asm_book/more/strlen_for_c

Determining the Length of Strings for C Functions

C string have no soul, is something I like to tell my students. Unlike C++ strings, you can't ask them to tell you how long they are. Instead you must use another function such as strlen().

When a string is fixed within your assembly code, you can let the assembler itself calculate the length for you.

Here is an example of a C function that requires you to specify a string's length:

write(1, "Hello, World!\n", 14);

This sends the familiar string to stdout.

In assembly language, the string would have been placed in a .data section using the .asciz directive. But! How to get the length of the string? You could:

• hard code the length as I did above, or

• go through the effort of calling strlen()

There is a third option as demonstrated by the following program:

        .global        main                                             // 1 
        .align         2                                                // 2 
        .text                                                           // 3 
                                                                        // 4 
main:    str           x30, [sp, -16]!                                  // 5 
        mov            w0, 1             // stdout                      // 6 
        ldr            x1, =s            // pointer to string           // 7 
        ldr            x2, =ssize        // pointer to computed length  // 8 
        ldr            w2, [x2]          // actual length of string     // 9 
        bl             write                                            // 10 
                                                                        // 11 
        ldr            x0, =fmt                                         // 12 
        ldr            x1, =s                                           // 13 
        ldr            x2, =ssize                                       // 14 
        ldr            w2, [x2]                                         // 15 
        bl             printf                                           // 16 
                                                                        // 17 
        ldr            x30, [sp], 16                                    // 18 
        mov            w0, wzr                                          // 19 
        ret                                                             // 20 
                                                                        // 21 
        .data                                                           // 22 
                                                                        // 23 
s:      .asciz         "Hello, World!\n"                                // 24 
ssize:  .word          ssize - s - 1        // accounts for null at end // 25 
fmt:    .asciz         "str: %slen: %d\n"   // accounts for newline     // 26 
                                                                        // 27 
        .end                                                            // 28 

Line 24 contains the string. It is null terminated.

Line 25 is the new learning. The assembler calculates the difference between the address of s and the address of ssize and puts it at the location of ssize. Then, it is used on Lines 8 and 9 like any other statically stored data.

Here is the output of the program:

Hello, World!
str: Hello, World!
len: 14