What are the possible values for file descriptors?

Question

I am interested to know the valid values which I can expect for a file descriptor.

Please let me explain a bit. I know that, for instance, when I use #include <unistd.h> on my linux system then a call to open a file for reading:

int fileDescriptor;
fileDescriptor = open("/some/filename",O_RDONLY);

an error might occur and I receive -1 as a result.
Incidently the (-1) negative one must have somewhat of a special meaning. Is it that all other values are valid file descriptors? i.e. also negative ones like -2 and -1023?

Assuming that int is 4 bytes (sizeof(int)==4), then would

(-1) = 10000000 0000000 00000000 00000001

would be the only detectable invalid file descriptor? Would others like:

• (0) = 00000000 0000000 00000000 00000000
• (-2) = 10000000 0000000 00000000 00000010
• (2) = 00000000 0000000 00000000 00000010

be ok? Since the file descriptor could store 4 bytes I could have therefore a maximum of (2^(8*4)-1) valid file descriptors and consequently this would be the maximum number of files I can have open, correct?

To put it plain again:

What should I expect a (valid) file descriptor to be?

any value but -1?

Answer 1

From the man page:

open() returns a file descriptor, a small, nonnegative integer.

and then:

open() and creat() return the new file descriptor, or -1 if an error occurred