Since the zlib is a C library, to use it in a c program one would need to use extern "C" when including the header, as in the following test program:

#include <iostream>

extern "C" {
#include <zlib.h>
}

int main()
{
  gzFile file;
  file = gzopen("test.gz", "w");
  gzwrite(file, "hello\0", 6);
  gzclose(file);

  char text[6];
  file = gzopen("test.gz", "r");
  gzread(file, text, 6);
  std::cout << text;
  
  return 0;
}

To my surprise, the code compiles and behaves correctly even if I remove the extern "C". Version of zlib is 1.2.13.

Why does the linkage works even without the extern "C"?

CodePudding user response：

If you had looked at zlib.h, you would have found very near the top:

#ifdef __cplusplus
extern "C" {
#endif

(and a corresponding close brace near the bottom).