Question

What actually happens after calling read:

n = read(fd, buf, try_read_size);

here fd is a TCP socket descriptor. buf is the buffer. try_read_size is the number of bytes that the program tries to read.

I guess this may finally invokes a system call to the kernel. But could anyone provide some details? say the source code implementation in glibc or kernel source?

Answer 1

From a high-level perspective, this is what happens:

• A wrapper function provided by glibc is called
• The wrapper function puts the parameters passed on the stack into registers and sets the syscall number in the register dedicated for that purpose (e.g. EAX on x86)
• The wrapper function executes a trap or equivalent instruction (e.g. SYSENTER)
• The CPU switches to ring0, and the trap handler is invoked
• The trap handler checks the syscall number for validity and looks it up in a jump table to kernel functions
• The respective kernel function checks whether arguments are valid (e.g. the range buf to buf+try_read_size refers to accessible memory pages, fd is really a file descriptor). If something is amiss, a negative error code (e.g. -EFAULT) is generated, the cpu is switched back to user mode and the call returns to the wrapper.
• Another function is called depending on the file descriptor’s type (in your case a socket, but one could read from a block device or a proc entry or something more exotic)
• The socket’s input buffer is checked:
  • If there is some data in the buffer, min(available, try_read_size) is copied to buf, the amount is written to the return code register (EAX on x86), the cpu is switched back to user mode and the call returns to the wrapper.
  • If the input buffer is empty
    • If the connection has been closed, zero is written to the return code register, the cpu is switched back to user mode and the call returns to the wrapper
    • If the connection has not been closed
      • A negative error code (-EAGAIN) is written to the return code register if the socket is nonblocking, the cpu is switched back to user mode and the call returns to the wrapper.
      • The process is suspended if the socket is not non-blocking
• The wrapper function checks whether the return value is negative (error).
  • If positive or zero, it returns the value.
  • If negative, it sets errno to the negated value (a positive error is reported) and returns -1