linux打开最大文件数量1,048,576

File descriptors are represented by the C int type. Not using a special type is often  considered odd, but is, historically, the Unix way. Each Linux process has a maximum number of files that it may open. File descriptors start at 0 and go up to one less than  this maximum value. By default, the maximum is 1,024, but it can be configured as high as 1,048,576. Because negative values are not legal file descriptors, −1 is often used to indicate an error from a function that would otherwise return a valid file descriptor.