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| .TH PROCESS_VM_READV 2 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| process_vm_readv, process_vm_writev \- transfer data between process address spaces |
| .SH SYNOPSIS |
| .nf |
| .B #include <sys/uio.h> |
| .PP |
| .BI "ssize_t process_vm_readv(pid_t " pid , |
| .BI " const struct iovec *" local_iov , |
| .BI " unsigned long " liovcnt , |
| .BI " const struct iovec *" remote_iov , |
| .BI " unsigned long " riovcnt , |
| .BI " unsigned long " flags ");" |
| .BI "ssize_t process_vm_writev(pid_t " pid , |
| .BI " const struct iovec *" local_iov , |
| .BI " unsigned long " liovcnt , |
| .BI " const struct iovec *" remote_iov , |
| .BI " unsigned long " riovcnt , |
| .BI " unsigned long " flags ");" |
| .fi |
| .PP |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| .BR process_vm_readv (), |
| .BR process_vm_writev (): |
| .nf |
| _GNU_SOURCE |
| .fi |
| .SH DESCRIPTION |
| These system calls transfer data between the address space |
| of the calling process ("the local process") and the process identified by |
| .IR pid |
| ("the remote process"). |
| The data moves directly between the address spaces of the two processes, |
| without passing through kernel space. |
| .PP |
| The |
| .BR process_vm_readv () |
| system call transfers data from the remote process to the local process. |
| The data to be transferred is identified by |
| .IR remote_iov |
| and |
| .IR riovcnt : |
| .IR remote_iov |
| is a pointer to an array describing address ranges in the process |
| .IR pid , |
| and |
| .IR riovcnt |
| specifies the number of elements in |
| .IR remote_iov . |
| The data is transferred to the locations specified by |
| .IR local_iov |
| and |
| .IR liovcnt : |
| .IR local_iov |
| is a pointer to an array describing address ranges in the calling process, |
| and |
| .IR liovcnt |
| specifies the number of elements in |
| .IR local_iov . |
| .PP |
| The |
| .BR process_vm_writev () |
| system call is the converse of |
| .BR process_vm_readv ()\(emit |
| transfers data from the local process to the remote process. |
| Other than the direction of the transfer, the arguments |
| .IR liovcnt , |
| .IR local_iov , |
| .IR riovcnt , |
| and |
| .IR remote_iov |
| have the same meaning as for |
| .BR process_vm_readv (). |
| .PP |
| The |
| .I local_iov |
| and |
| .I remote_iov |
| arguments point to an array of |
| .I iovec |
| structures, defined in |
| .IR <sys/uio.h> |
| as: |
| .PP |
| .in +4n |
| .EX |
| struct iovec { |
| void *iov_base; /* Starting address */ |
| size_t iov_len; /* Number of bytes to transfer */ |
| }; |
| .EE |
| .in |
| .PP |
| Buffers are processed in array order. |
| This means that |
| .BR process_vm_readv () |
| completely fills |
| .I local_iov[0] |
| before proceeding to |
| .IR local_iov[1] , |
| and so on. |
| Likewise, |
| .I remote_iov[0] |
| is completely read before proceeding to |
| .IR remote_iov[1] , |
| and so on. |
| .PP |
| Similarly, |
| .BR process_vm_writev () |
| writes out the entire contents of |
| .I local_iov[0] |
| before proceeding to |
| .IR local_iov[1] , |
| and it completely fills |
| .I remote_iov[0] |
| before proceeding to |
| .IR remote_iov[1] . |
| .PP |
| The lengths of |
| .I remote_iov[i].iov_len |
| and |
| .I 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. |
| .PP |
| The |
| .I flags |
| argument is currently unused and must be set to 0. |
| .PP |
| The values specified in the |
| .I liovcnt |
| and |
| .I riovcnt |
| arguments must be less than or equal to |
| .BR IOV_MAX |
| (defined in |
| .I <limits.h> |
| or accessible via the call |
| .IR sysconf(_SC_IOV_MAX) ). |
| .\" In time, glibc might provide a wrapper that works around this limit, |
| .\" as is done for readv()/writev() |
| .PP |
| The count arguments and |
| .IR local_iov |
| are checked before doing any transfers. |
| If the counts are too big, or |
| .I 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. |
| .PP |
| 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 |
| .I 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 |
| .I iovec |
| element. |
| (Instead, split the remote read into two |
| .I remote_iov |
| elements and have them merge back into a single write |
| .I local_iov |
| entry. |
| The first read entry goes up to the page boundary, |
| while the second starts on the next page boundary.) |
| .PP |
| Permission to read from or write to another process |
| is governed by a ptrace access mode |
| .B PTRACE_MODE_ATTACH_REALCREDS |
| check; see |
| .BR ptrace (2). |
| .SH RETURN VALUE |
| On success, |
| .BR process_vm_readv () |
| returns the number of bytes read and |
| .BR 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 |
| .I iovec |
| elements. |
| These system calls won't perform a partial transfer that splits a single |
| .I iovec |
| element.) |
| The caller should check the return value to determine whether |
| a partial read/write occurred. |
| .PP |
| On error, \-1 is returned and |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EFAULT |
| The memory described by |
| .I local_iov |
| is outside the caller's accessible address space. |
| .TP |
| .B EFAULT |
| The memory described by |
| .I remote_iov |
| is outside the accessible address space of the process |
| .IR pid . |
| .TP |
| .B EINVAL |
| The sum of the |
| .I iov_len |
| values of either |
| .I local_iov |
| or |
| .I remote_iov |
| overflows a |
| .I ssize_t |
| value. |
| .TP |
| .B EINVAL |
| .I flags |
| is not 0. |
| .TP |
| .B EINVAL |
| .I liovcnt |
| or |
| .I riovcnt |
| is too large. |
| .TP |
| .B ENOMEM |
| Could not allocate memory for internal copies of the |
| .I iovec |
| structures. |
| .TP |
| .B EPERM |
| The caller does not have permission to access the address space of the process |
| .IR pid . |
| .TP |
| .B ESRCH |
| No process with ID |
| .I pid |
| exists. |
| .SH VERSIONS |
| These system calls were added in Linux 3.2. |
| Support is provided in glibc since version 2.15. |
| .SH CONFORMING TO |
| These system calls are nonstandard Linux extensions. |
| .SH NOTES |
| The data transfers performed by |
| .BR process_vm_readv () |
| and |
| .BR process_vm_writev () |
| are not guaranteed to be atomic in any way. |
| .PP |
| 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). |
| .\" Original user is MPI, http://www.mcs.anl.gov/research/projects/mpi/ |
| .\" See also some benchmarks at http://lwn.net/Articles/405284/ |
| .\" and http://marc.info/?l=linux-mm&m=130105930902915&w=2 |
| .SH EXAMPLES |
| The following code sample demonstrates the use of |
| .BR process_vm_readv (). |
| It reads 20 bytes at the address 0x10000 from the process with PID 10 |
| and writes the first 10 bytes into |
| .I buf1 |
| and the second 10 bytes into |
| .IR buf2 . |
| .PP |
| .EX |
| #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; |
| } |
| .EE |
| .SH SEE ALSO |
| .BR readv (2), |
| .BR writev (2) |