1PROCESS_VM_READV(2)        Linux Programmer's Manual       PROCESS_VM_READV(2)
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NAME

6       process_vm_readv,  process_vm_writev  -  transfer  data between process
7       address spaces
8

SYNOPSIS

10       #include <sys/uio.h>
11
12       ssize_t process_vm_readv(pid_t pid,
13                                const struct iovec *local_iov,
14                                unsigned long liovcnt,
15                                const struct iovec *remote_iov,
16                                unsigned long riovcnt,
17                                unsigned long flags);
18
19       ssize_t process_vm_writev(pid_t pid,
20                                 const struct iovec *local_iov,
21                                 unsigned long liovcnt,
22                                 const struct iovec *remote_iov,
23                                 unsigned long riovcnt,
24                                 unsigned long flags);
25
26   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
27
28       process_vm_readv(), process_vm_writev():
29           _GNU_SOURCE
30

DESCRIPTION

32       These system calls transfer data between the address space of the call‐
33       ing  process  ("the  local  process") and the process identified by pid
34       ("the remote process").  The data moves directly  between  the  address
35       spaces of the two processes, without passing through kernel space.
36
37       The  process_vm_readv()  system  call  transfers  data  from the remote
38       process to the local process.  The data to be transferred is identified
39       by remote_iov and riovcnt: remote_iov is a pointer to an array describ‐
40       ing address ranges in the process pid, and riovcnt specifies the number
41       of  elements  in  remote_iov.  The data is transferred to the locations
42       specified by local_iov and liovcnt: local_iov is a pointer to an  array
43       describing address ranges in the calling process, and liovcnt specifies
44       the number of elements in local_iov.
45
46       The   process_vm_writev()   system   call   is    the    converse    of
47       process_vm_readv()—it  transfers  data  from  the  local process to the
48       remote process.  Other than the direction of the  transfer,  the  argu‐
49       ments liovcnt, local_iov, riovcnt, and remote_iov have the same meaning
50       as for process_vm_readv().
51
52       The local_iov and remote_iov arguments  point  to  an  array  of  iovec
53       structures, defined in <sys/uio.h> as:
54
55           struct iovec {
56               void  *iov_base;    /* Starting address */
57               size_t iov_len;     /* Number of bytes to transfer */
58           };
59
60       Buffers    are   processed   in   array   order.    This   means   that
61       process_vm_readv() completely fills local_iov[0] before  proceeding  to
62       local_iov[1],  and  so  on.  Likewise, remote_iov[0] is completely read
63       before proceeding to remote_iov[1], and so on.
64
65       Similarly,  process_vm_writev()  writes  out  the  entire  contents  of
66       local_iov[0] before proceeding to local_iov[1], and it completely fills
67       remote_iov[0] before proceeding to remote_iov[1].
68
69       The lengths of remote_iov[i].iov_len and  local_iov[i].iov_len  do  not
70       have to be the same.  Thus, it is possible to split a single local buf‐
71       fer into multiple remote buffers, or vice versa.
72
73       The flags argument is currently unused and must be set to 0.
74
75       The values specified in the liovcnt and riovcnt arguments must be  less
76       than  or  equal to IOV_MAX (defined in <limits.h> or accessible via the
77       call sysconf(_SC_IOV_MAX)).
78
79       The count arguments and local_iov are checked before doing  any  trans‐
80       fers.   If  the  counts  are  too  big, or local_iov is invalid, or the
81       addresses refer to regions that are inaccessible to the local  process,
82       none  of  the  vectors  will be processed and an error will be returned
83       immediately.
84
85       Note, however, that these system calls do not check the memory  regions
86       in  the  remote process until just before doing the read/write.  Conse‐
87       quently, a partial read/write (see RETURN VALUE) may result if  one  of
88       the  remote_iov  elements  points  to  an  invalid memory region in the
89       remote process.  No further reads/writes will be attempted beyond  that
90       point.   Keep  this  in  mind  when  attempting to read data of unknown
91       length (such as C strings  that  are  null-terminated)  from  a  remote
92       process,  by avoiding spanning memory pages (typically 4 KiB) in a sin‐
93       gle remote iovec element.  (Instead, split the  remote  read  into  two
94       remote_iov  elements  and  have  them  merge  back  into a single write
95       local_iov entry.  The first read entry goes up to  the  page  boundary,
96       while the second starts on the next page boundary.)
97
98       Permission  to  read  from or write to another process is governed by a
99       ptrace access mode PTRACE_MODE_ATTACH_REALCREDS check; see ptrace(2).
100

RETURN VALUE

102       On success, process_vm_readv() returns the number  of  bytes  read  and
103       process_vm_writev()  returns  the number of bytes written.  This return
104       value may be less than the total number of requested bytes, if  a  par‐
105       tial  read/write occurred.  (Partial transfers apply at the granularity
106       of iovec elements.  These system calls won't perform a partial transfer
107       that  splits  a  single  iovec  element.)   The caller should check the
108       return value to determine whether a partial read/write occurred.
109
110       On error, -1 is returned and errno is set appropriately.
111

ERRORS

113       EFAULT The memory described by local_iov is outside the caller's acces‐
114              sible address space.
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116       EFAULT The  memory  described  by  remote_iov is outside the accessible
117              address space of the process pid.
118
119       EINVAL The sum of the iov_len values of either local_iov or  remote_iov
120              overflows a ssize_t value.
121
122       EINVAL flags is not 0.
123
124       EINVAL liovcnt or riovcnt is too large.
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126       ENOMEM Could  not  allocate  memory  for  internal  copies of the iovec
127              structures.
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129       EPERM  The caller does not have permission to access the address  space
130              of the process pid.
131
132       ESRCH  No process with ID pid exists.
133

VERSIONS

135       These  system  calls  were  added in Linux 3.2.  Support is provided in
136       glibc since version 2.15.
137

CONFORMING TO

139       These system calls are nonstandard Linux extensions.
140

NOTES

142       The   data    transfers    performed    by    process_vm_readv()    and
143       process_vm_writev() are not guaranteed to be atomic in any way.
144
145       These  system  calls  were  designed  to permit fast message passing by
146       allowing messages to be exchanged with a single copy operation  (rather
147       than  the  double  copy that would be required when using, for example,
148       shared memory or pipes).
149

EXAMPLES

151       The following code sample demonstrates the use  of  process_vm_readv().
152       It  reads  20 bytes at the address 0x10000 from the process with PID 10
153       and writes the first 10 bytes into buf1 and the second  10  bytes  into
154       buf2.
155
156       #include <sys/uio.h>
157
158       int
159       main(void)
160       {
161           struct iovec local[2];
162           struct iovec remote[1];
163           char buf1[10];
164           char buf2[10];
165           ssize_t nread;
166           pid_t pid = 10;             /* PID of remote process */
167
168           local[0].iov_base = buf1;
169           local[0].iov_len = 10;
170           local[1].iov_base = buf2;
171           local[1].iov_len = 10;
172           remote[0].iov_base = (void *) 0x10000;
173           remote[0].iov_len = 20;
174
175           nread = process_vm_readv(pid, local, 2, remote, 1, 0);
176           if (nread != 20)
177               return 1;
178           else
179               return 0;
180       }
181

SEE ALSO

183       readv(2), writev(2)
184

COLOPHON

186       This  page  is  part of release 5.07 of the Linux man-pages project.  A
187       description of the project, information about reporting bugs,  and  the
188       latest     version     of     this    page,    can    be    found    at
189       https://www.kernel.org/doc/man-pages/.
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193Linux                             2020-06-09               PROCESS_VM_READV(2)
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