Returning a string properly in C, multiple times

Question

OK, the following way to return a string works fine:

char* SU(double V) {
    static char Str[80];
    // Do something
    return Str;
}
printf("%s", SU(A));

But the following will fail silently because the string space in memory is the same at the end of both calls:

printf("%s %s", SU(A), SU(B));

How can I do this cleanly and simply ? I was looking at alloca() but I don't think I can return a string allocated with alloca(), can I ?

Answer 1

Effectively, all current C compilers support compound literals and inline functions. Together it is possible to write thread-safe, memory-safe implementation that does not rely on static buffers (that can lead to unexpected bugs).

Basic idea is to combined a function that uses caller provided buffer, with a macro to abstract the creation of this temporary buffer. Negligent performance/memory impact for most applications.

For example - for the case of return the string-ified version of the a number the following itoa function can be implemented with macro/wrapper/inline function.

The limitation is that the wrapper need a way to estimate the size of returned string.

#include <stdio.h>

const char *itoa_wrapper(char *buffer, int buffer_sz, int val)
{
        int result = snprintf(buffer, buffer_sz, "%d", val) ;
                // Assuming formatting always works
        return buffer ;
}


#define itoa(val) itoa_wrapper( (char[20]) {}, 20, val)

int main(int argc, char **argv)
{
        printf("ARGC=%s, 1+2+3+4=%s\n", itoa(argc), itoa(1+2+3+4)) ;
}