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man pages section 3: Basic Library Functions

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Updated: Wednesday, July 27, 2022



walkcontext, addrtosymstr, printstack, backtrace, backtrace_symbols, backtrace_symbols_fd - walk stack pointed to by ucontext


#include <ucontext.h>

int walkcontext(const ucontext_t *uptr,
     int (*operate_func)(uintptr_t, int, void *), void *usrarg);
int addrtosymstr(void *addr, char *buffer, int len);
int printstack(int fd);
#include <execinfo.h>

int backtrace(void **buffer, int size);
char **backtrace_symbols(void *const *buffer, int size);
void backtrace_symbols_fd(void *const *buffer, int size, int fd);


The walkcontext() function walks the call stack pointed to by uptr, which can be obtained by a call to getcontext(2) or from a signal handler installed with the SA_SIGINFO flag. The walkcontext() function calls the user-supplied function operate_func for each routine found on the call stack and each signal handler invoked. The user function is passed three arguments: the PC at which the call or signal occurred, the signal number that occurred at this PC (0 if no signal occurred), and the third argument passed to walkcontext(). If the user function returns a non-zero value, walkcontext() returns without completing the callstack walk.

The addrtosymstr() function attempts to convert a PC into a symbolic representation of the form


where objname is the module in which the PC is located, funcname is the name of the function, and offset is the offset from the beginning of the function. The objname, funcname , and offset values are obtained from dladdr(3C) and might not always be present. The resulting string is written to the user-supplied buffer. Should the length of the string be larger than the user-supplied buffer, only the portion of the string that will fit is written and null-terminated.

The printstack() function uses walkcontext() to print a symbolic stack trace to the specified file descriptor. This is useful for reporting errors from signal handlers. The printstack() function uses dladdr1() (see dladdr(3C)) to obtain symbolic symbol names. As a result, only global symbols are reported as symbol names by printstack().

The backtrace() function uses walkcontext() to generate a stack's program counter values for the calling thread and place these values into the array specified by the buffer argument. The size argument specifies the maximum number of program counters that will be recorded. This function is provided for compatibility with the GNU libc used on Linux systems, glibc.

The backtrace_symbols() function translates the numerical program counter values previously recorded by a call to backtrace() in the buffer argument, and converts, where possible, each PC to a string indicating the module, function and offset of each call site. The number of symbols present in the array must be passed in with the size argument.

The set of strings is returned in an array obtained from a call to malloc(3C). It is the responsibility of the caller to pass the returned pointer to free(). The individual strings must not be freed. Since malloc() is used to obtain the needed space, this function is MT-Safe rather than Async-Signal-Safe and cannot be used reliably from a signal handler. This function is provided for glibc compatibility.

The backtrace_symbols_fd() function translates the numerical program counter values previously recorded by a call to backtrace() in the buffer argument, and converts, where possible, each PC to a string indicating the module, function, and offset of each call site. These strings are written to the file descriptor specified in the fd argument. This function is provided for glibc compatibility.

Return Values

Upon successful completion, walkcontext() and printstack() return 0. If walkcontext() cannot read the stack or the stack trace appears corrupted, both functions return -1.

The addrtosymstr() function returns the number of characters necessary to hold the entire string representation of the passed in address, irrespective of the size of the user-supplied buffer.

The backtrace() function returns the number of stack frames captured.

The backtrace_symbols() function returns a pointer to an array containing string representations of the calling sequence.


No error values are defined.


The walkcontext() function is typically used to obtain information about the call stack for error reporting, performance analysis, or diagnostic purposes. Many library functions are not Async-Signal-Safe and should not be used from a signal handler. If walkcontext() is to be called from a signal handler, careful programming is required. In particular, stdio(3C) and malloc(3C) cannot be used.

The walkcontext(), addrtosymstr(), printstack(), backtrace(), and backtrace_symbols_fd() functions are Async-Signal-Safe and can be called from a signal handler. The string representation generated by addrtosymstr() and displayed by printstack(), backtrace_symbols() and backtrace_symbols_fd() is unstable and will change depending on the information available in the modules that comprise the stack trace.

Tail-call optimizations on SPARC eliminate stack frames that would otherwise be present. For example, if the code is of the form

#include <stdio.h>
#include <ucontext.h>

foo(int file)

        int a;
        a = foo(fileno(stdout));
        return (a);

        return (0);

compiling without optimization will yield a stack trace of the form

/tmp/q'foo+0xe [0x8050b6e]
/tmp/q'bar+0x1d [0x8050b9d]
/tmp/q'main+0xb [0x8050bbb]
/tmp/q'_start+0x72 [0x80509e2]

whereas with higher levels of optimization the output is

/tmp/q'main+0x19 [0x8050ba9]
/tmp/q'_start+0x72 [0x80509e2]

since both the call to foo() in main and the call to bar() in foo() are handled as tail calls that perform a return or restore in the delay slot. For further information, see The SPARC Architecture Manual.


See attributes(7) for descriptions of the following attributes:

Interface Stability
See below.

The backtrace_symbols() function is MT-Safe. The remaining functions are Async-Signal-Safe.

See Also

getcontext(2), Intro(2), sigaction(2), dladdr(3C), siginfo.h(3HEAD), attributes(7)

Weaver, David L. and Tom Germond, eds. The SPARC Architecture Manual, Version 9. Santa Clara: Prentice Hall, 2000.