Doubts about mmap(2)
Hi!
I believe mmap(2) is confusing (see the emboldened parts):
DESCRIPTION
mmap() creates a new mapping in the virtual address space of the calling process. **The starting address for the new mapping is specified in addr.** The length argument specifies the length of the mapping (which must be greater than 0).
If addr is NULL, then the kernel chooses the (page‐aligned) ad‐ dress at which to create the mapping; this is the most portable method of creating a new mapping. **If addr is not NULL, then the kernel takes it as a hint about where to place the mapping; on Linux, the kernel will pick a nearby page boundary** (but al‐
ways above or equal to the value specified by
/proc/sys/vm/mmap_min_addr) and attempt to create the mapping
there. If another mapping already exists there, the kernel
picks a new address that may or may not depend on the hint. The address of the new mapping is returned as the result of the call.
The first emboldened sentence contradicts the second. It's not the starting address, but just a hint to find a nearby (>=) starting address. How about saying this?:
A hint for the starting address for the new mapping is specified in addr.
Cheers,
Alex
Signature · 2 lines
<http://www.alejandro-colomar.es/> GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
Quoted 23 lines · Alex Colomar
Hi!
I believe mmap(2) is confusing (see the emboldened parts):
DESCRIPTION
mmap() creates a new mapping in the virtual address space of the calling process. **The starting address for the new mapping is specified in addr.** The length argument specifies the length of the mapping (which must be greater than 0).If addr is NULL, then the kernel chooses the (page‐aligned) ad‐ dress at which to create the mapping; this is the most portable method of creating a new mapping. **If addr is not NULL, then the kernel takes it as a hint about where to place the mapping; on Linux, the kernel will pick a nearby page boundary** (but al‐ ways above or equal to the value specified by
/proc/sys/vm/mmap_min_addr) and attempt to create the mapping there. If another mapping already exists there, the kernel picks a new address that may or may not depend on the hint. The address of the new mapping is returned as the result of the call.
Quoted 2 lines
The first emboldened sentence contradicts the second. It's not the starting address, but just a hint to find a nearby (>=) starting
I was a bit wrong there. It's >= mmap_min_addr, not >= hint. But the suggestion below holds.
Quoted 8 lines
address. How about saying this?:
A hint for the starting address for the new mapping is specified in addr.
Cheers,
Alex
Signature · 2 lines
<http://www.alejandro-colomar.es/> GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
Quoted 24 lines · Alex Colomar
Quoted 23 lines · Alex Colomar
Hi!
I believe mmap(2) is confusing (see the emboldened parts):
DESCRIPTION
mmap() creates a new mapping in the virtual address space of the calling process. **The starting address for the new mapping is specified in addr.** The length argument specifies the length of the mapping (which must be greater than 0).If addr is NULL, then the kernel chooses the (page‐aligned) ad‐ dress at which to create the mapping; this is the most portable method of creating a new mapping. **If addr is not NULL, then the kernel takes it as a hint about where to place the mapping; on Linux, the kernel will pick a nearby page boundary** (but al‐ ways above or equal to the value specified by
/proc/sys/vm/mmap_min_addr) and attempt to create the mapping there. If another mapping already exists there, the kernel picks a new address that may or may not depend on the hint. The address of the new mapping is returned as the result of the call.
Quoted 11 lines
Quoted 2 lines
The first emboldened sentence contradicts the second. It's not the starting address, but just a hint to find a nearby (>=) starting
I was a bit wrong there. It's >=
mmap_min_addr, not >= hint. But the suggestion below holds.Quoted 5 lines
address. How about saying this?:
A hint for the starting address for the new mapping is specified in addr.
I think describing the default behavior first is a good idea. If a user needs an exact address they can read further for MAP_FIXED/_NOREPLACE (and the potential footguns involved).
- Oskari