本文深入分析了zlib库的使用方法，包括压缩、解压缩、同步解压、字典预设等功能的实现。通过具体代码实例，展示了如何在不同场景下灵活运用zlib库进行数据压缩与读写操作。
下面分析test/example.c，它示范了zlib库的各个函数的使用。
下面使用大缓冲区测试压缩、解压操作（deflate/deflate）：
下面测试同步方式的解压操作inflateSync：
下面代码定义要压缩的字符串、压缩时使用的字典、压缩/解压缩的内存分配策略等。
/* example.c -- usage example of the zlib compression library
* Copyright (C) 1995-2006, 2011 Jean-loup Gailly.
* For conditions of distribution and use, see copyright notice in zlib.h
*/
/* @(#) $Id$ */
#include "zlib.h"
#include <stdio.h>
#ifdef STDC
# include <string.h>
# include <stdlib.h>
#endif
#if defined(VMS) || defined(RISCOS)
# define TESTFILE "foo-gz"
#else
# define TESTFILE "foo.gz"
#endif
#define CHECK_ERR(err, msg) { \
if (err != Z_OK) { \
fprintf(stderr, "%s error: %d\n", msg, err); \
exit(1); \
} \
}
const char hello[] = "hello, hello!"; /* 字符长度为14（末尾还有一个null字符） */
/* "hello world" would be more standard, but the repeated "hello"
* stresses the compression code better, sorry...
*/
const char dictionary[] = "hello";
uLong dictId; /* 字典的Adler32校验值 */
void test_deflate OF((Byte *compr, uLong comprLen));
void test_inflate OF((Byte *compr, uLong comprLen,
Byte *uncompr, uLong uncomprLen));
void test_large_deflate OF((Byte *compr, uLong comprLen,
Byte *uncompr, uLong uncomprLen));
void test_large_inflate OF((Byte *compr, uLong comprLen,
Byte *uncompr, uLong uncomprLen));
void test_flush OF((Byte *compr, uLong *comprLen));
void test_sync OF((Byte *compr, uLong comprLen,
Byte *uncompr, uLong uncomprLen));
void test_dict_deflate OF((Byte *compr, uLong comprLen));
void test_dict_inflate OF((Byte *compr, uLong comprLen,
Byte *uncompr, uLong uncomprLen));
int main OF((int argc, char *argv[]));
/* Z_SOLO表示把zlib库编译成单独的不依赖第三方的库 */
#ifdef Z_SOLO
/* 使用自定义的内存分配策略 */
void *myalloc OF((void *, unsigned, unsigned));
void myfree OF((void *, void *));
void *myalloc(q, n, m)
void *q;
unsigned n, m;
{
q = Z_NULL;
return calloc(n, m);
}
void myfree(void *q, void *p)
{
q = Z_NULL;
free(p);
}
static alloc_func zalloc = myalloc;
static free_func zfree = myfree;
#else /* !Z_SOLO */
/* 使用zlib默认的内存分配策略 */
static alloc_func zalloc = (alloc_func)0;
static free_func zfree = (free_func)0; 下面测试compress和uncompress的用法：
void test_compress OF((Byte *compr, uLong comprLen,
Byte *uncompr, uLong uncomprLen));
void test_gzio OF((const char *fname,
Byte *uncompr, uLong uncomprLen));
/* ===========================================================================
* 测试compress()和uncompress()
*/
void test_compress(compr, comprLen, uncompr, uncomprLen)
Byte *compr, *uncompr;
uLong comprLen, uncomprLen;
{
int err;
uLong len = (uLong)strlen(hello)+1; /* 获取字符串长度 */
/* 压缩字符串 */
err = compress(compr, &comprLen, (const Bytef*)hello, len);
CHECK_ERR(err, "compress");
strcpy((char*)uncompr, "garbage");
/* 解压字符串 */
err = uncompress(uncompr, &uncomprLen, compr, comprLen);
CHECK_ERR(err, "uncompress");
/* 比较解压后的结果 */
if (strcmp((char*)uncompr, hello)) {
fprintf(stderr, "bad uncompress\n");
exit(1);
} else {
printf("uncompress(): %s\n", (char *)uncompr);
}
} 下面测试gzip文件的读写操作：
