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Leave the client running. It will be necessary to enter more requests later.
easy.c
Start another shell. Change directory (``cd``) so that it is the same as the directory that the client is running on.
Create a file. Name it ``easy.c``. Put these six lines in it.
#include "module.h"
int easy(box_function_ctx_t *ctx, const char *args, const char *args_end)
{
printf("hello world\n");
return 0;
}
int easy2(box_function_ctx_t *ctx, const char *args, const char *args_end)
{
printf("hello world -- easy2\n");
return 0;
}
Compile the program, producing a library file named ``easy.so``:
$ gcc -shared -o easy.so -fPIC easy.c
Now go back to the client and execute these requests:
box.schema.func.create('easy', {language = 'C'})
box.schema.user.grant('guest', 'execute', 'function', 'easy')
capi_connection:call('easy')
If these requests appear unfamiliar, re-read the descriptions of :doc:`/reference/reference_lua/box_schema/func_create`, :doc:`/reference/reference_lua/box_schema/user_grant` and :ref:`conn:call() <net_box-call>`.
The function that matters is ``capi_connection:call('easy')``.
Its first job is to find the 'easy' function, which should be easy because by default Tarantool looks on the current directory for a file named ``easy.so``.
Its second job is to call the 'easy' function. Since the ``easy()`` function in ``easy.c`` begins with ``printf("hello world\n")``, the words "hello world" will appear on the screen.
Its third job is to check that the call was successful. Since the ``easy()`` function in ``easy.c`` ends with :code:`return 0`, there is no error message to display and the request is over.
The result should look like this:
tarantool> capi_connection:call('easy')
hello world
---
- []
...
Now let's call the other function in easy.c -- ``easy2()``. This is almost the same as the ``easy()`` function, but there's a detail: when the file name is not the same as the function name, then we have to specify :samp:`{file-name}.{function-name}`.
box.schema.func.create('easy.easy2', {language = 'C'})
box.schema.user.grant('guest', 'execute', 'function', 'easy.easy2')
capi_connection:call('easy.easy2')
... and this time the result will be "hello world -- easy2".
Conclusion: calling a C function is easy.
harder.c
Go back to the shell where the ``easy.c`` program was created.
Create a file. Name it ``harder.c``. Put these 17 lines in it:
#include "module.h"
#include "msgpuck.h"
int harder(box_function_ctx_t *ctx, const char *args, const char *args_end)
{
uint32_t arg_count = mp_decode_array(&args);
printf("arg_count = %d\n", arg_count);
uint32_t field_count = mp_decode_array(&args);
printf("field_count = %d\n", field_count);
uint32_t val;
int i;
for (i = 0; i < field_count; ++i)
{
val = mp_decode_uint(&args);
printf("val=%d.\n", val);
}
return 0;
}
Compile the program, producing a library file named ``harder.so``:
$ gcc -shared -o harder.so -fPIC harder.c
box.schema.func.create('harder', {language = 'C'})
box.schema.user.grant('guest', 'execute', 'function', 'harder')
passable_table = {}
table.insert(passable_table, 1)
table.insert(passable_table, 2)
table.insert(passable_table, 3)
capi_connection:call('harder', {passable_table})
This time the call is passing a Lua table (``passable_table``) to the ``harder()`` function. The ``harder()`` function will see it, it's in the :code:`char *args` parameter.
At this point the ``harder()`` function will start using functions defined in `msgpuck.h <https://github.com/tarantool/msgpuck>`_. The routines that begin with "mp" are msgpuck functions that handle data formatted according to the `MsgPack <http://msgpack.org/>`_ specification. Passes and returns are always done with this format so one must become acquainted with msgpuck to become proficient with the C API.
For now, though, it's enough to know that ``mp_decode_array()`` returns the number of elements in an array, and ``mp_decode_uint`` returns an unsigned integer, from :code:`args`. And there's a side effect: when the decoding finishes, :code:`args` has changed and is now pointing to the next element.
Therefore the first displayed line will be "arg_count = 1" because there was only one item passed: ``passable_table``. |br| The second displayed line will be "field_count = 3" because there are three items in the table. |br| The next three lines will be "1" and "2" and "3" because those are the values in the items in the table.

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testing_project / Documentation: how-to/app/c_tutorial — English

tarantool> capi_connection:call('easy')
hello world
---
- []
...
2 years ago
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