I'm writing a MIPS assembly code that will ask the user for the file name and it will produce some statistics about the content of the file.

However, when I hard code the file name into a variable from the beginning it works just fine, but when I ask the user to input the file name it does not work.

after some debugging, I have discovered that the program adds 0x00 char and 0x0a char (check asciitable.com) at the end of user input in the memory and that's why it does not open the file based on the user input.

anyone has any idea about how to get rid of those extra chars, or how to open the file after getting its name from the user??

here is my complete code (it is working fine except for the file name from user thing, and anybody is free to use it for any purpose he/she wants to):

        .data
fin:   .ascii ""      # filename for input
msg0:   .asciiz "aaaa"
msg1:   .asciiz "Please enter the input file name:"
msg2:   .asciiz "Number of Uppercase Char: "
msg3:   .asciiz "Number of Lowercase Char: "
msg4:   .asciiz "Number of Decimal Char:   "
msg5:   .asciiz "Number of Words:          "
nline:  .asciiz "\n"
buffer: .asciiz ""
        .text

#-----------------------
    li $v0, 4
    la $a0, msg1
    syscall

    li $v0, 8
    la $a0, fin
    li $a1, 21
    syscall

    jal fileRead            #read from file

    move $s1, $v0           #$t0 = total number of bytes

    li $t0, 0   # Loop counter
    li $t1, 0   # Uppercase counter
    li $t2, 0   # Lowercase counter
    li $t3, 0   # Decimal counter
    li $t4, 0   # Words counter

loop:
    bge $t0, $s1, end           #if end of file reached OR if there is an error in the file
    lb $t5, buffer($t0)         #load next byte from file

    jal checkUpper              #check for upper case
    jal checkLower              #check for lower case
    jal checkDecimal            #check for decimal
    jal checkWord               #check for words


    addi $t0, $t0, 1            #increment loop counter

j loop

end:

    jal output
    jal fileClose

    li $v0, 10
    syscall







fileRead:
    # Open file for reading
    li   $v0, 13       # system call for open file
    la   $a0, fin      # input file name
    li   $a1, 0        # flag for reading
    li   $a2, 0        # mode is ignored
    syscall            # open a file 
    move $s0, $v0      # save the file descriptor 

    # reading from file just opened
    li   $v0, 14       # system call for reading from file
    move $a0, $s0      # file descriptor 
    la   $a1, buffer   # address of buffer from which to read
    li   $a2, 100000   # hardcoded buffer length
    syscall            # read from file

jr $ra

output:
    li $v0, 4
    la $a0, msg2
    syscall

    li $v0, 1
    move $a0, $t1
    syscall

    li $v0, 4
    la $a0, nline
    syscall

    li $v0, 4
    la $a0, msg3
    syscall

    li $v0, 1
    move $a0, $t2
    syscall

    li $v0, 4
    la $a0, nline
    syscall

    li $v0, 4
    la $a0, msg4
    syscall

    li $v0, 1
    move $a0, $t3
    syscall

    li $v0, 4
    la $a0, nline
    syscall

    li $v0, 4
    la $a0, msg5
    syscall

    addi $t4, $t4, 1
    li $v0, 1
    move $a0, $t4
    syscall

jr $ra

checkUpper:
    blt $t5, 0x41, L1           #branch if less than 'A'
    bgt $t5, 0x5a, L1           #branch if greater than 'Z'
    addi $t1, $t1, 1            #increment Uppercase counter

    L1:
jr $ra

checkLower:
    blt $t5, 0x61, L2           #branch if less than 'a'
    bgt $t5, 0x7a, L2           #branch if greater than 'z'
    addi $t2, $t2, 1            #increment Lowercase counter

    L2:
jr $ra

checkDecimal:
    blt $t5, 0x30, L3           #branch if less than '0'
    bgt $t5, 0x39, L3           #branch if greater than '9'
    addi $t3, $t3, 1            #increment Decimal counter

    L3:
jr $ra

checkWord:
    bne $t5, 0x20, L4           #branch if 'space'
    addi $t4, $t4, 1            #increment words counter

    L4:
jr $ra

fileClose:
    # Close the file 
    li   $v0, 16       # system call for close file
    move $a0, $s0      # file descriptor to close
    syscall            # close file
jr $ra

Note: I'm using MARS Simulator, if that makes any different