1process_vm_readv(2)           System Calls Manual          process_vm_readv(2)
2
3
4

NAME

6       process_vm_readv, process_vm_writev - transfer data between process ad‐
7       dress spaces
8

LIBRARY

10       Standard C library (libc, -lc)
11

SYNOPSIS

13       #include <sys/uio.h>
14
15       ssize_t process_vm_readv(pid_t pid,
16                              const struct iovec *local_iov,
17                              unsigned long liovcnt,
18                              const struct iovec *remote_iov,
19                              unsigned long riovcnt,
20                              unsigned long flags);
21       ssize_t process_vm_writev(pid_t pid,
22                              const struct iovec *local_iov,
23                              unsigned long liovcnt,
24                              const struct iovec *remote_iov,
25                              unsigned long riovcnt,
26                              unsigned long flags);
27
28   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
29
30       process_vm_readv(), process_vm_writev():
31           _GNU_SOURCE
32

DESCRIPTION

34       These system calls transfer data between the address space of the call‐
35       ing  process  ("the  local  process") and the process identified by pid
36       ("the remote process").  The data moves directly  between  the  address
37       spaces of the two processes, without passing through kernel space.
38
39       The  process_vm_readv()  system  call  transfers  data  from the remote
40       process to the local process.  The data to be transferred is identified
41       by remote_iov and riovcnt: remote_iov is a pointer to an array describ‐
42       ing address ranges in the process pid, and riovcnt specifies the number
43       of  elements  in  remote_iov.  The data is transferred to the locations
44       specified by local_iov and liovcnt: local_iov is a pointer to an  array
45       describing address ranges in the calling process, and liovcnt specifies
46       the number of elements in local_iov.
47
48       The   process_vm_writev()   system   call   is    the    converse    of
49       process_vm_readv()—it  transfers data from the local process to the re‐
50       mote process.  Other than the direction of the transfer, the  arguments
51       liovcnt,  local_iov,  riovcnt,  and remote_iov have the same meaning as
52       for process_vm_readv().
53
54       The local_iov and remote_iov arguments  point  to  an  array  of  iovec
55       structures, described in iovec(3type).
56
57       Buffers    are   processed   in   array   order.    This   means   that
58       process_vm_readv() completely fills local_iov[0] before  proceeding  to
59       local_iov[1],  and  so  on.  Likewise, remote_iov[0] is completely read
60       before proceeding to remote_iov[1], and so on.
61
62       Similarly, process_vm_writev() writes out the entire  contents  of  lo‐
63       cal_iov[0]  before  proceeding to local_iov[1], and it completely fills
64       remote_iov[0] before proceeding to remote_iov[1].
65
66       The lengths of remote_iov[i].iov_len and  local_iov[i].iov_len  do  not
67       have to be the same.  Thus, it is possible to split a single local buf‐
68       fer into multiple remote buffers, or vice versa.
69
70       The flags argument is currently unused and must be set to 0.
71
72       The values specified in the liovcnt and riovcnt arguments must be  less
73       than  or  equal to IOV_MAX (defined in <limits.h> or accessible via the
74       call sysconf(_SC_IOV_MAX)).
75
76       The count arguments and local_iov are checked before doing  any  trans‐
77       fers.   If  the counts are too big, or local_iov is invalid, or the ad‐
78       dresses refer to regions that are inaccessible to  the  local  process,
79       none of the vectors will be processed and an error will be returned im‐
80       mediately.
81
82       Note, however, that these system calls do not check the memory  regions
83       in  the  remote process until just before doing the read/write.  Conse‐
84       quently, a partial read/write (see RETURN VALUE) may result if  one  of
85       the  remote_iov  elements points to an invalid memory region in the re‐
86       mote process.  No further reads/writes will be  attempted  beyond  that
87       point.   Keep  this  in  mind  when  attempting to read data of unknown
88       length (such as C strings  that  are  null-terminated)  from  a  remote
89       process,  by avoiding spanning memory pages (typically 4 KiB) in a sin‐
90       gle remote iovec element.  (Instead, split the remote read into two re‐
91       mote_iov  elements  and  have  them  merge back into a single write lo‐
92       cal_iov entry.  The first read entry goes  up  to  the  page  boundary,
93       while the second starts on the next page boundary.)
94
95       Permission  to  read  from or write to another process is governed by a
96       ptrace access mode PTRACE_MODE_ATTACH_REALCREDS check; see ptrace(2).
97

RETURN VALUE

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

ERRORS

110       EFAULT The memory described by local_iov is outside the caller's acces‐
111              sible address space.
112
113       EFAULT The memory described by remote_iov is outside the accessible ad‐
114              dress space of the process pid.
115
116       EINVAL The sum of the iov_len values of either local_iov or  remote_iov
117              overflows a ssize_t value.
118
119       EINVAL flags is not 0.
120
121       EINVAL liovcnt or riovcnt is too large.
122
123       ENOMEM Could  not  allocate  memory  for  internal  copies of the iovec
124              structures.
125
126       EPERM  The caller does not have permission to access the address  space
127              of the process pid.
128
129       ESRCH  No process with ID pid exists.
130

STANDARDS

132       Linux.
133

HISTORY

135       Linux 3.2, glibc 2.15.
136

NOTES

138       The    data    transfers    performed    by    process_vm_readv()   and
139       process_vm_writev() are not guaranteed to be atomic in any way.
140
141       These system calls were designed to permit fast message passing by  al‐
142       lowing  messages  to  be exchanged with a single copy operation (rather
143       than the double copy that would be required when  using,  for  example,
144       shared memory or pipes).
145

EXAMPLES

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

SEE ALSO

182       readv(2), writev(2)
183
184
185
186Linux man-pages 6.05              2023-05-03               process_vm_readv(2)
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