Can I change the datatype of previously declared variable in C?

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Since the pointer returned by malloc() is sufficiently well aligned to be used as a pointer to any type, you could (but probably shouldn’t) use:

assert(sizeof(float) <= 10);
void *data = malloc(10);
char *str = data;
strcpy(str, "123.4");
float *vp = data;
*vp = strtof(str, NULL);

…use *vp…

free(data);

And now you can use *vp for the float value. However, that’s ridiculous compared with:

char *str = malloc(10);
strcpy(str, "123.4");
float value = strtof(str, NULL);

…use value…

free(str);

which is basically your original code — that you decided for unclear reasons was not what you wanted to do.

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