How to connect GnuPG pinentry to a C project

I need an MWE of use GnuPG pinentry with Assuan protocol in C project, i.e. Assuan context proper initialization, connection, etc.

I've tried to read docs, but both pinentry and Assuan documentation are not very helpful on this subject, especially for a beginner.

Well, I've read gpg-agent sources and have finally found some clues. The following code is working well for pinentry-qt5, though it definitely need some security tuning.

#include <assuan.h>
#include <gcrypt.h> // secure memory allocation for the passphrase
#include <string.h>

// This wrapper is for Assuan commands that return OK/<error> and nothing more
void pinentry_nooutput_cmd (assuan_context_t ctx_p, const char *cmd) {
    char *line;
    size_t linelen;
    gpg_error_t err;
    err = assuan_write_line (ctx_p, cmd);
    if (err) {
        fprintf (stderr, "assuan send error: %s\n", gpg_strerror (err));
        exit (1);
    }
    err = assuan_read_line (ctx_p, &line, &linelen);
    if (err) {
        fprintf (stderr, "assuan receive error: %s\n", gpg_strerror (err));
        exit (1);
    }
    if (strcmp (line, "OK")) {
        fprintf (stderr, "pinentry error: %s\n", line);
        exit (1);
    }
}

// This function is used to get a passphrase from the user and store it in secmem
// 'repeat' is 1 for a new passphrase, and 0 otherwise
char *pinentry_get_passphrase (const char *pinentry_exec, int repeat) {
    assuan_context_t ctx_p;
    gpg_error_t err;
    // pinentry command line options. --no-global-grab is used for debug
    const char *argv[] = { "--no-global-grab", NULL };
    // file descriptors that should stay open. Apparently none is needed
    assuan_fd_t fd_child_list[] = { ASSUAN_INVALID_FD };
    // context allocation
    err = assuan_new (&ctx_p);
    if (err) if (err) {
        fprintf (stderr, "cannot allocate assuan context: %s\n", gpg_strerror (err));
        exit (1);
    }
    // connect to pinentry.
    // here can be a callback for pinentry memory cleanup, but it is not necessary and can be NULL
    err = assuan_pipe_connect (ctx_p, pinentry_exec, argv, fd_child_list, NULL, NULL, 0);
    if (err) {
        fprintf (stderr, "cannot connect to pinentry: %s\n", gpg_strerror (err));
        exit (1);
    }
    // example pinentry tuning according to its docs
    pinentry_nooutput_cmd (ctx_p, "SETTIMEOUT 120");
    pinentry_nooutput_cmd (ctx_p, "SETTITLE Password");
    pinentry_nooutput_cmd (ctx_p, "SETDESC Please enter password:");
    // set repeat if needed
    if (repeat)
        pinentry_nooutput_cmd (ctx_p, "SETREPEAT");
    // get passphrase
    err = assuan_write_line (ctx_p, "GETPIN");
    if (err) {
        fprintf (stderr, "assuan send error: %s\n", gpg_strerror (err));
        exit (1);
    }
    char *line;
    size_t linelen;
    err = assuan_read_line (ctx_p, &line, &linelen);
    if (err) {
        fprintf (stderr, "assuan receive error: %s\n", gpg_strerror (err));
        exit (1);
    }
    // if repeat, check the answer and read one more line
    if (repeat) {
        if (strcmp (line, "S PIN_REPEATED")) {
            fprintf (stderr, "pinentry error: %s\n", line);
            exit (1);
        }
        err = assuan_read_line (ctx_p, &line, &linelen);
        if (err) {
            fprintf (stderr, "assuan receive error: %s\n", gpg_strerror (err));
            exit (1);
        }
    }
    // check for empty password if needed
    if (!strcmp (line, "OK")) {
        fputs ("error: empty password\n", stderr);
        exit (1);
    }
    // read the passphrase (answer is "D <passphrase>")
    if (*line != 'D') {
        fprintf (stderr, "pinentry error: %s\n", line);
        exit (1);
    }
    // move pointer
    line += 2;
    // get password length
    size_t length = strlen (line);
    // alloc secmem
    char *password = gcry_malloc_secure (length + 1);
    memmove (password, line, length);
    // add null termination
    *(line + length) = '\0';
    // release the context
    assuan_release (ctx_p);
    return password;
}

int main () {
    char *newpass = pinentry_get_passphrase ("/usr/bin/pinentry", 1);
    fprintf (stdout, "New pass: %s\n", newpass);
    gcry_free (newpass);
    char *pass = pinentry_get_passphrase ("/usr/bin/pinentry", 0);
    fprintf (stdout, "Pass: %s\n", pass);
    gcry_free (pass);
    return 0;
}

Compile:

gcc -x c -std=c17 \
    -o pinentry_test \
    `pkg-config --cflags libassuan gpg-error libgcrypt` \
    pinentry_test.c \
    `pkg-config --libs libassuan gpg-error libgcrypt`;