Added API to set mt environment variables.

Details:
- Renamed bli_env_get_nway() -> bli_thread_get_env().
- Added bli_thread_set_env() to allow setting environment variables
  pertaining to multithreading, such as BLIS_JC_NT or BLIS_NUM_THREADS.
- Added the following convenience wrapper routines:
    bli_thread_get_jc_nt()
    bli_thread_get_ic_nt()
    bli_thread_get_jr_nt()
    bli_thread_get_ir_nt()
    bli_thread_get_num_threads()
    bli_thread_set_jc_nt()
    bli_thread_set_ic_nt()
    bli_thread_set_jr_nt()
    bli_thread_set_ir_nt()
    bli_thread_set_num_threads()
- Added #include "errno.h" to bli_system.h.
- This commit addresses issue #140.
- Thanks to Chris Goodyer for inspiring these updates.

frame/base/bli_cntx.c

@@ -756,10 +756,10 @@ void bli_cntx_set_thrloop_from_env( opid_t l3_op, side_t side, cntx_t* cntx,
 
 #ifdef BLIS_ENABLE_MULTITHREADING
 
-	int nthread = bli_env_read_nway( "BLIS_NUM_THREADS", -1 );
+	int nthread = bli_thread_get_env( "BLIS_NUM_THREADS", -1 );
 
 	if ( nthread == -1 )
-	    nthread = bli_env_read_nway( "OMP_NUM_THREADS", -1 );
+	    nthread = bli_thread_get_env( "OMP_NUM_THREADS", -1 );
 
 	if ( nthread < 1 ) nthread = 1;
 
@@ -786,10 +786,10 @@ void bli_cntx_set_thrloop_from_env( opid_t l3_op, side_t side, cntx_t* cntx,
 
     pc = 1;
 
-    dim_t jc_env = bli_env_read_nway( "BLIS_JC_NT", -1 );
-    dim_t ic_env = bli_env_read_nway( "BLIS_IC_NT", -1 );
-    dim_t jr_env = bli_env_read_nway( "BLIS_JR_NT", -1 );
-    dim_t ir_env = bli_env_read_nway( "BLIS_IR_NT", -1 );
+    dim_t jc_env = bli_thread_get_env( "BLIS_JC_NT", -1 );
+    dim_t ic_env = bli_thread_get_env( "BLIS_IC_NT", -1 );
+    dim_t jr_env = bli_thread_get_env( "BLIS_JR_NT", -1 );
+    dim_t ir_env = bli_thread_get_env( "BLIS_IR_NT", -1 );
 
     if (jc_env != -1 || ic_env != -1 || jr_env != -1 || ir_env != -1)
     {

frame/include/bli_system.h

@@ -41,6 +41,7 @@
 #include <string.h>
 #include <stdarg.h>
 #include <float.h>
+#include <errno.h>
 
 // Determine if we are on a 64-bit or 32-bit architecture
 #if defined(_M_X64) || defined(__x86_64) || defined(__aarch64__) || \

frame/thread/bli_thread.c

@@ -1156,19 +1156,112 @@ void bli_partition_2x2( dim_t nthread, dim_t work1, dim_t work2,
 
 // -----------------------------------------------------------------------------
 
-// Some utilities
-dim_t bli_env_read_nway( const char* env, dim_t fallback )
+dim_t bli_thread_get_env( const char* env, dim_t fallback )
 {
-	dim_t num = fallback;
-	char* str = getenv( env );
+	dim_t r_val;
+	char* str;
 
+	// Query the environment variable and store the result in str.
+	str = getenv( env );
+
+	// Set the return value based on the string obtained from getenv().
 	if ( str != NULL )
-	{   
-		num = strtol( str, NULL, 10 );
-	}   
-	return num;
+	{
+		// If there was no error, convert the string to an integer and
+		// prepare to return that integer.
+		r_val = strtol( str, NULL, 10 );
+	}
+	else
+	{
+		// If there was an error, use the "fallback" as the return value.
+		r_val = fallback;
+	}
+
+	return r_val;
 }
 
