Implement strlen in Assembly

Topic Overview

Null-Terminated Walk

C strings end at the first 0x00 byte. strlen does not know the length upfront; it scans. That design powers every format string and buffer overflow class in the CVE database. glibc's production strlen uses SIMD to test 32 bytes per iteration. Your byte loop is the baseline it beats.

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• rdi holds the string pointer per the AMD64 ABI
• Return the count in rax, excluding the terminator
• Empty string: first byte is \0, return 0

rep scasb Exists, But Learn the Loop

x86 has rep scasb to compare AL against ES:[EDI], but manual cmp/inc teaches the contract.

For this exercise, you will implement my_strlen in NASM and return the length in rax. This task asks you to handle the empty string and preserve the pointer in rdi, because every hand-tuned libc string routine starts from this scan.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB. Skim the official documentation for the tool or ABI named in the exercise; the prose changes, but register roles, syscall numbers, and ownership rules stay stable across releases.

Your Task

Write strlen in x86-64 NASM:

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• Count bytes until the NUL terminator. The NUL is not counted.
• The empty string has length 0.
• Use only caller-saved registers (rax, rcx, rdx, rsi, rdi, r8-r11), or save and restore any callee-saved register you touch: the harness keeps its own state in rbx and r12-r15.

The harness reads one line, passes it to my_strlen as a NUL-terminated string without its line break, and prints the length.

Input

One line of text, possibly empty, possibly containing spaces.

Output

Its length in bytes: 5 for hello, 0 for an empty line.