt) == ids.end()) {    ids[pt] = nextindex++;  }  strm << ids[pt];}

Depending on the platform and the actual representation of pthread_t it might here be necessary to define an operator< for pthread_t, because std::map needs an ordering on the elements:

bool operator<(const pthread_t &left, const pthread_t &right) {  ...}

GDB uses the thread-id (aka kernel pid, aka LWP) for short numbers on Linux. Try:

  #include <syscall.h>  ...    printf("tid = %d\n", syscall(SYS_gettid));

In this case, it depends on the operating system, since the POSIX standard no longer requires pthread_t to be an arithmetic type:

IEEE Std 1003.1-2001/Cor 2-2004, item XBD/TC2/D6/26 is applied, adding pthread_t to the list of types that are not required to be arithmetic types, thus allowing pthread_t to be defined as a structure.

You will need to look in your sys/types.h header and see how pthread_t is implemented; then you can print it how you see fit. Since there isn't a portable way to do this and you don't say what operating system you are using, there's not a whole lot more to say.

Edit: to answer your new question, GDB assigns its own thread ids each time a new thread starts:

For debugging purposes, gdb associates its own thread number—always a single integer—with each thread in your program.

If you are looking at printing a unique number inside of each thread, your cleanest option would probably be to tell each thread what number to use when you start it.