The FORTRAN level-2 BLAS call for this operation is

SUBROUTINE TRSV ( UPLO, TRANS, DIAG, N, A, LDA, X, INCX )

Here is replaced by S, D, C, or Z
to indicate the datatype of the data (single, double, complex, double precision complex).
Parameter UPLO indicates whether A is
lower (UPLO = "Lower") or upper (UPLO = "Upper") triangular.
Parameter TRANS indicates if A is to be transposed as part of
the operation (TRANS = "No transpose", "Transpose", or "Conjugate transpose").
Parameter DIAG indicates if the diagonal of
A is to be taken to equal the identity matrix
(DIAG = "Unit" and or )
or the values in the matrix
(DIAG = "Non unit" and A = U or A = L ).
Parameter N holds the matrix dimension
n .
Parameters A and X equal the
addresses of where matrix A and vector x start in
memory. The leading dimensions of the matrix is given in
LDA and the strides for vector in INCX.