What? The person asked for Linux specific, and the equivalent of getpid(). Not BSD or Apple. The answer is gettid() and returns an integral type. You will have to call it using syscall(), like this:

#include <sys/types.h>
#include <unistd.h>
#include <sys/syscall.h>

 ....

 pid_t x = syscall(__NR_gettid);

While this may not be portable to non-linux systems, the threadid is directly comparable and very fast to acquire. It can be printed (such as for LOGs) like a normal integer.

pthread_self() function will give the thread id of current thread.

pthread_t pthread_self(void);

The pthread_self() function returns the Pthread handle of the calling thread. The pthread_self() function does NOT return the integral thread of the calling thread. You must use pthread_getthreadid_np() to return an integral identifier for the thread.

NOTE:

pthread_id_np_t   tid;
tid = pthread_getthreadid_np();

is significantly faster than these calls, but provides the same behavior.

pthread_id_np_t   tid;
pthread_t         self;
self = pthread_self();
pthread_getunique_np(&self, &tid);

As noted in other answers, pthreads does not define a platform-independent way to retrieve an integral thread ID.

On Linux systems, you can get thread ID thus:

#include <sys/types.h>
pid_t tid = gettid();

On many BSD-based platforms, this answer https://stackoverflow.com/a/21206357/316487 gives a non-portable way.

However, if the reason you think you need a thread ID is to know whether you're running on the same or different thread to another thread you control, you might find some utility in this approach

static pthread_t threadA;

// On thread A...
threadA = pthread_self();

// On thread B...
pthread_t threadB = pthread_self();
if (pthread_equal(threadA, threadB)) printf("Thread B is same as thread A.\n");
else printf("Thread B is NOT same as thread A.\n");

If you just need to know if you're on the main thread, there are additional ways, documented in answers to this question how can I tell if pthread_self is the main (first) thread in the process?.