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Full Discussion: Function main returning int?
Top Forums Programming Function main returning int? Post 302727693 by alister on Tuesday 6th of November 2012 04:20:01 PM
Old 11-06-2012
void exit(int status)
pid_t wait(int *stat_loc);

As already mentioned, only the low 8 bits of exit()'s int argument, status, are meaningful. The rest are practically ignored. However, it is an error to assume that the low 8 bits written by wait() to stat_loc are the low 8 of exit()'s status. This is often not the case. Since the layout of wait()'s *stat_loc bitmask is implementation dependent, POSIX specifies macros for examination and retrieval.

The 139 "exit status" in Gaurav's post isn't a true exit status. When a process exits abnormally, the bits which encode exit status have no meaningful value (which is why you are required to consult WIFEXITED() to confirm that the process exited normally before examining the exit status with WEXITSTATUS()).

If it's not an exit status, then what is it? If it wasn't an exit() argument, and if it wasn't main()'s return value, and if it's not provided by the kernel, then where does the 139 come from?

Before checking the exit status of it's child, the shell confirms that it exited normally, with WIFEXITED(). When the confirmation fails, the shell consults WIFSIGNALED(). Determining that the process was signaled and terminated abnormally, the signal number is retrieved with WTERMSIG(). By convention, the shell adds 128 to the signal number and stores that result in its ? parameter.

If your C code wait()ed on a process that was killed by that same signal, in the signal bits examined by WTERMSIG(), it would see 11 and not 139.

To avoid ambiguity, if you plan to invoke your binaries with the shell, it's a good idea to keep to exit values in the range 0 to 125 inclusive. The remaining values are spoken for: 126 (command found but not executable), 127 (command not found), and values larger than 128 (signal number + 128).

(Some of the following may be x86 specific.)

Returning to the original question: Where is the exit status stored? Inside the kernel.

When you call exit(n), the least significant 8 bits of the integer n are written to a cpu register. The kernel system call implementation will then copy it to a process-related data structure.

What if your code doesn't call exit()? The c runtime library responsible for invoking main() will call exit() (or some variant thereof) on your behalf. The return value of main(), which is passed to the c runtime in a register, is used as the argument to the exit() call.

When the parent calls wait(stat_loc), the exit status value (along with other status information) is copied from the kernel process structure to the address pointed to by wait()'s stat_loc.

Once a dead process is wait()'d on and its status information delivered, the kernel can destroy that process' data structure. Until then, the lingering data structure is the hallmark of a zombie.

Regards,
Alister

Last edited by alister; 11-06-2012 at 10:10 PM.. Reason: amend stack statement (register is used)
 

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Graph(3pm)						User Contributed Perl Documentation						Graph(3pm)

