In case anyone wants to see a multi-threaded FastCGI application written with the fcgiapp library.

libfcgi-example.c

# include <stdlib.h>
# include <stdio.h>

# include <sys/stat.h>

# include <pthread.h>

# include <fcgiapp.h>

const char* const sockpath = "/tmp/fcgicpp.sock";

void* start_fcgi_worker(void* arg);

struct FCGI_Info
{
    int fcgi_fd;
};

int main(void)
{
    int fcgifd = FCGX_OpenSocket(sockpath,128);

    chmod(sockpath,0777);

    if ( 0 > fcgifd )
    {
        printf("Error opening socket\n");
        exit(1);
    }

    /*
    if ( FCGX_IsCGI() )
    {
        printf("Please run this process as FastCGI process.\n");
        exit(1);
    }
    */

    const unsigned int n_threads = 4;

    pthread_t threads[n_threads];

    struct FCGI_Info info;
    info.fcgi_fd = fcgifd;

    for ( unsigned int i = 0; i < n_threads; i++ )
    {
        pthread_create(&threads[i],NULL,start_fcgi_worker,(void*)&info);
    }

    // Wait indefinitely
    for ( unsigned int i = 0; i < n_threads; i++ )
    {
        pthread_join(threads[i],NULL);
    }

    return 0;
}

void* start_fcgi_worker(void* arg)
{
    struct FCGI_Info* info = (struct FCGI_Info*)arg;

    FCGX_Init();
    
    FCGX_Request request;

    FCGX_InitRequest(&request,info->fcgi_fd,0);

    while ( 1 )
    {
        FCGX_Accept_r(&request);

        FCGX_PutStr("Content-type: text/plain\r\n\r\n",28,request.out);
        FCGX_PutStr("Hey!\n",5,request.out);

        FCGX_Finish_r(&request);
    }

}

Locking around calls to FCGX_Accept_r seem unnecessary. The header file says "(multi-thread safe)".

Thank you all for your comments! They are really helpful :)