/* ===========================================================================
* 测试.gz文件的读写操作
*/
void test_gzio(fname, uncompr, uncomprLen)
const char *fname; /* gz文件名 */
Byte *uncompr;
uLong uncomprLen;
{
#ifdef NO_GZCOMPRESS
fprintf(stderr, "NO_GZCOMPRESS -- gz* functions cannot compress\n");
#else
int err;
int len = (int)strlen(hello)+1;
gzFile file;
z_off_t pos;
file = gzopen(fname, "wb"); /* 打开要写入的gz文件 */
if (file == NULL) {
fprintf(stderr, "gzopen error\n");
exit(1);
}
gzputc(file, 'h'); /* 写入一个字符'h' */
if (gzputs(file, "ello") != 4) { /* 写入字符串"ello" */
fprintf(stderr, "gzputs err: %s\n", gzerror(file, &err));
exit(1);
}
if (gzprintf(file, ", %s!", "hello") != 8) { /* 按格式写入字符串", hello!" */
fprintf(stderr, "gzprintf err: %s\n", gzerror(file, &err));
exit(1);
}
gzseek(file, 1L, SEEK_CUR); /* 读写头向前移动1字节（即添加一个0字节） */
gzclose(file); /* 关闭gz文件 */
file = gzopen(fname, "rb"); /* 打开要读取的gz文件 */
if (file == NULL) {
fprintf(stderr, "gzopen error\n");
exit(1);
}
strcpy((char*)uncompr, "garbage");
/* 从压缩文件中读取给定大小的解压字节数 */
if (gzread(file, uncompr, (unsigned)uncomprLen) != len) {
fprintf(stderr, "gzread err: %s\n", gzerror(file, &err));
exit(1);
}
if (strcmp((char*)uncompr, hello)) { /* 比较解压后的结果 */
fprintf(stderr, "bad gzread: %s\n", (char*)uncompr);
exit(1);
} else {
printf("gzread(): %s\n", (char*)uncompr);
}
pos = gzseek(file, -8L, SEEK_CUR); /* 读写头向后移动8字节，应该停留在第6个字符处 */
if (pos != 6 || gztell(file) != pos) { /* 判断是否停留在第6个字符处 */
fprintf(stderr, "gzseek error, pos=%ld, gztell=%ld\n",
(long)pos, (long)gztell(file));
exit(1);
}
if (gzgetc(file) != ' ') { /* 从当前位置读取1个字符，应该为字符' ' */
fprintf(stderr, "gzgetc error\n");
exit(1);
}
if (gzungetc(' ', file) != ' ') { /* 推回这个字符到流中 */
fprintf(stderr, "gzungetc error\n");
exit(1);
}
/* 从压缩文件当前位置读取指定长度的解压字节数，直到len-1个字符被读取 */
gzgets(file, (char*)uncompr, (int)uncomprLen);
if (strlen((char*)uncompr) != 7) { /* " hello!" */
fprintf(stderr, "gzgets err after gzseek: %s\n", gzerror(file, &err));
exit(1);
}
if (strcmp((char*)uncompr, hello + 6)) {
fprintf(stderr, "bad gzgets after gzseek\n");
exit(1);
} else {
printf("gzgets() after gzseek: %s\n", (char*)uncompr);
}
gzclose(file); /* 关闭gz文件 */
#endif
}
#endif /* Z_SOLO */ 下面用小缓冲区测试压缩、解压操作（deflate/deflate）：
/* ===========================================================================
* 测试deflate()：使用小缓冲区
*/
void test_deflate(compr, comprLen)
Byte *compr;
uLong comprLen;
{
z_stream c_stream; /* 压缩流 */
int err;
uLong len = (uLong)strlen(hello)+1;
/* 这三个字段要在defalteInit之前初始化 */
c_stream.zalloc = zalloc;
c_stream.zfree = zfree;
c_stream.opaque = (voidpf)0;
/* 初始化压缩流的状态，使用默认压缩级别 */
err = deflateInit(&c_stream, Z_DEFAULT_COMPRESSION);
CHECK_ERR(err, "deflateInit");
/* 设置压缩操作的输入数据和输出缓冲区 */
c_stream.next_in = (Bytef*)hello; /* 输入缓冲区指向输入字符串 */
c_stream.next_out = compr;
/* 第一个循环：将flush设为Z_NO_FLUSH（表示还有输入数据未读完），将所有输入都读进去并进行压缩
根据avail_in和avail_out，不停地调用deflate将输入缓冲区的数据压缩
并写到输出缓冲区，直到输入字符串读完或输出缓冲区用完
*/
while (c_stream.total_in != len && c_stream.total_out < comprLen) {
c_stream.avail_in = c_stream.avail_out = 1; /* 强制小缓冲区 */
