1 | .\"Copyright 2006-2008 Sun Microsystems, Inc. |
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2 | .\" Copyright (c) 1996 Thinking Machines Corporation |
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3 | .TH MPI_Scan 3 "Dec 08, 2009" "1.4" "Open MPI" |
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4 | |
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5 | .SH NAME |
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6 | \fBMPI_Scan\fP \- Computes an inclusive scan (partial reduction) |
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7 | |
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8 | .SH SYNTAX |
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9 | .ft R |
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10 | |
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11 | .SH C Syntax |
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12 | .nf |
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13 | #include <mpi.h> |
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14 | int MPI_Scan(void *\fIsendbuf\fP, void *\fIrecvbuf\fP, int \fIcount\fP, |
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15 | MPI_Datatype \fIdatatype\fP, MPI_Op \fIop\fP, MPI_Comm \fIcomm\fP) |
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16 | |
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17 | .SH Fortran Syntax |
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18 | .nf |
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19 | INCLUDE 'mpif.h' |
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20 | MPI_SCAN(\fISENDBUF, RECVBUF, COUNT, DATATYPE, OP, COMM, IERROR\fP) |
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21 | <type> \fISENDBUF(*), RECVBUF(*)\fP |
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22 | INTEGER \fICOUNT, DATATYPE, OP, COMM, IERROR\fP |
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23 | |
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24 | .SH C++ Syntax |
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25 | .nf |
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26 | #include <mpi.h> |
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27 | void MPI::Intracomm::Scan(const void* \fIsendbuf\fP, void* \fIrecvbuf\fP, |
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28 | int \fIcount\fP, const MPI::Datatype& \fIdatatype\fP, |
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29 | const MPI::Op& \fIop\fP) const |
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30 | |
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31 | .SH INPUT PARAMETERS |
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32 | .ft R |
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33 | .TP 1i |
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34 | sendbuf |
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35 | Send buffer (choice). |
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36 | .TP 1i |
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37 | count |
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38 | Number of elements in input buffer (integer). |
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39 | .TP 1i |
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40 | datatype |
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41 | Data type of elements of input buffer (handle). |
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42 | .TP 1i |
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43 | op |
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44 | Operation (handle). |
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45 | .TP 1i |
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46 | comm |
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47 | Communicator (handle). |
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48 | |
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49 | .SH OUTPUT PARAMETERS |
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50 | .ft R |
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51 | .TP 1i |
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52 | recvbuf |
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53 | Receive buffer (choice). |
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54 | .ft R |
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55 | .TP 1i |
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56 | IERROR |
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57 | Fortran only: Error status (integer). |
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58 | |
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59 | .SH DESCRIPTION |
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60 | .ft R |
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61 | MPI_Scan is used to perform an inclusive prefix reduction on data |
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62 | distributed across the calling processes. The operation returns, in |
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63 | the \fIrecvbuf\fP of the process with rank i, the reduction |
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64 | (calculated according to the function \fIop\fP) of the values in the |
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65 | \fIsendbuf\fPs of processes with ranks 0, ..., i (inclusive). The type |
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66 | of operations supported, their semantics, and the constraints on send |
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67 | and receive buffers are as for MPI_Reduce. |
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68 | |
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69 | .SH EXAMPLE |
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70 | .ft R |
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71 | This example uses a user-defined operation to produce a segmented |
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72 | scan. A segmented scan takes, as input, a set of values and a set of |
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73 | logicals, where the logicals delineate the various segments of the |
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74 | scan. For example, |
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75 | .sp |
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76 | .nf |
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77 | values v1 v2 v3 v4 v5 v6 v7 v8 |
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78 | logicals 0 0 1 1 1 0 0 1 |
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79 | result v1 v1+v2 v3 v3+v4 v3+v4+v5 v6 v6+v7 v8 |
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80 | .fi |
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81 | .sp |
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82 | The result for rank j is thus the sum v(i) + ... + v(j), where i is |
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83 | the lowest rank such that for all ranks n, i <= n <= j, logical(n) = |
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84 | logical(j). The operator that produces this effect is |
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85 | .sp |
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86 | .nf |
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87 | [ u ] [ v ] [ w ] |
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88 | [ ] o [ ] = [ ] |
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89 | [ i ] [ j ] [ j ] |
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90 | .sp |
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91 | where |
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92 | .sp |
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93 | ( u + v if i = j |
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94 | w = ( |
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95 | ( v if i != j |
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96 | .fi |
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97 | .sp |
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98 | Note that this is a noncommutative operator. C code that implements it is |
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99 | given below. |
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100 | .sp |
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101 | .nf |
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102 | typedef struct { |
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103 | double val; |
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104 | int log; |
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105 | } SegScanPair; |
