Libzip - read file contents from zip - c++

Question

I using libzip to work with zip files and everything goes fine, until i need to read file from zip
I need to read just a whole text files, so it will be great to achieve something like PHP "file_get_contents" function.
To read file from zip there is a function "int
zip_fread(struct zip_file *file, void *buf, zip_uint64_t nbytes)".
Main problem what i don't know what size of buf must be and how many nbytes i must read (well i need to read whole file, but files have different size). I can just do a big buffer to fit them all and read all it's size, or do a while loop until fread return -1 but i don't think it's rational option.

Answer 1

You can try using zip_stat to get file size.
http://linux.die.net/man/3/zip_stat

Answer 2

I haven't used the libzip interface but from what you write it seems to look very similar to a file interface: once you got a handle to the stream you keep calling zip_fread() until this function return an error (ir, possibly, less than requested bytes). The buffer you pass in us just a reasonably size temporary buffer where the data is communicated.
Personally I would probably create a stream buffer for this so once the file in the zip archive is set up it can be read using the conventional I/O stream methods. This would look something like this:
struct zipbuf: std::streambuf {
zipbuf(???): file_(???) {}
private:
zip_file* file_;
enum { s_size = 8196 };
char buffer_[s_size];
int underflow() {
int rc(zip_fread(this->file_, this->buffer_, s_size));
this->setg(this->buffer_, this->buffer_,
this->buffer_ + std::max(0, rc));
return this->gptr() == this->egptr()
? traits_type::eof()
: traits_type::to_int_type(*this->gptr());
}
};
With this stream buffer you should be able to create an std::istream and read the file into whatever structure you need:
zipbuf buf(???);
std::istream in(&buf);
...
Obviously, this code isn't tested or compiled. However, when you replace the ??? with whatever is needed to open the zip file, I'd think this should pretty much work.

Answer 3

Here is a routine I wrote that extracts data from a zip-stream and prints out a line at a time. This uses zlib, not libzip, but if this code is useful to you, feel free to use it:
#
# compile with -lz option in order to link in the zlib library
#
#include <zlib.h>
#define Z_CHUNK 2097152
int unzipFile(const char *fName)
{
z_stream zStream;
char *zRemainderBuf = malloc(1);
unsigned char zInBuf[Z_CHUNK];
unsigned char zOutBuf[Z_CHUNK];
char zLineBuf[Z_CHUNK];
unsigned int zHave, zBufIdx, zBufOffset, zOutBufIdx;
int zError;
FILE *inFp = fopen(fName, "rbR");
if (!inFp) { fprintf(stderr, "could not open file: %s\n", fName); return EXIT_FAILURE; }
zStream.zalloc = Z_NULL;
zStream.zfree = Z_NULL;
zStream.opaque = Z_NULL;
zStream.avail_in = 0;
zStream.next_in = Z_NULL;
zError = inflateInit2(&zStream, (15+32)); /* cf. http://www.zlib.net/manual.html */
if (zError != Z_OK) { fprintf(stderr, "could not initialize z-stream\n"); return EXIT_FAILURE; }
*zRemainderBuf = '\0';
do {
zStream.avail_in = fread(zInBuf, 1, Z_CHUNK, inFp);
if (zStream.avail_in == 0)
break;
zStream.next_in = zInBuf;
do {
zStream.avail_out = Z_CHUNK;
zStream.next_out = zOutBuf;
zError = inflate(&zStream, Z_NO_FLUSH);
switch (zError) {
case Z_NEED_DICT: { fprintf(stderr, "Z-stream needs dictionary!\n"); return EXIT_FAILURE; }
case Z_DATA_ERROR: { fprintf(stderr, "Z-stream suffered data error!\n"); return EXIT_FAILURE; }
case Z_MEM_ERROR: { fprintf(stderr, "Z-stream suffered memory error!\n"); return EXIT_FAILURE; }
}
zHave = Z_CHUNK - zStream.avail_out;
zOutBuf[zHave] = '\0';
/* copy remainder buffer onto line buffer, if not NULL */
if (zRemainderBuf) {
strncpy(zLineBuf, zRemainderBuf, strlen(zRemainderBuf));
zBufOffset = strlen(zRemainderBuf);
}
else
zBufOffset = 0;
/* read through zOutBuf for newlines */
for (zBufIdx = zBufOffset, zOutBufIdx = 0; zOutBufIdx < zHave; zBufIdx++, zOutBufIdx++) {
zLineBuf[zBufIdx] = zOutBuf[zOutBufIdx];
if (zLineBuf[zBufIdx] == '\n') {
zLineBuf[zBufIdx] = '\0';
zBufIdx = -1;
fprintf(stdout, "%s\n", zLineBuf);
}
}
/* copy some of line buffer onto the remainder buffer, if there are remnants from the z-stream */
if (strlen(zLineBuf) > 0) {
if (strlen(zLineBuf) > strlen(zRemainderBuf)) {
/* to minimize the chance of doing another (expensive) malloc, we double the length of zRemainderBuf */
free(zRemainderBuf);
zRemainderBuf = malloc(strlen(zLineBuf) * 2);
}
strncpy(zRemainderBuf, zLineBuf, zBufIdx);
zRemainderBuf[zBufIdx] = '\0';
}
} while (zStream.avail_out == 0);
} while (zError != Z_STREAM_END);
/* close gzip stream */
zError = inflateEnd(&zStream);
if (zError != Z_OK) {
fprintf(stderr, "could not close z-stream!\n");
return EXIT_FAILURE;
}
if (zRemainderBuf)
free(zRemainderBuf);
fclose(inFp);
return EXIT_SUCCESS;
}

Answer 4

With any streaming you should consider the memory requirements of your app.
A good buffer size is large, but you do not want to have too much memory in use depending on your RAM usage requirements. A small buffer size will require you call your read and write operations more times which are expensive in terms of time performance. So, you need to find a buffer in the middle of those two extremes.
Typically I use a size of 4096 (4KB) which is sufficiently large for many purposes. If you want, you can go larger. But at the worst case size of 1 byte, you will be waiting a long time for you read to complete.
So to answer your question, there is no "right" size to pick. It is a choice you should make so that the speed of your app and the memory it requires are what you need.