err = deflate(&c_stream, Z_NO_FLUSH);
CHECK_ERR(err, "deflate");
}
/* 第二个循环：将flush设置为Z_FINISH，不再输入，让deflate()完成全部的压缩输出
注意因为deflate压缩时可能是异步的（为了加速压缩，读取一次输入后不一定立刻就会产生压缩输出，
可能读完K字节后才会产生输出），所以上一个循环可能还没产生全部输出，需要这个循环，让flush保持Z_FINISH
（表示输入数据已读完），多次调用deflate()，直到返回Z_STREAM_END，表示处理完全部输入并产生了全部的压缩输出
*/
for (;;) { /* 完成压缩流的刷新，仍然强制小缓冲区 */
c_stream.avail_out = 1;
err = deflate(&c_stream, Z_FINISH);
if (err == Z_STREAM_END) break;
CHECK_ERR(err, "deflate");
}
err = deflateEnd(&c_stream); /* 释放压缩流的资源 */
CHECK_ERR(err, "deflateEnd");
}
/* ===========================================================================
* 测试inflate()：使用小缓冲区
*/
void test_inflate(compr, comprLen, uncompr, uncomprLen)
Byte *compr, *uncompr;
uLong comprLen, uncomprLen;
{
int err;
z_stream d_stream; /* 解压流 */
strcpy((char*)uncompr, "garbage");
/* 这些个字段要在infalteInit之前初始化 */
d_stream.zalloc = zalloc;
d_stream.zfree = zfree;
d_stream.opaque = (voidpf)0;
d_stream.next_in = compr; /* 设置输入缓冲区 */
d_stream.avail_in = 0;
d_stream.next_out = uncompr; /* 设置输出缓冲区 */
/* 初始化解压流的状态 */
err = inflateInit(&d_stream);
CHECK_ERR(err, "inflateInit");
/* 只需一个循环：根据avail_in和avail_out，不停地调用inflate将输入缓冲区的数据
解压，直到返回Z_STREAM_END，表示处理完全部输入并产生了全部的解压输出
这里与flush参数是否为Z_FINISH无关
*/
while (d_stream.total_out < uncomprLen && d_stream.total_in < comprLen) {
d_stream.avail_in = d_stream.avail_out = 1; /* 强制小缓冲区 */
err = inflate(&d_stream, Z_NO_FLUSH);
if (err == Z_STREAM_END) break;
CHECK_ERR(err, "inflate");
}
err = inflateEnd(&d_stream); /* 释放解压流的资源 */
CHECK_ERR(err, "inflateEnd");
if (strcmp((char*)uncompr, hello)) { /* 比较解压后的数据 */
fprintf(stderr, "bad inflate\n");
exit(1);
} else {
printf("inflate(): %s\n", (char *)uncompr);
}
} 下面使用大缓冲区测试压缩、解压操作（deflate/deflate）：
/* ===========================================================================
* 测试deflate()：使用大缓冲区和动态改变的压缩级别
*/
void test_large_deflate(compr, comprLen, uncompr, uncomprLen)
Byte *compr, *uncompr;
uLong comprLen, uncomprLen;
{
z_stream c_stream; /* 压缩流 */
int err;
/* 这三个字段要在defalteInit之前初始化 */
c_stream.zalloc = zalloc;
c_stream.zfree = zfree;
c_stream.opaque = (voidpf)0;
/* 初始化压缩流的状态，使用最快速度压缩 */
err = deflateInit(&c_stream, Z_BEST_SPEED);
CHECK_ERR(err, "deflateInit");
c_stream.next_out = compr;
c_stream.avail_out = (uInt)comprLen;
/* 这里，uncompr几乎都为0，因此可以很好地被压缩 */
c_stream.next_in = uncompr;
c_stream.avail_in = (uInt)uncomprLen;
err = deflate(&c_stream, Z_NO_FLUSH); /* 压缩输入数据 */
CHECK_ERR(err, "deflate");
if (c_stream.avail_in != 0) {
fprintf(stderr, "deflate not greedy\n");
exit(1);
}
/* 把已压缩的数据转换成未压缩: */
/* 设置流的压缩级别（为未压缩）和压缩策略 */
deflateParams(&c_stream, Z_NO_COMPRESSION, Z_DEFAULT_STRATEGY);
c_stream.next_in = compr;
c_stream.avail_in = (uInt)comprLen/2;
err = deflate(&c_stream, Z_NO_FLUSH);
CHECK_ERR(err, "deflate");
/* 转换回压缩模式（最高压缩率）: */
deflateParams(&c_stream, Z_BEST_COMPRESSION, Z_FILTERED);
c_stream.next_in = uncompr;
c_stream.avail_in = (uInt)uncomprLen;
err = deflate(&c_stream, Z_NO_FLUSH);
CHECK_ERR(err, "deflate");
/* 流刷新，产生全部压缩输出 */
