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PROCESS_VM_READV(2)        Linux Programmer's ManualPROCESS_VM_READV(2)NAME       process_vm_readv, process_vm_writev - transfer data between process ad-       dress spacesSYNOPSIS       #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 (seefeature_test_macros(7)):       process_vm_readv(), process_vm_writev():           _GNU_SOURCEDESCRIPTION       These system calls transfer data between the address space of the call-       ing  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 describ-       ing 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 re-       mote 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  lo-       cal_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 buf-       fer 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  trans-       fers.   If  the counts are too big, or local_iov is invalid, or the ad-       dresses refer to regions that are inaccessible to  the  local  process,       none of the vectors will be processed and an error will be returned im-       mediately.       Note, however, that these system calls do not check the memory  regions       in  the  remote process until just before doing the read/write.  Conse-       quently, a partial read/write (see RETURN VALUE) may result if  one  of       the  remote_iov  elements points to an invalid memory region in the re-       mote 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 sin-       gle remote iovec element.  (Instead, split the remote read into two re-       mote_iov  elements  and  have  them  merge back into a single write lo-       cal_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; seeptrace(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  par-       tial  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 re-       turn 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 acces-              sible address space.       EFAULT The memory described by remote_iov is outside the accessible ad-              dress 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 al-       lowing 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).EXAMPLE       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;       }SEE ALSOreadv(2),writev(2)COLOPHON       This  page  is  part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs,  and  the       latest     version     of     this    page,    can    be    found    at       https://www.kernel.org/doc/man-pages/.Linux                             2017-09-15PROCESS_VM_READV(2)