Hello everyone: I append a simple syscall in kernel. and the function is as follows: asmlinkage long sys_mysyscall(long data) { printk("This is my syscall!\n"); return data; } and i test it sucessfully in user space . and the test program: #include <linux/unistd.h> #include <syscall.h> #include <sys/types.h> #include <stdio.h> int main(void) { long n = 0,m = 0,pid1,pid2; n = syscall(345,190);// #define __NR_mysyscall 345 printf("n = %ld\n",n); pid1 = syscall(SYS_getpid); //getpid printf("pid = %ld\n",pid1); pid2 = syscall(20); //getpid printf("pid = %ld\n",pid2); return 0; } and the result: n = 190 pid = 4097 pid = 4097 but if the test program is: #include <linux/unistd.h> #include <syscall.h> #include <sys/types.h> #include <stdio.h> int main(void) { long n = 0,m = 0,pid1,pid2; n = syscall(345,190);// #define __NR_mysyscall 345 printf("n = %ld\n",n); m = syscall(SYS_mysyscall,190); printf("m = %ld\n",m); pid1 = syscall(SYS_getpid); //getpid printf("pid = %ld\n",pid1); pid2 = syscall(20); //getpid printf("pid = %ld\n",pid2); return 0; } and the result: wanny@wanny-C-Notebook-XXXX:~/syscall/src$ gcc test1.c test1.c: In function ‘main’: test1.c:13:14: error: ‘SYS_mysyscall’ undeclared (first use in this function) test1.c:13:14: note: each undeclared identifier is reported only once for each function it appears in why i can't invoke my syscall with "SYS_mysyscall"? Thanks in advance!