process_vm_writev (2) - Linux Manuals

process_vm_writev: transfer data between process address spaces

Command to display process_vm_writev manual in Linux: $ man 2 process_vm_writev

NAME

process_vm_readv, process_vm_writev - transfer data between process address spaces

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

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).

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 appropriately.

ERRORS

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.

VERSIONS

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

CONFORMING TO

These system calls are nonstandard Linux extensions.

NOTES

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).

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; }

COLOPHON

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