How to decompress gzip files using zlib – a minimal example

zlib is a great library in use by hundreds of thousands of programs (including Python). However, it lacks documentation.

This minimal example shows you how to create a simple zcat-like program that decompresses a gzipped input file and prints its contents to stdout.

 * zzcat.c -- Minimal gzip decompression example using zlib
 * Written by Uli Köhler (
   Not copyrighted -- provided to the public domain
#include <stdio.h>
#include <zlib.h>

#define BUFSIZE 16384

/* compress or decompress from fin (command line argument) to stdout */
int main(int argc, char **argv)
    if(argc <= 1) { // <= (number of expected CLI arguments)
        fprintf(stderr, "Usage: %s <input file>\n", argv[0]);
        return -1;

    gzFile fin = gzopen(argv[1], "rb");

    char buf[BUFSIZE];
    int n;

    while((n = gzread(fin, buf, BUFSIZE)) > 0) {
        fwrite(buf, 1, n, stdout);

    return 0;

Compile using

g++ -o zzcat zzcat.c -lz

Usage example:

# Create test file
echo "foo" | gzip -c > test.txt.gz
# Uncompress using zzcat! 
./zzcat test.txt.gz # This will print "foo"