/tmp/ccDU0AnY.o: In function `thread_loop':
udpclient.c:(.text+0x492): undefined reference to `gettid'
collect2: error: ld returned 1 exit status
Makefile:8: recipe for target 'build' failed
make: *** [build] Error 1
 

在ubuntu 14.04上面编译程序，出现找不到gettid的问题。

在ubuntu 20.04上没问题

解决办法就是自己实现一个gettid 获取线程的id。

#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());
}

据说也有人这样解决：

I solved my similar problem by define _GNU_SOURCE. Last question :

gcc -D_GNU_SOURCE source.c solve problem. Adding #define _GNU_SOURCE in code, before #include <unistd.h> have no effect.

#define _GNU_SOURCE
#include <unistd.h>

参考

getting error in c program "undefined reference to gettid" - Stack Overflow

还是要懂英文呀