NAME
SVG::Graph - Visualize your data in Scalable Vector Graphics (SVG) format. SYNOPSIS
use SVG::Graph; use SVG::Graph::Data; use SVG::Graph::Data::Datum; #create a new SVG document to plot in... my $graph = SVG::Graph->new(width=>600,height=>600,margin=>30); #and create a frame to hold the data/glyphs my $frame = $graph->add_frame; #let's plot y = x^2 my @data = map {SVG::Graph::Data::Datum->new(x=>$_,y=>$_^2)} (1,2,3,4,5); my $data = SVG::Graph::Data->new(data => @data); #put the xy data into the frame $frame->add_data($data); #add some glyphs to apply to the data in the frame $frame->add_glyph('axis', #add an axis glyph 'x_absolute_ticks' => 1, #with ticks every one #unit on the x axis 'y_absolute_ticks' => 1, #and ticks every one #unit on the y axis 'stroke' => 'black', #draw the axis black 'stroke-width' => 2, #and 2px thick ); $frame->add_glyph('scatter', #add a scatterplot glyph 'stroke' => 'red', #the dots will be outlined #in red, 'fill' => 'red', #filled red, 'fill-opacity' => 0.5, #and 50% opaque ); #print the graphic print $graph->draw; DESCRIPTION
SVG::Graph is a suite of perl modules for plotting data. SVG::Graph currently supports plots of one-, two- and three-dimensional data, as well as N-ary rooted trees. Data may be represented as: Glyph Name Dimensionality supported 1d 2d 3d tree -------------------------------------------------------- Axis x Bar Graph x Bubble Plot x Heatmap Graph x Line Graph x Pie Graph x Scatter Plot x Spline Graph x Tree x SVG::Graph 0.02 is a pre-alpha release. Keep in mind that many of the glyphs are not very robust. PLOTTING
You need to create a SVG::Graph::Frame instance from the parent SVG::Graph instance for each set of data to be plotted. Datasets can be hierarchical, and to represent this, SVG::Graph::Frame instances can themselves contain subframes. SVG::Graph::Frame can contain: - multiple subframes as instances of SVG::Graph::Frame - a single SVG::Graph::Data instance - multiple SVG::Graph::Glyph instances with which to render the attached SVG::Graph::Data instance, and all SVG::Graph::Data instances attached to SVG::Graph::Frame subinstances See SVG::Graph::Frame and SVG::Graph::Glyph for details. ONE DATA SET 1. create an SVG::Graph instance 2. create an SVG::Graph::Frame instance by calling SVG::Graph::add_frame(); 3. create an SVG::Graph::Data instance, containing an SVG::Graph::Data::Datum instance for each data point. 4. Attach the SVG::Graph::Data instance to your SVG::Graph::Frame using SVG::Graph::Frame::add_data(); 5. Attach glyphs to the SVG::Graph::Frame instance using SVG::Graph::Frame::add_glyph(); 6. Call SVG::Graph::draw(); MULTIPLE DATA SETS 1. create an SVG::Graph instance 2. create an SVG::Graph::Frame instance by calling SVG::Graph::add_frame(); 3. create an SVG::Graph::Data instance, containing an SVG::Graph::Data::Datum instance for each data point. 4. Attach the SVG::Graph::Data instance to your SVG::Graph::Frame using SVG::Graph::Frame::add_data(); 5. Attach glyphs to the SVG::Graph::Frame instance using SVG::Graph::Frame::add_glyph(); 6. repeat [2-5] for each additional data set to be added. add_frame() can be called on SVG::Graph to add top-level data sets, or SVG::Graph::Frame to add hierarchical data sets. 7. Call SVG::Graph::draw(); FEEDBACK
Send an email to the svg-graph-developers list. For more info, visit the project page at http://www.sf.net/projects/svg-graph AUTHORS
Allen Day, <allenday@ucla.edu> Chris To, <crsto@ucla.edu> CONTRIBUTORS
James Chen, <chenj@seas.ucla.edu> Brian O'Connor, <boconnor@ucla.edu> SEE ALSO
SVG METHODS
new Title : new Usage : my $graph = SVG::Graph->new(width=>600, height=>600, margin=>20); Function: creates a new SVG::Graph object Returns : a SVG::Graph object Args : width => the width of the SVG height => the height of the SVG margin => margin for the root frame init Title : init Usage : Function: Example : Returns : Args : width Title : width Usage : $obj->width($newval) Function: Example : Returns : value of width (a scalar) Args : on set, new value (a scalar or undef, optional) height Title : height Usage : $obj->height($newval) Function: Example : Returns : value of height (a scalar) Args : on set, new value (a scalar or undef, optional) margin Title : margin Usage : $obj->margin($newval) Function: Example : Returns : value of margin (a scalar) Args : on set, new value (a scalar or undef, optional) svg Title : svg Usage : $obj->svg($newval) Function: Example : Returns : value of svg (a scalar) Args : on set, new value (a scalar or undef, optional) add_frame Title : add_frame Usage : my $frame = $graph->add_frame Function: adds a Frame to the current Graph Returns : a SVG::Graph::Frame object Args : a hash. usable keys: frame_transform (optional) 'top' default orientation 'bottom' rotates graph 180 deg (about the center) 'right' points top position towards right 'left' points top position towards left frames Title : frames Usage : get/set Function: Example : Returns : Args : xoffset Title : xoffset Usage : $obj->xoffset($newval) Function: Example : Returns : value of xoffset (a scalar) Args : on set, new value (a scalar or undef, optional) yoffset Title : yoffset Usage : $obj->yoffset($newval) Function: Example : Returns : value of yoffset (a scalar) Args : on set, new value (a scalar or undef, optional) draw Title : draw Usage : $graph=>draw Function: depends on child glyph implementations Returns : xmlifyied SVG object Args : none perl v5.10.0 2009-01-07 Graph(3pm)
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