err = deflate(&c_stream, Z_FINISH);
if (err != Z_STREAM_END) {
fprintf(stderr, "deflate should report Z_STREAM_END\n");
exit(1);
}
err = deflateEnd(&c_stream); /* 释放流的资源 */
CHECK_ERR(err, "deflateEnd");
}
/* ===========================================================================
* 测试inflate()：使用大缓冲区
*/
void test_large_inflate(compr, comprLen, uncompr, uncomprLen)
Byte *compr, *uncompr;
uLong comprLen, uncomprLen;
{
int err;
z_stream d_stream; /* 解压流 */
strcpy((char*)uncompr, "garbage");
/* 这些个字段要在infalteInit之前初始化 */
d_stream.zalloc = zalloc;
d_stream.zfree = zfree;
d_stream.opaque = (voidpf)0;
d_stream.next_in = compr;
d_stream.avail_in = (uInt)comprLen;
/* 初始化解压流 */
err = inflateInit(&d_stream);
CHECK_ERR(err, "inflateInit");
/* 解压 */
for (;;) {
d_stream.next_out = uncompr; /* 抛弃输出 */
d_stream.avail_out = (uInt)uncomprLen;
err = inflate(&d_stream, Z_NO_FLUSH); /* 解压输入数据 */
if (err == Z_STREAM_END) break;
CHECK_ERR(err, "large inflate");
}
err = inflateEnd(&d_stream);
CHECK_ERR(err, "inflateEnd");
if (d_stream.total_out != 2*uncomprLen + comprLen/2) {
fprintf(stderr, "bad large inflate: %ld\n", d_stream.total_out);
exit(1);
} else {
printf("large_inflate(): OK\n");
}
}
下面使用完全刷新模式测试压缩操作deflate：
/* ===========================================================================
* Test inflateSync()
*/
void test_sync(compr, comprLen, uncompr, uncomprLen)
Byte *compr, *uncompr;
uLong comprLen, uncomprLen;
{
int err;
z_stream d_stream; /* 解压流 */
strcpy((char*)uncompr, "garbage");
d_stream.zalloc = zalloc;
d_stream.zfree = zfree;
d_stream.opaque = (voidpf)0;
d_stream.next_in = compr; /* 设置输入缓冲区 */
d_stream.avail_in = 2; /* 只读取zlib头部信息 */
err = inflateInit(&d_stream);
CHECK_ERR(err, "inflateInit");
d_stream.next_out = uncompr; /* 设置输出缓冲区 */
d_stream.avail_out = (uInt)uncomprLen;
inflate(&d_stream, Z_NO_FLUSH);
CHECK_ERR(err, "inflate");
d_stream.avail_in = (uInt)comprLen-2; /* 读取所有压缩数据 */
err = inflateSync(&d_stream); /* 但忽略损坏的部分 */
CHECK_ERR(err, "inflateSync");
err = inflate(&d_stream, Z_FINISH); /* 完成解压 */
if (err != Z_DATA_ERROR) {
fprintf(stderr, "inflate should report DATA_ERROR\n");
/* 因为不正确的adler32 */
exit(1);
}
err = inflateEnd(&d_stream);
CHECK_ERR(err, "inflateEnd");
printf("after inflateSync(): hel%s\n", (char *)uncompr);
}
下面用预设的字典测试压缩、解压操作（deflate/inflate）:
/* ===========================================================================
* 测试：deflate()：使用预设的字典
*/
void test_dict_deflate(compr, comprLen)
Byte *compr;
uLong comprLen;
{
z_stream c_stream; /* 压缩流 */
int err;
c_stream.zalloc = zalloc;
c_stream.zfree = zfree;
c_stream.opaque = (voidpf)0;
err = deflateInit(&c_stream, Z_BEST_COMPRESSION);
CHECK_ERR(err, "deflateInit");
/* 设置压缩流要使用的字典 */
err = deflateSetDictionary(&c_stream,
(const Bytef*)dictionary, (int)sizeof(dictionary));
CHECK_ERR(err, "deflateSetDictionary");
dictId = c_stream.adler; /* 得到字典的Alder32校验值 */
c_stream.next_out = compr;
c_stream.avail_out = (uInt)comprLen;
c_stream.next_in = (Bytef*)hello; /* 输入要压缩的字符串 */
c_stream.avail_in = (uInt)strlen(hello)+1;
