带有 fseek 的 open_memstream 以用零结束填充缓冲区

Question

I using some C code that writes binary data to a file. In the process, it seeks around to different positions and then finally seeks to the end with fseeko(fp, 0, SEEK_END); .

However, in some cases, I want to work on a stream in memory instead. I use open_memstream for this, but seeking to the end pads the buffer with zeros and it ends up being twice as big as it should be.

An example just to demonstrate the effect of the fseek to the end of the stream is below. In the actual code, we also fseek to different parts of the stream, patching and editing bits of it, etc., as the stream is processed. Note also that writing the file at the end to the filesystem is just for demonstration to show the contents of the buffer – otherwise I wouldn't need the memory stream.

#include <stdio.h>
#include <stdlib.h>

#if (defined(BSD) || __APPLE__)
#include "open_memstream.h"
#endif

int main(void) {
    FILE *stream;
    FILE *outfile;
    char *buf;
    size_t buf_len;
    int i;

    stream = open_memstream(&buf, &buf_len);

    for(i = 0; i < 1000; i++) {
        fprintf(stream, "%d\n", i);
    }

    fseeko(stream, 0, SEEK_END);

    fclose(stream);

    outfile = fopen("out.txt", "w");
    fwrite(buf, buf_len, 1, outfile);
    fclose(outfile);

    return 0;
}

I was testing this out on Mac OS X with this implementation of open_memstream and it worked as I expected, but when I run this on Linux the file is twice the size with zeros at the end.

What's the best way to deal with this? I'm not sure if it's reliable to divide the buffer length by two and truncate it.

Answer 1

I've just ran into the same problem on Linux.

// It seams that SEEK_END does not work with open_memstream()
fseek(stream, 0, SEEK_END);

I've ended up doing this:

off_t o = ftell(stream);
/* do some things with the stream */
fseek(stream, o, SEEK_SET);