mxGetField (C and Fortran)

Get field value from structure array, given index and field name

C Syntax

#include "matrix.h"
mxArray *mxGetField(const mxArray *pm, mwIndex index,
         const char *fieldname);

Fortran Syntax

mwPointer mxGetField(pm, index, fieldname)
mwPointer pm
mwIndex index
character*(*) fieldname



Pointer to a structure mxArray


Index of the desired element.

In C, the first element of an mxArray has an index of 0. The index of the last element is N-1, where N is the number of elements in the array. In Fortran, the first element of an mxArray has an index of 1. The index of the last element is N, where N is the number of elements in the array.


Name of the field whose value you want to extract.


Pointer to the mxArray in the specified field at the specified fieldname, on success. Returns NULL in C (0 in Fortran) if passed an invalid argument or if there is no value assigned to the specified field. Common causes of failure include:

  • Specifying an array pointer pm that does not point to a structure mxArray. To determine whether pm points to a structure mxArray, call mxIsStruct.

  • Specifying an index to an element outside the bounds of the mxArray. For example, given a structure mxArray that contains 10 elements, you cannot specify an index greater than 9 in C (10 in Fortran).

  • Specifying a nonexistent fieldname. Call mxGetFieldNameByNumber or mxGetFieldNumber to get existing field names.

  • Insufficient heap space.

If the pointer in the field is NULL, mxGetField allocates an empty, double mxArray.


Call mxGetField to get the value held in the specified element of the specified field. In pseudo-C terminology, mxGetField returns the value at:


mxGetFieldByNumber is like mxGetField. Both functions return the same value. The only difference is in the way you specify the field. mxGetFieldByNumber takes a field number as its third argument, and mxGetField takes a field name as its third argument.

Do not call mxDestroyArray on an mxArray returned by the mxGetField function.

    Note   Inputs to a MEX-file are constant read-only mxArrays. Do not modify the inputs. Using mxSetCell* or mxSetField* functions to modify the cells or fields of a MATLAB® argument causes unpredictable results.

In C, calling:

mxGetField(pa, index, "field_name");

is equivalent to calling:

field_num = mxGetFieldNumber(pa, "field_name");
mxGetFieldByNumber(pa, index, field_num);

where, if you have a 1-by-1 structure, index is 0.

In Fortran, calling:

mxGetField(pm, index, 'fieldname')

is equivalent to calling:

fieldnum = mxGetFieldNumber(pm, 'fieldname')
mxGetFieldByNumber(pm, index, fieldnum)

where, if you have a 1-by-1 structure, index is 1.


See the following example in matlabroot/extern/examples/eng_mat.

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