Compile simple Apache Portable Runtime 1.5.2 program

Question

Following steps from here and here I tried to compile a simple program using apr 1.5.2 and apr-utils 1.5.4. Here is the code of the program:

    #include <apr.h>
    #include <stdlib.h>

    int main(int argc, char *argv[]) {
        apr_pool_t *pool;
        apr_file_t *out;

        apr_initialize();
        atexit(apr_terminate());

        apr_pool_create(&pool, NULL);
        apr_file_open_stdout(&out, pool);

        apr_file_printf(out, "Hello World\n");

        apr_pool_destroy(pool);
        return EXIT_SUCCESS;
    }

Now, when I try to compile it using the following Makefile, compiler returns errors.

Makefile:

    PREFIX?=/usr/local/apr
    CFLAGS=-g -Wall -I${PREFIX}/include/apr-1
    LDFLAGS=-L${PREFIX}/apr/lib -lapr-1 -pthread -laprutil-1
    CC=clang

    .PHONY: all clean
    all: hello

    clean:
            rm -f *.o

Compiler output:

 $ make
 clang -g -Wall -I/usr/local/apr/include/apr-1  -L/usr/local/apr/apr/lib -lapr-1 -pthread -laprutil-1  hello.c   -o hello
hello.c:5:5: error: use of undeclared identifier 'apr_pool_t'
    apr_pool_t *pool;
    ^
hello.c:5:17: error: use of undeclared identifier 'pool'
    apr_pool_t *pool;
                ^
hello.c:6:5: error: unknown type name 'apr_file_t'; did you mean 'apr_size_t'?
    apr_file_t *out;
    ^~~~~~~~~~
    apr_size_t
/usr/local/apr/include/apr-1/apr.h:356:26: note: 'apr_size_t' declared here
typedef  size_t          apr_size_t;
                         ^
hello.c:8:5: warning: implicit declaration of function 'apr_initialize' is
      invalid in C99 [-Wimplicit-function-declaration]
    apr_initialize();
    ^
hello.c:9:12: warning: implicit declaration of function 'apr_terminate' is
      invalid in C99 [-Wimplicit-function-declaration]
    atexit(apr_terminate());
           ^
hello.c:9:12: warning: incompatible integer to pointer conversion passing 'int'
      to parameter of type 'void (*)(void)' [-Wint-conversion]
    atexit(apr_terminate());
           ^~~~~~~~~~~~~~~
/usr/include/stdlib.h:519:27: note: passing argument to parameter '__func' here
extern int atexit (void (*__func) (void)) __THROW __nonnull ((1));
                          ^
hello.c:11:5: warning: implicit declaration of function 'apr_pool_create' is
      invalid in C99 [-Wimplicit-function-declaration]
    apr_pool_create(&pool, NULL);
    ^
hello.c:11:22: error: use of undeclared identifier 'pool'
    apr_pool_create(&pool, NULL);
                     ^
hello.c:12:5: warning: implicit declaration of function 'apr_file_open_stdout'
      is invalid in C99 [-Wimplicit-function-declaration]
    apr_file_open_stdout(&out, pool);
    ^
hello.c:12:32: error: use of undeclared identifier 'pool'
    apr_file_open_stdout(&out, pool);
                               ^
hello.c:13:5: warning: implicit declaration of function 'apr_file_printf' is
      invalid in C99 [-Wimplicit-function-declaration]
    apr_file_printf(out, "Hello World\n");
    ^
hello.c:14:5: warning: implicit declaration of function 'apr_pool_destroy' is
      invalid in C99 [-Wimplicit-function-declaration]
    apr_pool_destroy(pool);
    ^
hello.c:14:22: error: use of undeclared identifier 'pool'
    apr_pool_destroy(pool);
                     ^
7 warnings and 6 errors generated.
make: *** [hello] Error 1

Note: In the example, I took Makefile from, there is also a link -I${PREFIX}/apr/include/apr-util-1, but installation of the apr-utils did not create this folder.

Answer 1

The <apr.h> header is for the platform definitions. It defines what's available to APR (has WinAPI? has OpenSSL?) and the standard data types (apr_byte_t, apr_uint32_t.. etc.). It's generated automatically during build by ./configure.

Wat you need to include is apr_pools.h and apr_file_io.h for memory pools and file IO respectively.

Further atexit(3) expects a function pointer and you passed in a function. Instead it should say:

atexit(apr_terminate);

---

Errors like these are easy to debug on your own. When the compiler complains about an undeclared identifier from a library you are using, you almost certainly forgot to include the appropriate header.