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   ptrace    ( 2 )

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Возвращаемое значение (Return value)

On success, the PTRACE_PEEK* requests return the requested data
       (but see NOTES), the PTRACE_SECCOMP_GET_FILTER request returns
       the number of instructions in the BPF program, the
       PTRACE_GET_SYSCALL_INFO request returns the number of bytes
       available to be written by the kernel, and other requests return
       zero.

       On error, all requests return -1, and errno is set to indicate
       the error.  Since the value returned by a successful PTRACE_PEEK*
       request may be -1, the caller must clear errno before the call,
       and then check it afterward to determine whether or not an error
       occurred.