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Say I have 2 processes, ProcessA and ProcessB. If I perform int fd=open(somefile) in ProcessA, can I then pass the value of file descriptor fd over IPC to ProcessB and have it manipulate the same file?
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File descriptors are generally unique to each process, but they can be shared by child processes created with a fork subroutine or copied by the fcntl, dup, and dup2 subroutines.
The file descriptor table itself contains pointers to the file objects which do all the resource handling. Each process (a process being an instance of an application) has it's own file descriptor table which is pointed to by an entry in it's process description table.
A file descriptor is a number that uniquely identifies an open file in a computer's operating system. It describes a data resource, and how that resource may be accessed. When a program asks to open a file — or another data resource, like a network socket — the kernel: Grants access.
Pointers to the Open File Table: One process can have multiple file descriptors point to the same entry (e.g., as a result of a call to dup() ) Multiple processes (e.g., a parent and child) can have file descriptors that point to the same entry.
You can pass a file descriptor to another process over unix domain sockets. Here's the code to pass such a file descriptor, taken from Unix Network Programming
ssize_t write_fd(int fd, void *ptr, size_t nbytes, int sendfd) { struct msghdr msg; struct iovec iov[1]; #ifdef HAVE_MSGHDR_MSG_CONTROL union { struct cmsghdr cm; char control[CMSG_SPACE(sizeof(int))]; } control_un; struct cmsghdr *cmptr; msg.msg_control = control_un.control; msg.msg_controllen = sizeof(control_un.control); cmptr = CMSG_FIRSTHDR(&msg); cmptr->cmsg_len = CMSG_LEN(sizeof(int)); cmptr->cmsg_level = SOL_SOCKET; cmptr->cmsg_type = SCM_RIGHTS; *((int *) CMSG_DATA(cmptr)) = sendfd; #else msg.msg_accrights = (caddr_t) &sendfd; msg.msg_accrightslen = sizeof(int); #endif msg.msg_name = NULL; msg.msg_namelen = 0; iov[0].iov_base = ptr; iov[0].iov_len = nbytes; msg.msg_iov = iov; msg.msg_iovlen = 1; return(sendmsg(fd, &msg, 0)); } /* end write_fd */ And here's the code to receive the file descriptor
ssize_t read_fd(int fd, void *ptr, size_t nbytes, int *recvfd) { struct msghdr msg; struct iovec iov[1]; ssize_t n; int newfd; #ifdef HAVE_MSGHDR_MSG_CONTROL union { struct cmsghdr cm; char control[CMSG_SPACE(sizeof(int))]; } control_un; struct cmsghdr *cmptr; msg.msg_control = control_un.control; msg.msg_controllen = sizeof(control_un.control); #else msg.msg_accrights = (caddr_t) &newfd; msg.msg_accrightslen = sizeof(int); #endif msg.msg_name = NULL; msg.msg_namelen = 0; iov[0].iov_base = ptr; iov[0].iov_len = nbytes; msg.msg_iov = iov; msg.msg_iovlen = 1; if ( (n = recvmsg(fd, &msg, 0)) <= 0) return(n); #ifdef HAVE_MSGHDR_MSG_CONTROL if ( (cmptr = CMSG_FIRSTHDR(&msg)) != NULL && cmptr->cmsg_len == CMSG_LEN(sizeof(int))) { if (cmptr->cmsg_level != SOL_SOCKET) err_quit("control level != SOL_SOCKET"); if (cmptr->cmsg_type != SCM_RIGHTS) err_quit("control type != SCM_RIGHTS"); *recvfd = *((int *) CMSG_DATA(cmptr)); } else *recvfd = -1; /* descriptor was not passed */ #else /* *INDENT-OFF* */ if (msg.msg_accrightslen == sizeof(int)) *recvfd = newfd; else *recvfd = -1; /* descriptor was not passed */ /* *INDENT-ON* */ #endif return(n); } /* end read_fd */ If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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