SystemVerilog file operations |
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System Verilog allows us to read and write into files in the disk. How to open and close a file ?A file can be opened for either read or write using the $fopen() system task. This task will return a 32-bit integer handle called a file descriptor. This handle should be used to read and write into that file until it is closed. The file descriptor can be closed with the $fclose() system task. No further reads or writes to the file descriptor is allowed once it is closed. ExampleIn the code shown below, we will declare a int variable called fd to hold the file descriptor. fd is initially zero, and gets a valid value from $fopen() and can be checked to see if the file opened successfully. The file is finally closed when $fclose() is executed. module tb; initial begin // 1. Declare an integer variable to hold the file descriptor int fd; // 2. Open a file called "note.txt" in the current folder with a "read" permission // If the file does not exist, then fd will be zero fd = $fopen ("./note.txt", "r"); if (fd) $display("File was opened successfully : %0d", fd); else $display("File was NOT opened successfully : %0d", fd); // 2. Open a file called "note.txt" in the current folder with a "write" permission // "fd" now points to the same file, but in write mode fd = $fopen ("./note.txt", "w"); if (fd) $display("File was opened successfully : %0d", fd); else $display("File was NOT opened successfully : %0d", fd); // 3. Close the file descriptor $fclose(fd); end endmoduleIt is important to close all open files before end of simulation to completely write contents into the file Simulation Log ncsim> run Open failed on file "./note.txt". No such file or directory File was NOT opened successfully : 0 File was opened successfully : -2147483645 ncsim: *W,RNQUIE: Simulation is complete. How to open in read and append modes ?By default a file is opened in the write w mode. The file can also be opened in other modes by providing the correct mode type. The following table shows all the different modes a file can be opened in. ArgumentDescription"r"Open for reading"w"Create for writing, overwrite if it exists"a"Create if file does not exist, else append; open for writing at end of file"r+"Open for update (reading and writing)"w+"Truncate or create for update"a+"Append, open or create for update at EOFExampleIn the following code, we will see how to use the different file access modes as described in the table above. module tb; initial begin int fd_w, fd_r, fd_a, fd_wp, fd_rp, fd_ap; fd_w = $fopen ("./todo.txt", "w"); // Open a new file in write mode and store file descriptor in fd_w fd_r = $fopen ("./todo.txt", "r"); // Open in read mode fd_a = $fopen ("./todo.txt", "a"); // Open in append mode if (fd_w) $display("File was opened successfully : %0d", fd_w); else $display("File was NOT opened successfully : %0d", fd_w); if (fd_r) $display("File was opened successfully : %0d", fd_r); else $display("File was NOT opened successfully : %0d", fd_r); if (fd_a) $display("File was opened successfully : %0d", fd_a); else $display("File was NOT opened successfully : %0d", fd_a); // Close the file descriptor $fclose(fd_w); $fclose(fd_r); $fclose(fd_a); end endmoduleAs you can see from the log shown below, all three variables have a different value and each one points to the same file, but with different access permissions. Simulation Log ncsim> run ncsim: *W,VFOPTW: File ./todo.txt being opened by Initial stmt (file: ./testbench.sv, line: 2 in worklib.tb [module]) has been opened earlier. File was opened successfully : -2147483645 File was opened successfully : -2147483644 File was opened successfully : -2147483643 ncsim: *W,RNQUIE: Simulation is complete. How to read and write to a file ?Files should be opened in the write w mode, or append a mode. System tasks like $fdisplay() and $fwrite() can be used to write a formatted string into the file. The first argument of these tasks is the file descriptor handle and the second will be the data to be stored. To read a file, it has to be opened in either read r mode or read-write r+ mode. $fgets() is a system task that will read a single line from the file. If this task is called 10 times, then it will read 10 lines. ExampleThe code shown below demonstrates how to open a file and write contents into it using $fdisplay(). The file is then opened in read mode and contents are read back using $fgets() into a local variable. This is then printed out using the standard display task $display() module tb; int fd; // Variable for file descriptor handle string line; // String value read from the file initial begin // 1. Lets first open a new file and write some contents into it fd = $fopen ("trial", "w"); // Write each index in the for loop to the file using $fdisplay // File handle should be the first argument for (int i = 0; i Simulation Log ncsim> run Line read : Iteration = 0 Line read : Iteration = 1 ncsim: *W,RNQUIE: Simulation is complete. How to read until end of file ?In the previous example, we used $fgets() system task twice to read two lines from the file. SystemVerilog has another task called $feof() that returns true when end of the file has reached. This can be used in a loop as follows to read the entire contents of a file. Example module tb; int fd; // Variable for file descriptor handle string line; // String value read from the file initial begin // 1. Lets first open a new file and write some contents into it fd = $fopen ("trial", "w"); for (int i = 0; i Simulation Log ncsim> run Line: Iteration = 0 Line: Iteration = 1 Line: Iteration = 2 Line: Iteration = 3 Line: Iteration = 4 Line: ncsim: *W,RNQUIE: Simulation is complete. How to parse a line for values ?SystemVerilog has another system task called $fscanf() that allows us to scan and get certain values. Example module tb; int fd; // Variable for file descriptor handle int idx; string str; initial begin // 1. Lets first open a new file and write some contents into it fd = $fopen ("trial", "w"); for (int i = 0; i Simulation Log ncsim> run Line: Iteration = 0 Line: Iteration = 1 Line: Iteration = 2 Line: Iteration = 3 Line: Iteration = 4 ncsim: *W,RNQUIE: Simulation is complete. What are multichannel file descriptors ?The mcd is a 32-bit packed array value in which a single bit is set indicating which file is opened. The LSB of an mcd always refers to the standard output. Output is directed to two or more files opened with multichannel descriptors by bitwise OR-ing together their mcds, and writing to the resultant value. |
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