LINUX_VERSION_CODE where the value from？ - linux

Question

I just make a mistake about the macro LINUX_VERSION_CODE.
Here is what I want to do,when execute the code,the code run different branch due to the machine ,which is running.
I just realized,the result is not change,since I compile my code always in the same server.For example I compile the belw code in kernel version of 2.6.18,then the LINUX_VERSION_CODE marco is always 132626 .
If there is any way ,let the run different branch due to the version of which it runs?
#include <stdio.h>
#include <linux/version.h>
int main()
{
printf("kernel version(2.6.32) = %d\n",KERNEL_VERSION(2,6,32));
printf("LINUX_VERSION_CODE = %d\n",LINUX_VERSION_CODE);
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,32)
printf("Hello,world.\n");
#else
printf("Fine,thank you.\n");
#endif
return 0;
}

Answer 1

So you want to run a different code depending on kernel version. But you don't want to decide on that using a compile time constant - you want that at runtime.
Nothing simpler, but a call to uname:
#include <sys/utsname.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
struct utsname name;
if (uname(&name)) {
fprintf(stderr, "Uname failed!\n");
exit(-1);
}
printf("%s\n", name.release);
if (!memcmp(&name.release, "4.18.9-", sizeof("4.18.9-") - 1)) {
printf("Och, kernel 4.18.9 found!\n");
} else {
printf("Och, you've got a different kernel...\n");
}
}
On my machine:
$ cat 1.c | gcc -xc - && ./a.out
4.18.9-arch1-1-ARCH
Och, kernel 4.18.9 found!
On my friends machine:
cat 1.c | ssh friend 'gcc -xc - && ./a.out'
4.12.14-lp150.11-default
Och, you've got a different kernel...
I will leave it to the OP, to call strtoll or sscanf(&name.release, "%d.%d.%d-", &major, &minor, &release) to get the kernel version as integer number.
But you can get way more hardcore than that. On runtime, you can just do anything, so just read the content of /usr/include/linux/version.h file:
#define _GNU_SOURCE 1
#include <linux/version.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
FILE *f;
f = fopen("/usr/include/linux/version.h", "r");
if (f == NULL) return -__LINE__;
char *line = NULL;
size_t linelen = 0;
char *found = NULL;
while (getline(&line, &linelen, f) > 0) {
if ((found = strstr(line, "LINUX_VERSION_CODE")) != NULL) {
break;
}
}
if (found == NULL) return -__LINE__;
fclose(f);
found += sizeof("LINUX_VERSION_CODE") - 1;
errno = 0;
const long long kv = strtoll(found, NULL, 10);
if (errno) return -__LINE__;
free(line);
printf("%ld.%ld.%ld\n", kv>>16&0xff, kv>>8&0xff, kv&0xff);
if (kv > KERNEL_VERSION(4,17,0)) {
printf("Och, kernel api greater then 4.17.0 found!\n");
} else {
printf("Och, kernel api below 4.17.0 found!\n");
}
}
And on my machine this outputs:
$ cat 1.c | gcc -xc - && ./a.out
4.17.11
Och, kernel api greater then 4.17.0 found!
And on my friends:
$ cat 1.c | ssh friend 'gcc -xc - && ./a.out'
4.15.0
Och, kernel api below 4.17.0 found!
We can also see, that uname -a != grep "LINUX_VERSION_CODE" /usr/include/linux/version.h.