wrap ungetc() without puts() gets() and streams in general - c

Question

I'm porting net-snmp to an embedded platform that only has limited access to the filesystem and I stumbled upon a big problem. There's a part of the core code that uses the ungetc() function, which I don't have. There are of course 2 solutions:
A) write my own ungetc() using what I have
B) modify net-snmp code in order to achieve the same result without ungetc()
Solution (B) will be eventually discussed in the net-snmp coders mailing list since requires deep understanding of the library's internals, so let's please focus on feasibility of (A)
What I have on my embedded system is:
fopen()
fclose()
fcreate()
fwrite()
fread()
fdelete()
fclear()
fcopy()
ffindfirst()
ffindnext()
frename()
fgetsize()
ftell()
fseek()
fgetc()
fgets()
The main difference is that my file functions work with INT32* file handles instead of FILE* types. I don't have the FILE* type.
What the ungetc() function does is to basically "put back the char in the stream" , either the char that it just read or another one.
In the first case the solution is easy, I rewind the pointer with fseek() one position backwards.
But in the second case I have a problem. I would be modifying the stream and not the file, except I don't have streams! I'm reading the file directly.
With ungetc() you can do something like
FILE *fp = fopen("file.txt", "r");
int c = getc (fp);
if( c == 'a' ) ungetc ('b', fp);
If "file.txt" contains "abcdefghi", a subsequent read with gets() will read "bbcdefghi" and not "abcdefghi" because the content IN THE STREAM has been changed, but not the file!
How can I replicate this behavior if I don't have "streams" ? My getc() and gets() read from an INT32* file handle and I don't have a puts() or putc() equivalent.
I can only write with fwrite() but that alters the content on the NV memory.
Thank you for your insight

Answer 1

Here is how I solved it. I created a more complex struct for the file handle that contains not only the handle itself but also the file name, the file size and a buffer that holds the whole content of the file. It should only load the part of the file that I need but mine is an embedded application and I know I won't be opening big files so I didn't bother.
Then once you have the "stream" it's trivial to pop chars in and out.
typedef struct _myfile {
_FS_HANDLE handle; /* file descriptor */
CHAR* fname; /* file name */
UINT32 fsize; /* file size */
CHAR* buffer; /* file buffer */
} *my_FILE;
int my_ungetc(int c, my_FILE stream)
{
if (stream)
{
UINT32 pointer = _fs_tell(stream->handle);
if (pointer > 0)
{
_fs_seek(stream->handle,pointer - 1);
stream->buffer[pointer - 1] = c;
return c;
}
}
else
{
printf("ERROR! stream is NULL!\r\n");
}
return EOF;
}
void *my_fopen(const char *filename, const char *mode)
{
my_FILE fp = _mem_alloc(sizeof(struct _myfile));
fp->fname = strdup(filename);
if (mode == "r")
{
fp->handle = _fs_open((CHAR*)filename, OPEN_READ);
if (fp->handle) fp->fsize = _get_size_with_handle(fp->handle);
if (fp->fsize)
{
fp->buffer = _mem_alloc(fp->fsize);
if (fp->buffer)
{
if (_fs_read(fp->handle,fp->buffer,fp->fsize))
{
_fs_seek(fp->handle,0);
}
else
{
printf("ERROR: unable to read %d bytes from %s\r\n",fp->fsize,filename);
}
}
else
{
printf("ERROR in my_fopen(\"%s\",\"r\"): could not alloc %d bytes for buffer\r\n",filename,fp->fsize);
}
}
else
{
fp->buffer = NULL;
printf("File \"%s\" is empty\r\n");
}
return fp;
}
else if (mode == "w")
{
fp->handle = _fs_open((CHAR*)filename, OPEN_WRITE);
if (fp->handle) fp->fsize = _get_size_with_handle(fp->handle);
fp->buffer = NULL;
return fp;
}
else
{
printf("File open mode %s not supported\r\n",mode);
return NULL;
}
}