hi, when my target board run a period of time, the alloc_mem() return NULL in the interupt_ethernet __irq routine. what's the problems?
Increase heap size
Indeed - probably the most obvious thing that could cause a dynamic memory allocation to fail is that there isn't enough memory to allocate!
But you don't know what size heap he already has; maybe it should be OK - yet the allocation still fails.
So the OP still needs to think about what's happening here...