Standard I/O functions as wrappers around File I/O functions?-CodePudding

I have read somewhere that functions like scanf and printf are actually wrappers around functions like fscanf and fprintf, with the FILE pointer set to standard input and standard output respectively. Is this guaranteed to be true for all implementations of C, or is that a feature of a specific implementation? I am unable to find any definitive answer to this question, one way or the other.

CodePudding user response：

It's more likely that they're wrappers around vfscanf() and vfprintf(), because the 'obvious' implementation is:

int scanf(const char * restrict fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    int rv = vfscanf(stdin, fmt, args);
    va_end(args);
    return rv;
}

int printf(const char * restrict fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    int rv = vfprintf(stdout, fmt, args);
    va_end(args);
    return rv;
}

However, it is certainly true at the calling level that these are equivalent:

printf("The data is %d, %d, %d\n", i, j, k);
fprintf(stdout, "The data is %d, %d, %d\n", i, j, k);

and so are these:

int rv = scanf("%d %d %d", &i, &j, &k);
int rv = fscanf(stdin, "%d %d %d", &i, &j, &k);

This could well be why it is claimed that scanf() is a wrapper around fscanf(), and printf() is a wrapper around fprintf(), but when you actually have to get into the nitty-gritty details of how to implement it, you have to write code similar to what I showed to create portable code. There may be compiler-specific shortcuts that a library implementation could take, but it's relatively unlikely that they'd be used.

I suppose another option is a macro implementation:

#define printf(...) fprintf(stdout, __VA_ARGS__)
#define scanf(...) fscanf(stdin, __VA_ARGS__)

However, the implementation must also provide an actual function printf() and an actual function scanf() to meet the requirements of the standard — see §7.1.4 Use of library functions.