Question

Why is the code below working? Should that be a compilation error (or at least a run-time error)?

#include <stdio.h>

int main(int argc, char** argv){
        float *buf = "happy holiday";        // notice the float
        printf("content of buf = %s\n",buf); //its working
        return 0;
}

I compiled it and got just a warning:

~/Desktop/cTest>gcc -o run run.c
run.c: In function `main':
run.c:4: warning: initialization from incompatible pointer type

Answer 1

You should always compile with -Wall -Werror -Wextra (at a minimum). Then you get this:

cc1: warnings being treated as errors
test.c: In function 'main':
test.c:4: warning: initialization from incompatible pointer type
test.c:5: warning: format '%s' expects type 'char *', but argument 2 has type 'float *'
test.c: At top level:
test.c:3: warning: unused parameter 'argc'
test.c:3: warning: unused parameter 'argv'

It “works” because in practice, there’s no difference between a char * and a float * under the hood on your platform. Your code is really no different to:

#include <stdio.h>

int main(int argc, char** argv){
        float *buf = (float *)"happy holiday";
        printf("content of buf = %s\n",(char *)buf);
        return 0;
}

This is well-defined behaviour, unless the alignment requirements of float and char differ, in which case it results in undefined behaviour (see C99, 6.3.2.3 p7).