Program termination and exit codes

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Every program returns an exit status upon termination, supplied as the single argument to the exit system call.

Although system calls can accept multiple arguments, exit requires only one: the exit code.

x86_64 Linux,

On x86_64 Linux, the first argument to any system call is placed in the rdi register.

An 8-bit status value passed to the kernel when a process terminates (e.g., via the exit syscall). The ABI dictates:

• How this status is encoded in the wait/waitpid status word
• That only the low 8 bits are reported to the parent process or shell