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added to string length chapter

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Perry Kivolowitz 2022-08-07 19:29:13 -05:00
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more/strlen_for_c/README.md
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@ -1,14 +1,12 @@
# Determining the Length of Strings for C Functions # Determining the Length of Strings for C Functions
C string have no soul, is something I like to tell my students. Unlike C string have no soul, is something I like to tell my students. For
C++ strings, you can't ask them to tell you how long they are. Instead example, uUnlike C++ strings, you can't ask them to tell you about
you must use another function such as `strlen()`. themselves. Instead you must use other functions such as `strlen()`.
When a string is fixed within your assembly code, you can let the When working with APIs that use C strings, you must often tell the
assembler itself calculate the length for you. API the length of those strings (because, well, C string have no
soul). Here is an example:
Here is an example of a C function that requires you to specify a
string's length:
```c ```c
write(1, "Hello, World!\n", 14); write(1, "Hello, World!\n", 14);
@ -16,9 +14,14 @@ write(1, "Hello, World!\n", 14);
This sends the familiar string to `stdout`. This sends the familiar string to `stdout`.
In assembly language, the string would have been placed in a `.data` ## In Assembly Language
section using the `.asciz` directive. But! How to get the length of the
string? You could: When a string is fixed within your assembly code, you can let the
assembler itself calculate the length for you.
In assembly language, the string would likely 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 * hard code the length as I did above, or
@ -62,7 +65,11 @@ fmt: .asciz "str: %slen: %d\n" // accounts for newline // 26
`Line 25` is the new learning. The assembler calculates the difference `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 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 location of `ssize`. One is subtracted from the length because of the
null termination of the string. You might not see the null terminator
but it takes up space.
An example of using the stored length is on `Lines 8` and `9` like any
other statically stored data. other statically stored data.
Here is the output of the program: Here is the output of the program: