CODEWITHSUNDEEP
cgcclinkerld
foxcub
foxcub

Reputation: 2567

Link and call two separate mains

Is it possible through some linker magic to link together two files with two separate main()s, then add a third controlling main() that would decide at run time which of the other two mains to call?

So imagine:

/* test1.c */
include <stdio.h>

int main()
{
    printf("Test1\n");
}

/* test2.c */
include <stdio.h>

int main()
{
    printf("Test2\n");
}

/* controller.c */
int main()
{
    int x;
    // x gets set somehow
    if (x == 1)
        // call main from test1.c
    else if (x == 2)
        // call main from test2.c
}

I realize that this may be a weird question, but I'm trying to work around a constraint on a Cray supercomputer that allows me to run only a single executable per node. And I explicitly don't want to modify test1.c and test2.c.

Upvotes: 1

Views: 103

Answers (2)

4566976
4566976

Reputation: 2499

You could try it with PIEs and dynamic linking:

/* controller.c */

#include <stdio.h>
#include <errno.h>
#include <dlfcn.h>

int main(int argc, char *argv[])
{
    int retval, (*pmain)(int argc, char *argv[]);
    void *prg = dlopen(argv[1], RTLD_LAZY);

    pmain = dlsym(prg, "main");
    retval = (*pmain)(argc - 1, &argv[1]);
    dlclose(prg);
    return retval;
}

Compile and run, it works on my Linux:

gcc controller.c -o controller -ldl
gcc -fPIE -pie -Wl,--export-dynamic test1.c -o test1
gcc -fPIE -pie -Wl,--export-dynamic test2.c -o test2
./controller ./test1
Test1
./controller ./test2
Test2

Upvotes: 2

Some programmer dude
Some programmer dude

Reputation: 409136

In short, there's no possible way. Not directly anyway.

A possible solution is to use e.g. system (or fork and exec yourself) to run the specific program externally in a new process.

Another solution is to put the functionality from the different main functions in separate functions, and have main be conditionally compiled in if a specific preprocessor macro is defined, and the main functions just call the special functions that do the actual work.

Upvotes: 0

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