Hello all,
I've recently been working on improving documentation of the
dlopen API, and as part of that work, I wrote a page documenting
dlinfo(3). I would be happy to receive review comments, suggestions
for fixes or improvements, or offers of beer in Prague next month.
Page source provided inline below. (Extract and use "man -l file"
to render.)
Cheers,
Michael
'\" t
.\" Copyright (C) 2015 Michael Kerrisk <mtk.manpages@xxxxxxxxx>
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.TH DLINFO 3 2015-07-06 "Linux" "Linux Programmer's Manual"
.SH NAME
dlinfo \- obtain information about a dynamically loaded object
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <link.h>
.B #include <dlfcn.h>
.BR "int dlinfo(void *" handle ", int " request ", void *" info );
Link with \fI\-ldl\fP.
.fi
.SH DESCRIPTION
The
.BR dlinfo ()
function obtains information about the dynamically loaded object
referred to by
.IR handle
(typically obtained by an earlier call to
.BR dlopen (3)
or
.BR dlmopen (3)).
The
.I request
argument specifies which information is to be returned.
The
.I info
argument is a pointer to a buffer used to store information
returned by the call; the type of this argument depends on
.IR request .
The following values are supported for
.IR request
(with the corresponding type for
.IR info
shown in parentheses):
.TP
.BR RTLD_DI_LMID " (\fILmid_t *\fP)"
Obtain the ID of the link-map list (namespace) in which
.I handle
is loaded.
.TP
.BR RTLD_DI_LINKMAP " (\fIstruct link_map **\fP)"
Obtain a pointer to the
.I link_map
structure corresponding to
.IR handle .
The
.IR info
argument points to a pointer to a
.I link_map
structure, defined in
.I <link.h>
as:
.in +4n
.nf
struct link_map {
ElfW(Addr) l_addr; /* Difference between the
address in the ELF file and
the address in memory */
char *l_name; /* Absolute pathname where
object was found */
ElfW(Dyn) *l_ld; /* Dynamic section of the
shared object */
struct link_map *l_next, *l_prev;
/* Chain of loaded objects */
/* Plus additional fields private to the
implementation */
};
.fi
.in
.TP
.BR RTLD_DI_ORIGIN " (\fIchar *\fP)"
Copy the pathname of the origin of the shared object corresponding to
.IR handle
to the location pointed to by
.IR info .
.TP
.BR RTLD_DI_SERINFO " (\fIDl_serinfo *\fP)"
Obtain the library search paths for the shared object referred to by
.IR handle .
The
.I info
argument is a pointer to a
.I Dl_serinfo
that contains the search paths.
Because the number of search paths may vary,
the size of the structure pointed to by
.IR info
can vary.
The
.B RTLD_DI_SERINFOSIZE
request described below allows applications to size the buffer suitably.
The caller must perform the following steps:
.RS
.IP 1. 3
Use a
.B RTLD_DI_SERINFOSIZE
request to populate a
.I Dl_serinfo
structure with the size
.RI ( dls_size )
of the structure needed for the subsequent
.B RTLD_DI_SERINFO
request.
.IP 2.
Allocate a
.I Dl_serinfo
buffer of the correct size
.RI ( dls_size ).
.IP 3.
Use a further
.B RTLD_DI_SERINFOSIZE
request to populate the
.I dls_size
and
.I dls_cnt
fields of the buffer allocated in the previous step.
.IP 4.
Use a
.B RTLD_DI_SERINFO
to obtain the library search paths.
.IP
.RE
.IP
The
.I Dl_serinfo
structure is defined as follows:
.in +4n
.nf
typedef struct {
size_t dls_size; /* Size in bytes of
the whole buffer */
unsigned int dls_cnt; /* Number of elements
in 'dls_serpath' */
Dl_serpath dls_serpath[1]; /* Actually longer,
'dls_cnt' elements */
} Dl_serinfo;
.fi
.in
Each of the
.I dls_serpath
elements in the above structure is a structure of the following form:
.in +4n
.nf
typedef struct {
char *dls_name; /* Name of library search
path directory */
unsigned int dls_flags; /* Indicates where this
directory came from */
} Dl_serpath;
.fi
.in
The
.I dls_flags
field is currently unused, and always contains zero.
