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# Wasm memcheck (wmemcheck)
wmemcheck provides the ability to check for invalid mallocs, reads, and writes
inside a Wasm module, as long as Wasmtime is able to make certain assumptions
(`malloc` and `free` functions are visible and your program uses only the
default allocator). This is analogous to the Valgrind tool's memory checker
(memcheck) tool for native programs.
How to use:
1. When building Wasmtime, add the CLI flag "--features wmemcheck" to compile with wmemcheck configured.
> cargo build --features wmemcheck
2. When running your wasm module, add the CLI flag "-W wmemcheck".
> wasmtime run -W wmemcheck test.wasm
If your program executes an invalid operation (load or store to non-allocated
address, double-free, or an internal error in malloc that allocates the same
memory twice) you will see an error that looks like a Wasm trap. For example, given the program
```c
#include <stdlib.h>
int main() {
char* p = malloc(1024);
*p = 0;
free(p);
*p = 0;
}
```
compiled with WASI-SDK via
```plain
$ /opt/wasi-sdk/bin/clang -o test.wasm test.c
```
you can observe the memory checker working like so:
```plain
$ wasmtime run -W wmemcheck ./test.wasm
Error: failed to run main module `./test.wasm`
Caused by:
0: failed to invoke command default
1: error while executing at wasm backtrace:
0: 0x103 - <unknown>!__original_main
1: 0x87 - <unknown>!_start
2: 0x2449 - <unknown>!_start.command_export
2: Invalid store at addr 0x10610 of size 1
```
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