Question

C# has a neat feature of being able to write to a memory stream using the MemoryStream object.

I’m looking for similar functionality from C using a FILE* pointer.

I want to be able to sprintf() but with the added functionality of having C remember “where I was” in the buffer I’m writing to.

Answer 1

There’s also an ugly hack which works with plain ISO-C: You can use fopen() to open a null file (/dev/null on *nix, NUL on Windows) and set the array as the file’s buffer via

setvbuf(file, buffer, _IOFBF, buffer_size)

This should work fine as long as fflush() isn’t called anywhere in the code. Also, the programmer has to explicitly take care of the string delimiter.

I don’t really see a need to do this, though: As snprintf() returns the number of characters written, it’s trivial to keep track of a buffer’s position.

One can even write a function to automagically resize the buffer on overflow: bufprintf.c

The function’s prototype is

int bufprintf(char **buffer, size_t *size, size_t *offset,
    const char *format, ...);

An example program could look like this:

#include <stdio.h>

extern int bufprintf(char **buffer, size_t *size, size_t *offset,
    const char *format, ...);

int main(void)
{
    size_t size = 0; // must be set!
    size_t offset;
    char * buffer;

    for(int i = 0; i < 100; ++i)
        bufprintf(&buffer, &size, &offset, "we rock %i\n", i);

    puts(buffer);
    printf("size:\t%u\noffset:\t%u\n", (unsigned)size, (unsigned)offset);
}