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106 | |
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107 | /* |
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108 | * the user-defined function |
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109 | */ |
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110 | void segScan(SegScanPair *in, SegScanPair *inout, int *len, |
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111 | MPI_Datatype *dptr) |
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112 | { |
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113 | int i; |
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114 | SegScanPair c; |
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115 | |
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116 | for (i = 0; i < *len; ++i) { |
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117 | if (in->log == inout->log) |
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118 | c.val = in->val + inout->val; |
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119 | else |
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120 | c.val = inout->val; |
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121 | |
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122 | c.log = inout->log; |
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123 | *inout = c; |
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124 | in++; |
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125 | inout++; |
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126 | } |
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127 | } |
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128 | .fi |
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129 | .sp |
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130 | Note that the inout argument to the user-defined function corresponds |
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131 | to the right-hand operand of the operator. When using this operator, |
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132 | we must be careful to specify that it is noncommutative, as in the |
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133 | following: |
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134 | .sp |
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135 | .nf |
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136 | int i, base; |
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137 | SeqScanPair a, answer; |
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138 | MPI_Op myOp; |
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139 | MPI_Datatype type[2] = {MPI_DOUBLE, MPI_INT}; |
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140 | MPI_Aint disp[2]; |
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141 | int blocklen[2] = {1, 1}; |
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142 | MPI_Datatype sspair; |
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143 | |
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144 | /* |
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145 | * explain to MPI how type SegScanPair is defined |
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146 | */ |
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147 | MPI_Get_address(a, disp); |
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148 | MPI_Get_address(a.log, disp + 1); |
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149 | base = disp[0]; |
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150 | for (i = 0; i < 2; ++i) |
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151 | disp[i] -= base; |
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152 | MPI_Type_struct(2, blocklen, disp, type, &sspair); |
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153 | MPI_Type_commit(&sspair); |
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154 | |
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155 | /* |
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156 | * create the segmented-scan user-op |
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157 | * noncommutative - set commute (arg 2) to 0 |
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158 | */ |
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159 | MPI_Op_create((MPI_User_function *)segScan, 0, &myOp); |
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160 | \&... |
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161 | MPI_Scan(a, answer, 1, sspair, myOp, comm); |
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162 | .fi |
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163 | |
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164 | .SH USE OF IN-PLACE OPTION |
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165 | WHen the communicator is an intracommunicator, you can perform a scanning operation in place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the \fIsendbuf\fR argument. The input data is taken from the receive buffer and replaced by the output data. |
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166 | |
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167 | .SH NOTES ON COLLECTIVE OPERATIONS |
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168 | .ft R |
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169 | The reduction functions of type MPI_Op do not return an error value. |
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170 | As a result, if the functions detect an error, all they can do is |
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171 | either call MPI_Abort or silently skip the problem. Thus, if the |
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172 | error handler is changed from MPI_ERRORS_ARE_FATAL to something else |
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173 | (e.g., MPI_ERRORS_RETURN), then no error may be indicated. |
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174 | .sp |
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175 | The reason for this is the performance problems in ensuring that |
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176 | all collective routines return the same error value. |
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177 | |
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178 | .SH ERRORS |
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179 | .ft R |
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180 | Almost all MPI routines return an error value; C routines as |
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181 | the value of the function and Fortran routines in the last argument. C++ |
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182 | functions do not return errors. If the default error handler is set to |
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183 | MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism |
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184 | will be used to throw an MPI:Exception object. |
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185 | .sp |
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186 | Before the error value is returned, the current MPI error handler is |
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187 | called. By default, this error handler aborts the MPI job, except for |
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188 | I/O function errors. The error handler may be changed with |
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189 | MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN |
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190 | may be used to cause error values to be returned. Note that MPI does not |
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191 | guarantee that an MPI program can continue past an error. |
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192 | .sp |
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193 | See the MPI man page for a full list of MPI error codes. |
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194 | |
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195 | .SH SEE ALSO |
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196 | .ft R |
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197 | .nf |
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198 | MPI_Exscan |
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199 | MPI_Op_create |
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200 | MPI_Reduce |
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201 | |
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