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   posix_spawn.3p    ( 3 )

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Возвращаемое значение (Return value)

Upon successful completion, posix_spawn() and posix_spawnp()
       shall return the process ID of the child process to the parent
       process, in the variable pointed to by a non-NULL pid argument,
       and shall return zero as the function return value.  Otherwise,
       no child process shall be created, the value stored into the
       variable pointed to by a non-NULL pid is unspecified, and an
       error number shall be returned as the function return value to
       indicate the error. If the pid argument is a null pointer, the
       process ID of the child is not returned to the caller.