/* 直接进行压缩 */
err = deflate(&c_stream, Z_FINISH);
if (err != Z_STREAM_END) {
fprintf(stderr, "deflate should report Z_STREAM_END\n");
exit(1);
}
err = deflateEnd(&c_stream);
CHECK_ERR(err, "deflateEnd");
}
/* ===========================================================================
* 测试inflate()：使用预设的字典
*/
void test_dict_inflate(compr, comprLen, uncompr, uncomprLen)
Byte *compr, *uncompr;
uLong comprLen, uncomprLen;
{
int err;
z_stream d_stream; /* 解压流 */
strcpy((char*)uncompr, "garbage");
d_stream.zalloc = zalloc;
d_stream.zfree = zfree;
d_stream.opaque = (voidpf)0;
d_stream.next_in = compr;
d_stream.avail_in = (uInt)comprLen;
err = inflateInit(&d_stream);
CHECK_ERR(err, "inflateInit");
d_stream.next_out = uncompr;
d_stream.avail_out = (uInt)uncomprLen;
for (;;) { /* 解压 */
err = inflate(&d_stream, Z_NO_FLUSH);
if (err == Z_STREAM_END) break;
if (err == Z_NEED_DICT) { /* 如果需要字典 */
if (d_stream.adler != dictId) { /* 校验是否与压缩时的字典值一致 */
fprintf(stderr, "unexpected dictionary");
exit(1);
}
/* 设置解压需要的字典 */
err = inflateSetDictionary(&d_stream, (const Bytef*)dictionary,
(int)sizeof(dictionary));
}
CHECK_ERR(err, "inflate with dict");
}
err = inflateEnd(&d_stream);
CHECK_ERR(err, "inflateEnd");
if (strcmp((char*)uncompr, hello)) { /* 比较解压后的字符串 */
fprintf(stderr, "bad inflate with dict\n");
exit(1);
} else {
printf("inflate with dictionary: %s\n", (char *)uncompr);
}
} 下面是命令行程序：
/* ===========================================================================
* Usage: example [output.gz [input.gz]]
*/
int main(argc, argv)
int argc;
char *argv[];
{
Byte *compr, *uncompr;
uLong comprLen = 10000*sizeof(int); /* 在MSDOS上不会溢出 */
uLong uncomprLen = comprLen;
static const char* myVersion = ZLIB_VERSION;
/* 检查zlib版本是否一致 */
if (zlibVersion()[0] != myVersion[0]) {
fprintf(stderr, "incompatible zlib version\n");
exit(1);
} else if (strcmp(zlibVersion(), ZLIB_VERSION) != 0) {
fprintf(stderr, "warning: different zlib version\n");
}
/* 打印版本和zlib编译信息 */
printf("zlib version %s = 0x%04x, compile flags = 0x%lx\n",
ZLIB_VERSION, ZLIB_VERNUM, zlibCompileFlags());
/* 分配输入、输出缓冲区的内存 */
compr = (Byte*)calloc((uInt)comprLen, 1);
uncompr = (Byte*)calloc((uInt)uncomprLen, 1);
/* 清空compr和uncompr，以避免读到未初始化的数据，并且确保uncompr能很好
* 地被压缩
*/
if (compr == Z_NULL || uncompr == Z_NULL) {
printf("out of memory\n");
exit(1);
}
/* 下面运行各个测试函数 */
#ifdef Z_SOLO
argc = strlen(argv[0]);
#else
test_compress(compr, comprLen, uncompr, uncomprLen);
test_gzio((argc > 1 ? argv[1] : TESTFILE),
uncompr, uncomprLen);
#endif
test_deflate(compr, comprLen);
test_inflate(compr, comprLen, uncompr, uncomprLen);
test_large_deflate(compr, comprLen, uncompr, uncomprLen);
test_large_inflate(compr, comprLen, uncompr, uncomprLen);
test_flush(compr, &comprLen);
test_sync(compr, comprLen, uncompr, uncomprLen);
comprLen = uncomprLen;
test_dict_deflate(compr, comprLen);
test_dict_inflate(compr, comprLen, uncompr, uncomprLen);
/* 释放缓冲区资源 */
free(compr);
free(uncompr);
return 0;
} 
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