glGetActiveUniform 3G
glGetActiveUniform return information about an active uniform variable

## C Specification

void `glGetActiveUniform` 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 uniform 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 uniform variable.

type

  Returns the data type of the uniform variable.

name

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

## Description

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

Shaders may use either built-in uniform variables, user-defined uniform variables, or both. Built-in uniform variables have a prefix of "gl_" and reference existing OpenGL state or values derived from such state (e.g., gl_DepthRange). User-defined uniform variables have arbitrary names and obtain their values from the application through calls to glUniform. A uniform 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 uniform variable name in program can be obtained by calling glGetProgramiv with the value `GL_ACTIVE_UNIFORM_MAX_LENGTH`. This value should be used to allocate a buffer of sufficient size to store the returned uniform variable name. The size of this character buffer is passed in bufSize, and a pointer to this character buffer is passed in name.

`glGetActiveUniform` returns the name of the uniform 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 will return a pointer to the uniform variable's data type. The symbolic constants `GL_FLOAT`, `GL_FLOAT_VEC2`, `GL_FLOAT_VEC3`, `GL_FLOAT_VEC4`, `GL_INT`, `GL_INT_VEC2`, `GL_INT_VEC3`, `GL_INT_VEC4`, `GL_BOOL`, `GL_BOOL_VEC2`, `GL_BOOL_VEC3`, `GL_BOOL_VEC4`, `GL_FLOAT_MAT2`, `GL_FLOAT_MAT3`, `GL_FLOAT_MAT4`, `GL_SAMPLER_2D`, or `GL_SAMPLER_CUBE` may be returned.

If one or more elements of an array are active, the name of the array is returned in name, the type is returned in type, and the size parameter returns the highest array element index used, plus one, as determined by the compiler and/or linker. Only one active uniform variable will be reported for a uniform array.

Uniform variables that are declared as structures or arrays of structures will not be returned directly by this function. Instead, each of these uniform variables will be reduced to its fundamental components containing the "." and "[]" operators such that each of the names is valid as an argument to glGetUniformLocation. Each of these reduced uniform variables is counted as one active uniform variable and is assigned an index. A valid name cannot be a structure, an array of structures, or a subcomponent of a vector or matrix.

The size of the uniform variable will be returned in size. Uniform variables other than arrays will have a size of 1. Structures and arrays of structures will be reduced as described earlier, such that each of the names returned will be a data type in the earlier list. If this reduction results in an array, the size returned will be as described for uniform arrays; otherwise, the size returned will be 1.

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

This function will return as much information as it can about the specified active uniform 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 uniform variables in program.

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

## Associated Gets

glGet with argument `GL_MAX_VERTEX_UNIFORM_VECTORS` or `GL_MAX_FRAGMENT_UNIFORM_VECTORS`.

glGetProgramiv with argument `GL_ACTIVE_UNIFORMS` or `GL_ACTIVE_UNIFORM_MAX_LENGTH`.

glIsProgram

## See Also

glGetActiveAttrib, glGetUniform, glGetUniformLocation, glLinkProgram, glUniform, glUseProgram

## Copyright

Copyright 2003-2005 3Dlabs Inc. Ltd. 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/.