system calls - C - syscall - 64-bit - pointer

Question

I am on 64-bit Linux x86. I need to execute mmap syscall using syscall function. mmap syscall number is 9:

printf("mmap-1: %lli
", syscall(9,    0, 10, 3, 2 | 32, -1, 0));
printf("mmap-2: %lli
", mmap(         0, 10, 3, 2 | 32, -1, 0));

However, when I run it, the syscall function gives wrong results.

mmap-1: 2236940288
mmap-2: 140503502090240

mmap-1: 3425849344
mmap-2: 140612065181696

mmap-1: 249544704
mmap-2: 139625341366272

mmap works just fine, but the "addresses" returned syscall result in Segmentation fault. The values from syscall seem to be cast to 32 bits or something.

What am I doing wrong?

Answer 1

In your syscall(), instead of passing in 0 (NULL) for the first parameter, addr, pass some pointer you have declared previously. This way you can access the memory mapped by mmap. mmap function declaration:

void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);