.TP
.BR RTLD_DI_SERINFOSIZE " (\fIDl_serinfo *\fP)"
Populate the
.I dls_size
and
.I dls_cnt
fields of the
.I Dl_serinfo
structure pointed to by
.IR info
with values suitable for allocating a buffer for use in a subsequent
.B RTLD_DI_SERINFO
request.
.TP
.BR RTLD_DI_TLS_MODID " (\fIsize_t *\fP, since glibc 2.4)"
Obtain the module ID of this shared object's TLS (thread-local storage)
segment, as used in TLS relocations.
If this object does not define a TLS segment, zero is placed in
.IR *info .
.TP
.BR RTLD_DI_TLS_DATA " (\fIvoid **\fP, since glibc 2.4)"
Obtain a pointer to the calling
thread's TLS block corresponding to this shared object's TLS segment.
If this object does not define a PT_TLS segment,
or if the calling thread has not allocated a block for it,
NULL is placed in
.IR *info .
.SH RETURN VALUE
On success,
.BR dlinfo ()
returns 0.
On failure, it returns \-1; the cause of the error can be diagnosed using
.BR dlerror (3).
.SH VERSIONS
.BR dlinfo ()
first appeared in glibc 2.3.3.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface Attribute Value
T{
.BR dlinfo ()
T} Thread safety MT-Safe
.TE
.SH CONFORMING TO
This function is a nonstandard GNU extension.
.SH NOTES
This function derives from the Solaris function of the same name
and also appears on some other systems.
The sets of requests supported by the various implementations
overlaps only partially.
.SH EXAMPLE
The program below opens a shared objects using
.BR dlopen ()
and then uses the
.B RTLD_DI_SERINFOSIZE
and
.B RTLD_DI_SERINFO
requests to obtain the library search path list for the library.
Here is an example of what we might see when running the program:
.in +4n
.nf
.fi
$ \fB./a.out /lib64/libm.so.6\fP
dls_serpath[0].dls_name = /lib64
dls_serpath[1].dls_name = /usr/lib64
.in
.SS Program source
\&
.nf
#define _GNU_SOURCE
#include <dlfcn.h>
#include <link.h>
#include <stdio.h>
#include <stdlib.h>
int
main(int argc, char *argv[])
{
void *handle;
Dl_serinfo serinfo;
Dl_serinfo *sip;
int j;
if (argc != 2) {
fprintf(stderr, "Usage: %s <libpath>\\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Obtain a handle for shared objects specified on command line */
handle = dlopen(argv[1], RTLD_NOW);
if (handle == NULL) {
fprintf(stderr, "dlopen() failed: %s\\n", dlerror());
exit(EXIT_FAILURE);
}
/* Discover the size of the buffer that we must pass to
RTLD_DI_SERINFO */
if (dlinfo(handle, RTLD_DI_SERINFOSIZE, &serinfo) == \-1) {
fprintf(stderr, "RTLD_DI_SERINFOSIZE failed: %s\\n", dlerror());
exit(EXIT_FAILURE);
}
/* Allocate the buffer for use with RTLD_DI_SERINFO */
sip = malloc(serinfo.dls_size);
if (sip == NULL) {
perror("malloc");
exit(EXIT_FAILURE);
}
/* Initialize the \(aqdls_size\(aq and \(aqdls_cnt\(aq fields in the newly
allocated buffer */
if (dlinfo(handle, RTLD_DI_SERINFOSIZE, sip) == \-1) {
fprintf(stderr, "RTLD_DI_SERINFOSIZE failed: %s\\n", dlerror());
exit(EXIT_FAILURE);
}
/* Fetch and print library search list */
if (dlinfo(handle, RTLD_DI_SERINFO, sip) == \-1) {
fprintf(stderr, "RTLD_DI_SERINFO failed: %s\\n", dlerror());
exit(EXIT_FAILURE);
}
for (j = 0; j < serinfo.dls_cnt; j++)
printf("dls_serpath[%d].dls_name = %s\\n",
j, sip\->dls_serpath[j].dls_name);
exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR dl_iterate_phdr (3),
.BR dladdr (3),
.BR dlerror (3),
.BR dlopen (3),
.BR dlsym (3),
.BR ld.so (8)
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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