not a valid l-value - verilog compiler error - verilog

Question

module fronter ( arc, length, clinic ) ;
input [7:0] arc;
output reg [7:0] length ;
input [1:0] clinic;
input en0, en1, en2, en3; // 11
// clock generator is here
g_cal A( en0) ;
g_cal B( en1) ;
g_cal C( en2) ;
g_cal D( en3) ;
always #( negedge arc, posedge clk )
case ( clinic )
2'b00 : { en3, en2, en1, en0 } = 4'b0001; // 23
2'b01 : { en3, en2, en1, en0 } = 4'b0010; // 24
2'b10 : { en3, en2, en1, en0 } = 4'b0100; // 25
2'b11 : { en3, en2, en1, en0 } = 4'b1000; // 26
default : { en3, en2, en1, en0 } = 4'bxxxx; // 27
endcase
// I am trying to change value of en to call corresponding intance with that
//corresponding en value
endmodule
module g_cal ( en ) ;
input en ;
// some other jobs, calling another instances after making some job
endmodule
when I compile, compiler gives me ;
verilog.v:23: error: en0 is not a valid l-value in Numerator.
verilog.v:11: : en0 is declared here as wire.
verilog.v:24: error: en1 is not a valid l-value in Numerator.
verilog.v:11: : en1 is declared here as wire.
verilog.v:25: error: en2 is not a valid l-value in Numerator.
verilog.v:11: : en2 is declared here as wire.
verilog.v:26: error: en3 is not a valid l-value in Numerator.
verilog.v:11: : en3 is declared here as wire.
verilog.v:27: error: en3 is not a valid l-value in Numerator.
verilog.v:11: : en3 is declared here as wire.
segmentation fault
How can I fix it ?
Why it gives error?
EDIT:
I have solved problem as ;
// I erased that line "input en0, en1, en2, en3; // 11"
// clock generator is here
g_cal A( 1'b0) ;
g_cal B( 1'b0) ;
g_cal C( 1'b0) ;
g_cal D( 1'b0) ;
always #( negedge arc, posedge clk )
/* erasing all those line
case ( clinic )
2'b00 : { en3, en2, en1, en0 } = 4'b0001; // 23
2'b01 : { en3, en2, en1, en0 } = 4'b0010; // 24
2'b10 : { en3, en2, en1, en0 } = 4'b0100; // 25
2'b11 : { en3, en2, en1, en0 } = 4'b1000; // 26
default : { en3, en2, en1, en0 } = 4'bxxxx; // 27
endcase
I will use if and else structure, and calling corresponding instance with 1'b1*/
// I am trying to change value of en to call corresponding intance with that
//corresponding en value
endmodule

Answer 1

You're trying assign to an input (which is bad). Change input en0, en1, en2, en3; to output reg en0, en1, en2, en3;. The reg is necessary since you are assigning to that variable within a procedural block (ie, an always or initial). The "not a valid l-value" message is trying to tell you this.
Also, I'm assuming that the 11, 23, 24, etc are stray line numbers from a copy-paste...

Answer 2

Problem has solved when I write ;
reg en0, en1, en2, en3 ;
initial begin
en0 <= 1'b0; en1 <= 1'b0;
en2 <= 1'b0; en3 <= 1'b0;
end
g_cal A( en0) ;
g_cal B( en1) ;
g_cal C( en2) ;
g_cal D( en3) ;
#Marty have emphasized important thing "The reg is necessary since you are assigning to that variable within a procedural block (ie, an always or initial)."