glGetActiveAttrib 3G
glGetActiveAttrib Returns information about an active attribute variable for the specified program object

## C Specification

void `glGetActiveAttrib` GLuint program GLuint index GLsizei bufSize GLsizei *length GLint *size GLenum *type GLchar *name

## Parameters

program

  Specifies the program object to be queried.

index

  Specifies the index of the attribute variable to be queried.

bufSize

  Specifies the maximum number of characters OpenGL is allowed to write in the character buffer indicated by name.

length

  Returns the number of characters actually written by OpenGL in the string indicated by name (excluding the null terminator) if a value other than `NULL` is passed.

size

  Returns the size of the attribute variable.

type

  Returns the data type of the attribute variable.

name

  Returns a null terminated string containing the name of the attribute variable.

## Description

`glGetActiveAttrib` returns information about an active attribute variable in the program object specified by program. The number of active attributes can be obtained by calling glGetProgram with the value `GL_ACTIVE_ATTRIBUTES`. A value of 0 for index selects the first active attribute variable. Permissible values for index range from 0 to the number of active attribute variables minus 1.

A vertex shader may use either built-in attribute variables, user-defined attribute variables, or both. Built-in attribute variables have a prefix of "gl_" and reference conventional OpenGL vertex attribtes (e.g., gl_Vertex, gl_Normal, etc., see the OpenGL Shading Language specification for a complete list.) User-defined attribute variables have arbitrary names and obtain their values through numbered generic vertex attributes. An attribute variable (either built-in or user-defined) is considered active if it is determined during the link operation that it may be accessed during program execution. Therefore, program should have previously been the target of a call to glLinkProgram, but it is not necessary for it to have been linked successfully.

The size of the character buffer required to store the longest attribute variable name in program can be obtained by calling glGetProgram with the value `GL_ACTIVE_ATTRIBUTE_MAX_LENGTH`. This value should be used to allocate a buffer of sufficient size to store the returned attribute name. The size of this character buffer is passed in bufSize, and a pointer to this character buffer is passed in name.

`glGetActiveAttrib` returns the name of the attribute variable indicated by index, storing it in the character buffer specified by name. The string returned will be null terminated. The actual number of characters written into this buffer is returned in length, and this count does not include the null termination character. If the length of the returned string is not required, a value of `NULL` can be passed in the length argument.

The type argument specifies a pointer to a variable into which the attribute variable's data type will be written. The symbolic constants `GL_FLOAT`, `GL_FLOAT_VEC2`, `GL_FLOAT_VEC3`, `GL_FLOAT_VEC4`, `GL_FLOAT_MAT2`, `GL_FLOAT_MAT3`, `GL_FLOAT_MAT4`, `GL_FLOAT_MAT2x3`, `GL_FLOAT_MAT2x4`, `GL_FLOAT_MAT3x2`, `GL_FLOAT_MAT3x4`, `GL_FLOAT_MAT4x2`, `GL_FLOAT_MAT4x3`, `GL_INT`, `GL_INT_VEC2`, `GL_INT_VEC3`, `GL_INT_VEC4`, `GL_UNSIGNED_INT`, `GL_UNSIGNED_INT_VEC2`, `GL_UNSIGNED_INT_VEC3`, `GL_UNSIGNED_INT_VEC4`, `DOUBLE`, `DOUBLE_VEC2`, `DOUBLE_VEC3`, `DOUBLE_VEC4`, `DOUBLE_MAT2`, `DOUBLE_MAT3`, `DOUBLE_MAT4`, `DOUBLE_MAT2x3`, `DOUBLE_MAT2x4`, `DOUBLE_MAT3x2`, `DOUBLE_MAT3x4`, `DOUBLE_MAT4x2`, or `DOUBLE_MAT4x3` may be returned. The size argument will return the size of the attribute, in units of the type returned in type.

The list of active attribute variables may include both built-in attribute variables (which begin with the prefix "gl_") as well as user-defined attribute variable names.

This function will return as much information as it can about the specified active attribute variable. If no information is available, length will be 0, and name will be an empty string. This situation could occur if this function is called after a link operation that failed. If an error occurs, the return values length, size, type, and name will be unmodified.

## Errors

`GL_INVALID_VALUE` is generated if program is not a value generated by OpenGL.

`GL_INVALID_OPERATION` is generated if program is not a program object.

`GL_INVALID_VALUE` is generated if index is greater than or equal to the number of active attribute variables in program.

`GL_INVALID_VALUE` is generated if bufSize is less than 0.

## Associated Gets

glGet with argument `GL_MAX_VERTEX_ATTRIBS`.

glGetProgram with argument `GL_ACTIVE_ATTRIBUTES` or `GL_ACTIVE_ATTRIBUTE_MAX_LENGTH`.

glIsProgram

## See Also

glBindAttribLocation, glLinkProgram, glVertexAttrib, glVertexAttribPointer

## Copyright

Copyright 2003-2005 3Dlabs Inc. Ltd. Copyright 2010 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.