Try catch statements in C

I was thinking today about the try/catch blocks existent in another languages. Googled for a while this but with no result. From what I know, there is not such a thing as try/catch in C. However, is there a way to "simulate" them?
Sure, there is assert and other tricks but nothing like try/catch, that also catch the raised exception. Thank you

C itself doesn't support exceptions but you can simulate them to a degree with setjmp and longjmp calls.

static jmp_buf s_jumpBuffer;

void Example() { 
  if (setjmp(s_jumpBuffer)) {
    // The longjmp was executed and returned control here
    printf("Exception happened here\n");
  } else {
    // Normal code execution starts here
    Test();
  }
}

void Test() {
  // Rough equivalent of `throw`
  longjmp(s_jumpBuffer, 42);
}

This website has a nice tutorial on how to simulate exceptions with setjmp and longjmp

http://www.di.unipi.it/~nids/docs/longjump_try_trow_catch.html

You use goto in C for similar error handling situations.
That is the closest equivalent of exceptions you can get in C.

Ok, I couldn't resist replying to this. Let me first say I don't think it's a good idea to simulate this in C as it really is a foreign concept to C.

We can ~~use~~ abuse the preprocessor and local stack variables to give use a limited version of C++ try/throw/catch.

Version 1 (local scope throws)

#include <stdbool.h>

#define try bool __HadError=false;
#define catch(x) ExitJmp:if(__HadError)
#define throw(x) __HadError=true;goto ExitJmp;

Version 1 is a local throw only (can't leave the function's scope). It does rely on C99's ability to declare variables in code (it should work in C89 if the try is first thing in the function).

This function just makes a local var so it knows if there was an error and uses a goto to jump to the catch block.

For example:

#include <stdio.h>
#include <stdbool.h>

#define try bool __HadError=false;
#define catch(x) ExitJmp:if(__HadError)
#define throw(x) __HadError=true;goto ExitJmp;

int main(void)
{
    try
    {
        printf("One\n");
        throw();
        printf("Two\n");
    }
    catch(...)
    {
        printf("Error\n");
    }
    return 0;
}

This works out to something like:

int main(void)
{
    bool HadError=false;
    {
        printf("One\n");
        HadError=true;
        goto ExitJmp;
        printf("Two\n");
    }
ExitJmp:
    if(HadError)
    {
        printf("Error\n");
    }
    return 0;
}

Version 2 (scope jumping)

#include <stdbool.h>
#include <setjmp.h>

jmp_buf *g__ActiveBuf;

#define try jmp_buf __LocalJmpBuff;jmp_buf *__OldActiveBuf=g__ActiveBuf;bool __WasThrown=false;g__ActiveBuf=&__LocalJmpBuff;if(setjmp(__LocalJmpBuff)){__WasThrown=true;}else
#define catch(x) g__ActiveBuf=__OldActiveBuf;if(__WasThrown)
#define throw(x) longjmp(*g__ActiveBuf,1);

Version 2 is a lot more complex but basically works the same way. It uses a long jump out of the current function to the try block. The try block then uses an if/else to skip the code block to the catch block which check the local variable to see if it should catch.

The example expanded again:

jmp_buf *g_ActiveBuf;

int main(void)
{
    jmp_buf LocalJmpBuff;
    jmp_buf *OldActiveBuf=g_ActiveBuf;
    bool WasThrown=false;
    g_ActiveBuf=&LocalJmpBuff;

    if(setjmp(LocalJmpBuff))
    {
        WasThrown=true;
    }
    else
    {
        printf("One\n");
        longjmp(*g_ActiveBuf,1);
        printf("Two\n");
    }
    g_ActiveBuf=OldActiveBuf;
    if(WasThrown)
    {
        printf("Error\n");
    }
    return 0;
}

This uses a global pointer so the longjmp() knows what try was last run. We are ~~using~~ abusing the stack so child functions can also have a try/catch block.

Using this code has a number of down sides (but is a fun mental exercise):

It will not free allocated memory as there are no deconstructors being called.
You can't have more than 1 try/catch in a scope (no nesting)
You can't actually throw exceptions or other data like in C++
Not thread safe at all
You are setting up other programmers for failure because they will likely not notice the hack and try using them like C++ try/catch blocks.

~~In C99,~~ you can use setjmp/longjmp for non-local control flow.

Within a single scope, the generic, structured coding pattern for C in the presence of multiple resource allocations and multiple exits uses goto, like in this example. This is similar to how C++ implements destructor calls of automatic objects under the hood, and if you stick to this diligently, it should allow you for a certain degree of cleanness even in complex functions.

Try catch statements in C

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