What is the meaning of adding 1 on socket file descriptor when using in select function?
I create socket file descriptor like below,
JavaScript
x
int sock_file_descriptor;
sock_file_descriptor = socket(AF_INET, SOCK_DGRAM, 0);
and use it in select function like below,
JavaScript
result = select(sock_file_descriptor+1, &readfd, NULL, NULL, 0);
What is the meaning of +1 in select function? It even does not work when I remove the calculation adding the value.
Thanks in advance.
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Answer
RTFM! The first parameter of select
is the number of file descriptors to considere:
The nfds argument specifies the range of descriptors to be tested. The first nfds descriptors shall be checked in each set; that is, the descriptors from zero through nfds-1 in the descriptor sets shall be examined.
Here is an example of use:
JavaScript
// create socket
int sock_file_descriptor;
sock_file_descriptor = socket(AF_INET, SOCK_DGRAM, 0);
// initialize the read fd_set
fd_set read;
FD_ZERO(&read);
FD_SET(sock_file_descriptor, &read);
// ok we can select
result = select(sock_file_descriptor+1, &readfd, NULL, NULL, 0);