+dim_t bli_thread_get_jc_nt( void )
+{
+	return bli_thread_get_env( "BLIS_JC_NT", 1 );
+}
+
+dim_t bli_thread_get_ic_nt( void )
+{
+	return bli_thread_get_env( "BLIS_IC_NT", 1 );
+}
+
+dim_t bli_thread_get_jr_nt( void )
+{
+	return bli_thread_get_env( "BLIS_JR_NT", 1 );
+}
+
+dim_t bli_thread_get_ir_nt( void )
+{
+	return bli_thread_get_env( "BLIS_IR_NT", 1 );
+}
+
+dim_t bli_thread_get_num_threads( void )
+{
+	return bli_thread_get_env( "BLIS_NUM_THREADS", 1 );
+}
+
+void bli_thread_set_env( const char* env, dim_t value )
+{
+	dim_t       r_val;
+	char        value_str[32];
+	const char* fs_32 = "%u";
+	const char* fs_64 = "%lu";
+
+	// Convert the string to an integer, but vary the format specifier
+	// depending on the integer type size.
+	if ( bli_info_get_int_type_size() == 32 ) sprintf( value_str, fs_32, value );
+	else                                      sprintf( value_str, fs_64, value );
+
+	// Set the environment variable using the string we just wrote to via
+	// sprintf(). (The 'TRUE' argument means we want to overwrite the current
+	// value if the environment variable already exists.)
+	r_val = setenv( env, value_str, TRUE );
+
+	// Check the return value in case something went horribly wrong.
+	if ( r_val == -1 )
+	{
+		char err_str[128];
+
+		// Query the human-readable error string corresponding to errno.
+		strerror_r( errno, err_str, 128 );
+
+		// Print the error message.
+		bli_print_msg( err_str, __FILE__, __LINE__ );
+	}
+}
+
+void bli_thread_set_jc_nt( dim_t value )
+{
+	bli_thread_set_env( "BLIS_JC_NT", value );
+}
+
+void bli_thread_set_ic_nt( dim_t value )
+{
+	bli_thread_set_env( "BLIS_IC_NT", value );
+}
+
+void bli_thread_set_jr_nt( dim_t value )
+{
+	bli_thread_set_env( "BLIS_JR_NT", value );
+}
+
+void bli_thread_set_ir_nt( dim_t value )
+{
+	bli_thread_set_env( "BLIS_IR_NT", value );
+}
+
+void bli_thread_set_num_threads( dim_t value )
+{
+	bli_thread_set_env( "BLIS_NUM_THREADS", value );
+}
+
+// -----------------------------------------------------------------------------
+
 dim_t bli_gcd( dim_t x, dim_t y )
 {
 	while ( y != 0 )

frame/thread/bli_thread.h

@@ -164,6 +164,8 @@ void bli_l3_thread_decorator
        cntl_t* cntl
      );
 
+// -----------------------------------------------------------------------------
+
 // Factorization and partitioning prototypes
 typedef struct
 {
@@ -178,8 +180,26 @@ dim_t bli_next_prime_factor(bli_prime_factors_t* factors);
 
 void bli_partition_2x2(dim_t nthread, dim_t work1, dim_t work2, dim_t* nt1, dim_t* nt2);
 
-// Miscellaneous prototypes
-dim_t bli_env_read_nway( const char* env, dim_t fallback );
+// -----------------------------------------------------------------------------
+
+dim_t bli_thread_get_env( const char* env, dim_t fallback );
+
+dim_t bli_thread_get_jc_nt( void );
+dim_t bli_thread_get_ic_nt( void );
+dim_t bli_thread_get_jr_nt( void );
+dim_t bli_thread_get_ir_nt( void );
+dim_t bli_thread_get_num_threads( void );
+
+void  bli_thread_set_env( const char* env, dim_t value );
+
+void  bli_thread_set_jc_nt( dim_t value );
+void  bli_thread_set_ic_nt( dim_t value );
+void  bli_thread_set_jr_nt( dim_t value );
+void  bli_thread_set_ir_nt( dim_t value );
+void  bli_thread_set_num_threads( dim_t value );
+
+// -----------------------------------------------------------------------------
+
 dim_t bli_gcd( dim_t x, dim_t y );
 dim_t bli_lcm( dim_t x, dim_t y );
 dim_t bli_ipow( dim_t base, dim_t power );