Longjmp

`longjmp` and `setjmp` in WASIX

This sample project demonstrates how longjmp and setjmp works in WASIX.

Prerequisites

The project requires the following tools to be installed on your system:

Start a new project

Create a new directory for your project:

$ mkdir wasix-jump
$ cd wasix-jump

Make a new file called jump.c:

$ touch jump.c

Link the SysRoot

ℹ️

You need to link the SysRoot to your project. The SysRoot is a directory that contains the WASI libc headers and libraries. The SysRoot is located in the Wasix-libc binary or you can compile it yourself.

Writing the Application

Basic Application Setup

We want to write a simple C program that demonstrates how to use the opendir and jump functions to read the contents of a directory. It takes a single command-line argument, which is the path to the directory to read.

Let's write code for the above:

jump.c

#include <stdio.h>
#include <setjmp.h>
 
jmp_buf bufferA, bufferB;
 
__attribute__((noinline)) void routineB(); // forward declaration
 
__attribute__((noinline)) void routineA()
{
    int r = 1;
 
    printf("(A1)\n");
 
    r = setjmp(bufferA); // save the current context in bufferA
    if (r == 0)
        routineB(); // call routineB() and pass control to it
 
    printf("(A2) r=%d\n", r); // r will be 10001
 
    r = setjmp(bufferA);
    if (r == 0)
        longjmp(bufferB, 20001); // pass control to bufferB and return 20001
 
    printf("(A3) r=%d\n", r); // r will be 10002
 
    r = setjmp(bufferA);
    if (r == 0)
        longjmp(bufferB, 20002); // pass control to bufferB and return 20002
 
    printf("(A4) r=%d\n", r); // r will be 10003
}
 
__attribute__((noinline)) void routineB()
{
    int r;
 
    printf("(B1)\n");
 
    r = setjmp(bufferB); // save the current context in bufferB
    if (r == 0)
        longjmp(bufferA, 10001); // pass control to bufferA and return 10001
 
    printf("(B2) r=%d\n", r); // r will be 20001
 
    r = setjmp(bufferB);
    if (r == 0)
        longjmp(bufferA, 10002); // pass control to bufferA and return 10002
 
    printf("(B3) r=%d\n", r); // r will be 20002
 
    r = setjmp(bufferB);
    if (r == 0)
        longjmp(bufferA, 10003); // pass control to bufferA and return 10003
}
 
int main(int argc, char **argv)
{
    routineA(); // call routineA() and pass control to it
    return 0;
}

Compiling the Application

Let's compile the application with WASIX:

$ /path/to/wasix-sdk/clang jump.c --target="wasm32-wasi" -o jump.wasm

It's compiling! Now, let's try to run it:

$ wasmer run jump.wasm /
(A1)
(B1)
(A2) r=10001
(B2) r=20001
(A3) r=10002
(B3) r=20002
(A4) r=10003

Yaay, It works!

Conclusion

In this tutorial we learned:

How to use longjmp and setjmp in C
Compiling a C program with WASIX

wasix-libc/examples/longjmp.c

File Copy Readdir

longjmp and setjmp in WASIX