Compute the (Moore-Penrose) pseudo-inverse of a libgsl matrix in plain C.

moore_penrose_pseudoinverse.c

/**
 * Compute the (Moore-Penrose) pseudo-inverse of a libgsl matrix in plain C.
 *
 * Compile uding:
 *
 *     gcc moore_penrose_pseudoinverse.c -lgsl -lblas
 *
 * Dependencies:
 * - libgsl (GNU Scientific Library) 
 * - libblas (Basic Linear Algebra Subprograms)
 *
 * Charl Linssen <charl@itfromb.it>
 * Feb 2016
 * PUBLIC DOMAIN
**/

#include <stdio.h>
#include <gsl/gsl_blas.h>
#include <gsl/gsl_linalg.h>


typedef double realtype;

#define max(a,b)		((a) > (b) ? (a) : (b))
#define min(a,b)		((a) < (b) ? (a) : (b))


typedef unsigned int bool;
#define false		0
#define true 		1




void print_matrix(const gsl_matrix *m) {
	size_t i, j;

	for (i = 0; i < m->size1; i++) {
		for (j = 0; j < m->size2; j++) {
			printf("%f\t", gsl_matrix_get(m, i, j));
		}
		printf("\n");
	}
}

void print_vector (const gsl_vector * v) {
	size_t i;

	for (i = 0; i < v->size; i++) {
		printf("%f\t", gsl_vector_get (v, i));
	}
}


/**
 * Compute the (Moore-Penrose) pseudo-inverse of a matrix.
 *
 * If the singular value decomposition (SVD) of A = UΣVᵀ then the pseudoinverse A⁻¹ = VΣ⁻¹Uᵀ, where ᵀ indicates transpose and Σ⁻¹ is obtained by taking the reciprocal of each nonzero element on the diagonal, leaving zeros in place. Elements on the diagonal smaller than ``rcond`` times the largest singular value are considered zero.
 *
 * @parameter A		Input matrix. **WARNING**: the input matrix ``A`` is destroyed. However, it is still the responsibility of the caller to free it.
 * @parameter rcond		A real number specifying the singular value threshold for inclusion. NumPy default for ``rcond`` is 1E-15.
 *
 * @returns A_pinv		Matrix containing the result. ``A_pinv`` is allocated in this function and it is the responsibility of the caller to free it.
**/
gsl_matrix* moore_penrose_pinv(gsl_matrix *A, const realtype rcond) {

	gsl_matrix *V, *Sigma_pinv, *U, *A_pinv;
	gsl_matrix *_tmp_mat = NULL;
	gsl_vector *_tmp_vec;
	gsl_vector *u;
	realtype x, cutoff;
	size_t i, j;
	unsigned int n = A->size1;
	unsigned int m = A->size2;
	bool was_swapped = false;


	if (m > n) {
		/* libgsl SVD can only handle the case m <= n - transpose matrix */
		was_swapped = true;
		_tmp_mat = gsl_matrix_alloc(m, n);
		gsl_matrix_transpose_memcpy(_tmp_mat, A);
		A = _tmp_mat;
		i = m;
		m = n;
		n = i;
	}

	/* do SVD */
	V = gsl_matrix_alloc(m, m);
	u = gsl_vector_alloc(m);
	_tmp_vec = gsl_vector_alloc(m);
	gsl_linalg_SV_decomp(A, V, u, _tmp_vec);
	gsl_vector_free(_tmp_vec);

	/* compute Σ⁻¹ */
	Sigma_pinv = gsl_matrix_alloc(m, n);
	gsl_matrix_set_zero(Sigma_pinv);
	cutoff = rcond * gsl_vector_max(u);

	for (i = 0; i < m; ++i) {
		if (gsl_vector_get(u, i) > cutoff) {
			x = 1. / gsl_vector_get(u, i);
		}
		else {
			x = 0.;
		}
		gsl_matrix_set(Sigma_pinv, i, i, x);
	}

	/* libgsl SVD yields "thin" SVD - pad to full matrix by adding zeros */
	U = gsl_matrix_alloc(n, n);
	gsl_matrix_set_zero(U);

	for (i = 0; i < n; ++i) {
		for (j = 0; j < m; ++j) {
			gsl_matrix_set(U, i, j, gsl_matrix_get(A, i, j));
		}
	}

	if (_tmp_mat != NULL) {
		gsl_matrix_free(_tmp_mat);
	}

	/* two dot products to obtain pseudoinverse */
	_tmp_mat = gsl_matrix_alloc(m, n);
	gsl_blas_dgemm(CblasNoTrans, CblasNoTrans, 1., V, Sigma_pinv, 0., _tmp_mat);

	if (was_swapped) {
		A_pinv = gsl_matrix_alloc(n, m);
		gsl_blas_dgemm(CblasNoTrans, CblasTrans, 1., U, _tmp_mat, 0., A_pinv);
	}
	else {
		A_pinv = gsl_matrix_alloc(m, n);
		gsl_blas_dgemm(CblasNoTrans, CblasTrans, 1., _tmp_mat, U, 0., A_pinv);
	}

	gsl_matrix_free(_tmp_mat);
	gsl_matrix_free(U);
	gsl_matrix_free(Sigma_pinv);
	gsl_vector_free(u);
	gsl_matrix_free(V);

	return A_pinv;
}


int main() {

	const unsigned int N = 2;
	const unsigned int M = 3;
	const realtype rcond = 1E-15;


	gsl_matrix *A = gsl_matrix_alloc(N, M);
	gsl_matrix *A_pinv;

	gsl_matrix_set(A, 0, 0, 1.);
	gsl_matrix_set(A, 0, 1, 3.);
	gsl_matrix_set(A, 0, 2, 5.);
	gsl_matrix_set(A, 1, 0, 2.);
	gsl_matrix_set(A, 1, 1, 4.);
	gsl_matrix_set(A, 1, 2, 6.);

	printf("A matrix:\n");
	print_matrix(A);
	A_pinv = moore_penrose_pinv(A, rcond);
	printf("\nPseudoinverse of A:\n");
	print_matrix(A_pinv);

	gsl_matrix_free(A);
	gsl_matrix_free(A_pinv);

	return 0;
}

You forgot:
#include <gsl/gsl_linalg.h>
For:
gsl_linalg_SV_decomp(A, V, u, _tmp_vec);
https://gist.github.com/turingbirds/5e99656e08dbe1324c99#file-moore_penrose_pseudoinverse-c-L92

Thank you @GhostNaN! Gist has been updated.

No problem @turingbirds! It works great for getting my Savitzky Golay coefficients.