SYMBOL(2) SYMBOL(2)
NAME
syminit, getsym, symbase, pc2sp, pc2line, textseg,
line2addr, lookup, findlocal, getauto, findsym, localsym,
globalsym, textsym, file2pc, fileelem, filesym, fileline,
fnbound - symbol table access functions
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <bio.h>
#include <mach.h>
int syminit(int fd, Fhdr *fp)
Sym *getsym(int index)
Sym *symbase(long *nsyms)
int fileelem(Sym **fp, uchar *encname, char *buf, int n)
int filesym(int index, char *buf, int n)
long pc2sp(ulong pc)
long pc2line(ulong pc)
void textseg(ulong base, Fhdr *fp)
long line2addr(ulong line, ulong basepc)
int lookup(char *fn, char *var, Symbol *s)
int findlocal(Symbol *s1, char *name, Symbol *s2)
int getauto(Symbol *s1, int off, int class, Symbol *s2)
int findsym(long addr, int class, Symbol *s)
int localsym(Symbol *s, int index)
int globalsym(Symbol *s, int index)
int textsym(Symbol *s, int index)
long file2pc(char *file, ulong line)
int fileline(char *str, int n, ulong addr)
int fnbound(long addr, ulong *bounds)
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SYMBOL(2) SYMBOL(2)
DESCRIPTION
These functions provide machine-independent access to the
symbol table of an executable file or executing process.
The latter is accessible by opening the device
/proc/pid/text as described in proc(3). Mach(2) and
object(2) describe additional library functions for process-
ing executable and object files.
Syminit, getsym, symbase, fileelem, pc2sp, pc2line, and
line2addr process the symbol table contained in an exe-
cutable file or the text image of an executing program. The
symbol table is stored internally as an array of Sym data
structures as defined in a.out(6).
Syminit uses the data in the Fhdr structure filled by
crackhdr (see mach(2)) to read the raw symbol tables from
the open file descriptor fd. It returns the count of the
number of symbols or -1 if an error occurs.
Getsym returns the address of the ith Sym structure or zero
if index is out of range.
Symbase returns the address of the first Sym structure in
the symbol table. The number of entries in the symbol table
is returned in nsyms.
Fileelem converts a file name, encoded as described in
a.out(6), to a character string. Fp is the base of an array
of pointers to file path components ordered by path index.
Encname is the address of an array of encoded file path com-
ponents in the form of a z symbol table entry. Buf and n
specify the address of a receiving character buffer and its
length. Fileelem returns the length of the null-terminated
string that is at most n-1 bytes long.
Filesym is a higher-level interface to fileelem. It fills
buf with the name of the ith file and returns the length of
the null-terminated string that is at most n-1 bytes long.
File names are retrieved in no particular order, although
the order of retrieval does not vary from one pass to the
next. A zero is returned when index is too large or too
small or an error occurs during file name conversion.
Pc2sp returns an offset associated with a given value of the
program counter. Adding this offset to the current value of
the stack pointer gives the address of the current stack
frame. This approach only applies to the 68020 architec-
ture; other architectures use a fixed stack frame offset by
a constant contained in a dummy local variable (called
.frame) in the symbol table.
Pc2line returns the line number of the statement associated
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SYMBOL(2) SYMBOL(2)
with the instruction address pc. The line number is the
absolute line number in the source file as seen by the com-
piler after pre-processing; the original line number in the
source file may be derived from this value using the history
stacks contained in the symbol table.
Pc2sp and pc2line must know the start and end addresses of
the text segment for proper operation. These values are
calculated from the file header by function syminit. If the
text segment address is changed, the application program
must invoke textseg to recalculate the boundaries of the
segment. Base is the new base address of the text segment
and fp points to the Fhdr data structure filled by crackhdr.
Line2addr converts a line number to an instruction address.
The first argument is the absolute line number in a file.
Since a line number does not uniquely identify an instruc-
tion location (e.g., every source file has line 1), a second
argument specifies a text address from which the search
begins. Usually this is the address of the first function
in the file of interest.
Pc2sp, pc2line, and line2addr return -1 in the case of an
error.
Lookup, findlocal, getauto, findsym, localsym, globalsym,
textsym, file2pc, and fileline operate on data structures
riding above the raw symbol table. These data structures
occupy memory and impose a startup penalty but speed
retrievals and provide higher-level access to the basic sym-
bol table data. Syminit must be called prior to using these
functions. The Symbol data structure:
typedef struct {
void *handle; /* private */
struct {
char *name;
long value;
char type;
char class;
};
} Symbol;
describes a symbol table entry. The value field contains
the offset of the symbol within its address space: global
variables relative to the beginning of the data segment,
text beyond the start of the text segment, and automatic
variables and parameters relative to the stack frame. The
type field contains the type of the symbol as defined in
a.out(6). The class field assigns the symbol to a general
class; CTEXT, CDATA, CAUTO, and CPARAM are the most popular.
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SYMBOL(2) SYMBOL(2)
Lookup fills a Symbol structure with symbol table informa-
tion. Global variables and functions are represented by a
single name; local variables and parameters are uniquely
specified by a function and variable name pair. Arguments
fn and var contain the name of a function and variable,
respectively. If both are non-zero, the symbol table is
searched for a parameter or automatic variable. If only var
is zero, the text symbol table is searched for function fn.
If only fn is zero, the global variable table is searched
for var.
Findlocal fills s2 with the symbol table data of the auto-
matic variable or parameter matching name. S1 is a Symbol
data structure describing a function or a local variable;
the latter resolves to its owning function.
Getauto searches the local symbols associated with function
s1 for an automatic variable or parameter located at stack
offset off. Class selects the class of variable: CAUTO or
CPARAM. S2 is the address of a Symbol data structure to
receive the symbol table information of the desired symbol.
Findsym returns the symbol table entry of type class stored
near addr. The selected symbol is a global variable or func-
tion with address nearest to and less than or equal to addr.
Class specification CDATA searches only the global variable
symbol table; class CTEXT limits the search to the text sym-
bol table. Class specification CANY searches the text table
first, then the global table.
Localsym returns the ith local variable in the function
associated with s. S may reference a function or a local
variable; the latter resolves to its owning function. If
the ith local symbol exists, s is filled with the data
describing it.
Globalsym loads s with the symbol table information of the
ith global variable.
Textsym loads s with the symbol table information of the ith
text symbol. The text symbols are ordered by increasing
address.
File2pc returns a text address associated with line in file
file, or -1 on an error.
Fileline converts text address addr to its equivalent line
number in a source file. The result, a null terminated
character string of the form `file:line', is placed in
buffer str of n bytes.
Fnbound returns the start and end addresses of the function
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SYMBOL(2) SYMBOL(2)
containing the text address supplied as the first argument.
The second argument is an array of two unsigned longs;
fnbound places the bounding addresses of the function in the
first and second elements of this array. The start address
is the address of the first instruction of the function; the
end address is the address of the start of the next function
in memory, so it is beyond the end of the target function.
Fnbound returns 1 if the address is within a text function,
or zero if the address selects no function.
Functions file2pc and fileline may produce inaccurate
results when applied to optimized code.
Unless otherwise specified, all functions return 1 on suc-
cess, or 0 on error. When an error occurs, a message
describing it is stored in the system error buffer where it
is available via errstr.
SOURCE
/sys/src/libmach
SEE ALSO
mach(2), object(2), errstr(2), proc(3), a.out(6)
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