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If you read the man page bugs section: https://man7.org/linux/man-pages/man2/select.2.html https://man7.org/linux/man-pages/man2/select.2.html "The Linux kerne

If you read the man page bugs section: https://man7.org/linux/man-pages/man2/select.2.html https://man7.org/linux/man-pages/man2/select.2.html "The Linux kernel imposes no fixed limit, but the glibc implementation makes fd_set a fixed-size type, with FD_SETSIZE defined as 1024" Everyone using select, with very few exceptions, is using glibc. That limitation does appear to exist in glibc if you look at the source code. The source code requires you to pass a glibc defined 'fd_set' type https://man7.org/linux/man-pages/man2/select.2.html https://man7.org/linux/man-pages/man2/select.2.html That struct is defined here: https://github.com/bminor/glibc/blob/595c22ecd8e87a27fd19270ed30fdbae9ad25426/misc/sys/select.h#L58-L70 https://github.com/bminor/glibc/blob/595c22ecd8e87a27fd19270... It does seem to have the limits described by the blogpost.

ksherlock 5y ago
The glibc/musl select library call doesn't verify nfds is < FD_SETSIZE so you could create your own fd_set as large as you want it. glibc: https://github.com/bminor/glibc/blob/master/sysdeps/unix/sysv/linux/select.c https://github.com/bminor/glibc/blob/master/sysdeps/unix/sys... musl: https://git.musl-libc.org/cgit/musl/tree/src/select/select.c https://git.musl-libc.org/cgit/musl/tree/src/select/select.c *BSD allows you to define your own FD_SETSIZE before including system headers.
toast0 5y ago
On FreeBSD's select(2) man page, the NOTES section has the much friendlier > The default size of FD_SETSIZE is currently 1024. In order to accommodate programs which might potentially use a larger number of open files with select(), it is possible to increase this size by having the program define FD_SETSIZE before the inclusion of any header which includes <sys/types.h>. Although, if you link to a library which uses select and doesn't check that fds it uses are valid with the FD_SETSIZE it was compiled with, you're still in for a bad time, as suggested in the article. And still, probably use something better than select.
Joker_vD 5y ago
Yep, and if you pass an fd >= 1024, it will corrupt the stack by using FD_SET and your program will crash. I've seen that in production, was solved by forcing glibc to use epoll.

ksherlock 5y ago

The glibc/musl select library call doesn't verify nfds is < FD_SETSIZE so you could create your own fd_set as large as you want it. glibc: https://github.com/bminor/glibc/blob/master/sysdeps/unix/sysv/linux/select.c https://github.com/bminor/glibc/blob/master/sysdeps/unix/sys... musl: https://git.musl-libc.org/cgit/musl/tree/src/select/select.c https://git.musl-libc.org/cgit/musl/tree/src/select/select.c *BSD allows you to define your own FD_SETSIZE before including system headers.

toast0 5y ago

On FreeBSD's select(2) man page, the NOTES section has the much friendlier > The default size of FD_SETSIZE is currently 1024. In order to accommodate programs which might potentially use a larger number of open files with select(), it is possible to increase this size by having the program define FD_SETSIZE before the inclusion of any header which includes <sys/types.h>. Although, if you link to a library which uses select and doesn't check that fds it uses are valid with the FD_SETSIZE it was compiled with, you're still in for a bad time, as suggested in the article. And still, probably use something better than select.

Joker_vD 5y ago

Yep, and if you pass an fd >= 1024, it will corrupt the stack by using FD_SET and your program will crash. I've seen that in production, was solved by forcing glibc to use epoll.