getting error in c program "undefined reference to gettid"

Try

#include <unistd.h>#include <sys/syscall.h>#ifdef SYS_gettidpid_t tid = syscall(SYS_gettid);#else#error "SYS_gettid unavailable on this system"#endif

Macro to be pasted (improved over previous answer):

#include <unistd.h>#include <sys/syscall.h>#ifndef SYS_gettid#error "SYS_gettid unavailable on this system"#endif#define gettid() ((pid_t)syscall(SYS_gettid))

Example:

#include <unistd.h>#include <sys/syscall.h>#ifndef SYS_gettid#error "SYS_gettid unavailable on this system"#endif#define gettid() ((pid_t)syscall(SYS_gettid))#include <stdio.h>void main(){        printf("tid is %d\n", gettid());}

I have followed the suggestions provided on errro and corrected the source*below is the error free code *

#include <pthread.h>#include <stdio.h>#include <sys/types.h>#include <unistd.h>#include <sys/syscall.h>#define ARRAYSIZE 17#define NUMTHREADS 4struct ThreadData {    int start, stop;    int* array; };void* squarer(void* td) { struct ThreadData* data=(struct ThreadData*) td; int start=data->start; int stop=data->stop; int* array=data->array; int i; pid_t tid1; tid1 = syscall(SYS_gettid); // here is the correct statement // printf("tid : %d\n",tid1); for (i=start; i<stop; i++) {         sleep(1);         array[i]=i*i;         printf("arr[%d] = [%d]\n",i,array[i]); }  return NULL;}int main(void) {    int array[ARRAYSIZE];    pthread_t thread[NUMTHREADS];    struct ThreadData data[NUMTHREADS];    int i;    int tasksPerThread=(ARRAYSIZE+NUMTHREADS-1)/NUMTHREADS;    for (i=0; i<NUMTHREADS; i++) {        data[i].start=i*tasksPerThread;        data[i].stop=(i+1)*tasksPerThread;        data[i].array=array;    }    data[NUMTHREADS-1].stop=ARRAYSIZE;    for (i=0; i<NUMTHREADS; i++) {            pthread_create(&thread[i], NULL, squarer, &data[i]);    }    for (i=0; i<NUMTHREADS; i++) {            pthread_join(thread[i], NULL);    }    for (i=0; i<ARRAYSIZE; i++) {            printf("%d ", array[i]);    }    printf("\n");    return 0;}