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   process_vm_writev    ( 2 )

передавать данные между адресными пространствами процесса (transfer data between process address spaces)

Имя (Name)

process_vm_readv, process_vm_writev - transfer data between
       process address spaces

Синопсис (Synopsis)

#include <sys/uio.h>

ssize_t process_vm_readv(pid_t pid, const struct iovec *local_iov, unsigned long liovcnt, const struct iovec *remote_iov, unsigned long riovcnt, unsigned long flags); ssize_t process_vm_writev(pid_t pid, const struct iovec *local_iov, unsigned long liovcnt, const struct iovec *remote_iov, unsigned long riovcnt, unsigned long flags);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

process_vm_readv(), process_vm_writev(): _GNU_SOURCE


Описание (Description)

These system calls transfer data between the address space of the
       calling process ("the local process") and the process identified
       by pid ("the remote process").  The data moves directly between
       the address spaces of the two processes, without passing through
       kernel space.

The process_vm_readv() system call transfers data from the remote process to the local process. The data to be transferred is identified by remote_iov and riovcnt: remote_iov is a pointer to an array describing address ranges in the process pid, and riovcnt specifies the number of elements in remote_iov. The data is transferred to the locations specified by local_iov and liovcnt: local_iov is a pointer to an array describing address ranges in the calling process, and liovcnt specifies the number of elements in local_iov.

The process_vm_writev() system call is the converse of process_vm_readv()—it transfers data from the local process to the remote process. Other than the direction of the transfer, the arguments liovcnt, local_iov, riovcnt, and remote_iov have the same meaning as for process_vm_readv().

The local_iov and remote_iov arguments point to an array of iovec structures, defined in <sys/uio.h> as:

struct iovec { void *iov_base; /* Starting address */ size_t iov_len; /* Number of bytes to transfer */ };

Buffers are processed in array order. This means that process_vm_readv() completely fills local_iov[0] before proceeding to local_iov[1], and so on. Likewise, remote_iov[0] is completely read before proceeding to remote_iov[1], and so on.

Similarly, process_vm_writev() writes out the entire contents of local_iov[0] before proceeding to local_iov[1], and it completely fills remote_iov[0] before proceeding to remote_iov[1].

The lengths of remote_iov[i].iov_len and local_iov[i].iov_len do not have to be the same. Thus, it is possible to split a single local buffer into multiple remote buffers, or vice versa.

The flags argument is currently unused and must be set to 0.

The values specified in the liovcnt and riovcnt arguments must be less than or equal to IOV_MAX (defined in <limits.h> or accessible via the call sysconf(_SC_IOV_MAX)).

The count arguments and local_iov are checked before doing any transfers. If the counts are too big, or local_iov is invalid, or the addresses refer to regions that are inaccessible to the local process, none of the vectors will be processed and an error will be returned immediately.

Note, however, that these system calls do not check the memory regions in the remote process until just before doing the read/write. Consequently, a partial read/write (see RETURN VALUE) may result if one of the remote_iov elements points to an invalid memory region in the remote process. No further reads/writes will be attempted beyond that point. Keep this in mind when attempting to read data of unknown length (such as C strings that are null-terminated) from a remote process, by avoiding spanning memory pages (typically 4 KiB) in a single remote iovec element. (Instead, split the remote read into two remote_iov elements and have them merge back into a single write local_iov entry. The first read entry goes up to the page boundary, while the second starts on the next page boundary.)

Permission to read from or write to another process is governed by a ptrace access mode PTRACE_MODE_ATTACH_REALCREDS check; see ptrace(2).


Возвращаемое значение (Return value)

On success, process_vm_readv() returns the number of bytes read
       and process_vm_writev() returns the number of bytes written.
       This return value may be less than the total number of requested
       bytes, if a partial read/write occurred.  (Partial transfers
       apply at the granularity of iovec elements.  These system calls
       won't perform a partial transfer that splits a single iovec
       element.)  The caller should check the return value to determine
       whether a partial read/write occurred.

On error, -1 is returned and errno is set to indicate the error.


Ошибки (Error)

EFAULT The memory described by local_iov is outside the caller's
              accessible address space.

EFAULT The memory described by remote_iov is outside the accessible address space of the process pid.

EINVAL The sum of the iov_len values of either local_iov or remote_iov overflows a ssize_t value.

EINVAL flags is not 0.

EINVAL liovcnt or riovcnt is too large.

ENOMEM Could not allocate memory for internal copies of the iovec structures.

EPERM The caller does not have permission to access the address space of the process pid.

ESRCH No process with ID pid exists.


Версии (Versions)

These system calls were added in Linux 3.2.  Support is provided
       in glibc since version 2.15.

Стандарты (Conforming to)

These system calls are nonstandard Linux extensions.

Примечание (Note)

The data transfers performed by process_vm_readv() and
       process_vm_writev() are not guaranteed to be atomic in any way.

These system calls were designed to permit fast message passing by allowing messages to be exchanged with a single copy operation (rather than the double copy that would be required when using, for example, shared memory or pipes).


Примеры (Examples)

The following code sample demonstrates the use of
       process_vm_readv().  It reads 20 bytes at the address 0x10000
       from the process with PID 10 and writes the first 10 bytes into
       buf1 and the second 10 bytes into buf2.

#include <sys/uio.h>

int main(void) { struct iovec local[2]; struct iovec remote[1]; char buf1[10]; char buf2[10]; ssize_t nread; pid_t pid = 10; /* PID of remote process */

local[0].iov_base = buf1; local[0].iov_len = 10; local[1].iov_base = buf2; local[1].iov_len = 10; remote[0].iov_base = (void *) 0x10000; remote[0].iov_len = 20;

nread = process_vm_readv(pid, local, 2, remote, 1, 0); if (nread != 20) return 1; else return 0; }


Смотри также (See also)

readv(2), writev(2)