В glibc он определен в dlfcn / dlfcn.h и выглядит как
/* Structure containing information about object searched using
`dladdr'. */
typedef struct
{
__const char *dli_fname; /* File name of defining object. */
void *dli_fbase; /* Load address of that object. */
__const char *dli_sname; /* Name of nearest symbol. */
void *dli_saddr; /* Exact value of nearest symbol. */
} Dl_info;