Why would linking against the FastCGI library cause a segfault?

Question

I have a program that is using a library called "wjelement", whenever I try to use this library with FastCGI I get a segfault. I have made a simplified test case below. If I compile the code without fcgi_stdio.h and do not link against the library, the code works fine, if I add the fastcgi header and link against it I get a segfault, even if I don't use any fast cgi calls.

In My FastCGI code the opposite is also true, if I remove the WJelement code the rest of the program works fine.

I'm not sure if I need to blame my program, the FastCGI Library, or the WJElement library...

#include <stdio.h>
#include <fcgi_stdio.h>
#include <wjreader.h>

int main (int argc, char *argv[]) {

    FILE *my_schema_file;
    my_schema_file = fopen("test_schema.json", "rb");

    if (my_schema_file == NULL) {
        printf("Failed to open test schema file\n");
        return 1;
    } else {
        printf("Opened test schema file\n");
    }

    WJReader my_schema_reader;
    my_schema_reader = WJROpenFILEDocument(my_schema_file, NULL, 0);

    if (my_schema_reader  == NULL) {
        printf("Failed to open test schema reader\n");
        return 1;
    } else {
        printf("Opened test schema reader\n");
    }

    return 0;
}

GDB Backtrace:

Program received signal SIGSEGV, Segmentation fault.
0x0000003e19e6c85f in __GI__IO_fread (buf=0x6023c4, size=1, count=2731, fp=0x602250) at iofread.c:41
41    _IO_acquire_lock (fp);
(gdb) backtrace
#0  0x0000003e19e6c85f in __GI__IO_fread (buf=0x6023c4, size=1, count=2731, fp=0x602250) at iofread.c:41
#1  0x00007ffff7dde5d9 in WJRFileCallback () from /lib/libwjreader.so.0
#2  0x00007ffff7dde037 in WJRFillBuffer () from /lib/libwjreader.so.0
#3  0x00007ffff7dde4e9 in _WJROpenDocument () from /lib/libwjreader.so.0
#4  0x000000000040081f in main (argc=1, argv=0x7fffffffdeb8) at test.c:20

Answer 1

Found the answer here: http://www.fastcgi.com/devkit/doc/fcgi-devel-kit.htm

If your application passes FILE * to functions implemented in libraries for which you do not have source code, then you'll need to include the headers for these libraries before you include fcgi_stdio.h

I then had to convert from FCGI_FILE * to FILE * with FCGI_ToFILE(FCGI_FILE *);

#include <stdio.h>
#include <wjreader.h>
#include <fcgi_stdio.h>

int main (int argc, char *argv[]) {

    FILE *my_schema_file;
    my_schema_file = fopen("test_schema.json", "rb");

    if (my_schema_file == NULL) {
        printf("Failed to open test schema file\n");
        return 1;
    } else {
        printf("Opened test schema file\n");
    }

    WJReader my_schema_reader;
    my_schema_reader = WJROpenFILEDocument(FCGI_ToFILE(my_schema_file), NULL, 0);

    if (my_schema_reader  == NULL) {
        printf("Failed to open test schema reader\n");
        return 1;
    } else {
        printf("Opened test schema reader\n");
    }

    return 0;
}