.TL
Netpbm subroutine library: pm_system() subroutine
.SH 1
pm_system()
.LP
Updated: 26 August 2006
.br
.SH 2
Name
.LP
pm_system - run a Netpbm program with program input and output
.SH 2
Synopsis
.LP
.DS L
#include <netpbm/pm_system.h>
pm_system(void                  stdinFeeder(int, void *),
          void *          const feederParm,
          void                  stdoutAccepter(int, void *),
          void *          const accepterParm,
          const char *    const shellCommand);
.DE
.SH 2
Example
.LP
.LP
This simple example converts a PNM image on Standard Input to a
JFIF (JPEG) image on Standard Output.  In this case,
\fBpm_system()\fR is doing no more than \fBsystem()\fR would do.
.DS L
    pm_system(NULL, NULL, NULL, NULL, "pnmtojpeg");
.DE
.LP
This example does the same thing, but moves the data through memory
buffers to illustrate use with memory buffers, and we throw in a stage
to shrink the image too:
.DS L
#include <netpbm/pm_system.h>
char              pnmData[100*1024];   /* Input file better be < 100K */
char              jfifData[100*1024];
struct bufferDesc pnmBuffer;
struct bufferDesc jfifBuffer;
unsigned int      jfifSize;
pnmBuffer.size = fread(pnmData, 1, sizeof(pnmData), stdin);
pnmBuffer.buffer = pnmData;
pnmBuffer.bytesTransferredP = NULL;
jfifBuffer.size = sizeof(jfifData);
jfifBuffer.buffer = jfifData;
jfifBuffer.bytesTransferredP = &jfifSize; 
pm_system(&pm_feed_from_memory, &pnmBuffer,
          &pm_accept_to_memory, &jfifBuffer,
          "pamscale .5 | pnmtojpeg");
fwrite(jfifData, 1, jfifSize, stdout);
.DE
.LP
This example reads an image into libnetpbm PAM structures, then
brightens it, then writes it out, to illustrate use of \fBpm_system\fR
with PAM structures.
.DS L
#include <netpbm/pam.h>
#include <netpbm/pm_system.h>
struct pam       inpam;
struct pam       outpam;
tuples **        inTuples;
tuples **        outTuples;
struct pamtuples inPamtuples;
struct pamtuples outPamtuples;
inTuples = pnm_readpam(stdin, &inpam, sizeof(inpam));
outpam = inpam;
inPamtuples.pamP = &inpam;
inPamtuples.tuplesP = &inTuples;
outPamtuples.pamP = &outpam;
outPamtuples.tuplesP = &outTuples;
pm_system(&pm_feed_from_pamtuples, &inPamtuples,
          &pm_accept_to_pamtuples, &outPamtuples,
          "ppmbrighten -v 100");
outpam.file = stdout;
pnm_writepam(&outpam, outTuples);
.DE
.SH 2
DESCRIPTION
.LP
.LP
This library function is part of Netpbm.
.LP
\fBpm_system()\fR is a lot like the standard C library
\fBsystem()\fR subroutine.  It runs a shell and has that shell
execute a shell command that you specify.  But \fBpm_system()\fR
gives you more control over the Standard Input and Standard Output of
that shell command than \fBsystem()\fR.  \fBsystem()\fR passes to the
shell command as Standard Input and Output whatever is the Standard Input
and Output of the process that calls \fBsystem()\fR.  But with
\fBpm_system()\fR, you specify as arguments subroutines to execute to
generate the shell command's Standard Input stream and to process the
shell command's Standard Output stream.
.LP
Your Standard Input feeder subroutine can generate the stream in
limitless ways.  \fBpm_system()\fR gives it a file descriptor of a
pipe to which to write the stream it generates.  \fBpm_system()\fR
hooks up the other end of that pipe to the shell command's Standard
Input.
.LP
Likewise, your Standard Output accepter subroutine can do anything
it wants with the stream it gets.  \fBpm_system()\fR gives it a file
descriptor of a pipe from which to read the stream.
\fBpm_system()\fR hooks up the other end of that pipe to the shell
command's Standard Output.
.LP
The argument \fIstdinFeeder\fR is a function pointer that
identifies your Standard Input feeder subroutine.  \fBpm_system()\fR
runs it in a child process and waits for that process to terminate (and
accepts its completion status) before returning.  \fIfeederParm\fR is
the argument that \fBpm_system()\fR passes to the subroutine; it is
opaque to \fBpm_system()\fR.
.LP
If you pass \fIstdinFeeder\fR = NULL, \fBpm_system()\fR simply
passes your current Standard Input stream to the shell command (as
\fBsystem()\fR would do), and does not create a child process.
.LP
The argument \fIstdoutAccepter\fR is a function pointer that
identifies your Standard Output accepter subroutine.
\fBpm_system()\fR calls it in the current process.
\fIaccepterParm\fR is an argument analogous to \fIfeederParm\fR.
.LP
If you pass \fIstdoutAccepter\fR = NULL, \fBpm_system()\fR simply
passes your current Standard Output stream to the shell command (as
\fBsystem()\fR would do.
.LP
The argument \fIshellCommand\fR is a null-terminated string
containing the shell command that the shell is to execute.  It can be
any command that means something to the shell and can take a pipe for
Standard Input and Output.  Example:
.DS L
\f(CWppmbrighten -v 100 | pamdepth 255 | pamscale .5
\fR.DE
\fBpm_system()\fR creates a child process to run the shell and waits
for that process to terminate (and accepts its completion status)
before returning.
.SH 2
Applications
.LP
.LP
The point of \fBpm_system()\fR is to allow you write a C program that
uses other programs internally, as a shell script would.  This is particularly
desirable with Netpbm, because Netpbm consists of a lot of programs that
perform basic graphic manipulations and you'd like to be able to build a
program that does a more sophisticated graphic manipulation by calling the
more basic Netpbm programs.  These building block programs typically take
input from Standard Input and write output to Standard Output.
.LP
The obvious alternative is to use a higher level language -- Bourne Shell
or Perl, for example.  But often you want your program to do manipulations
of your graphical data that are easier and more efficient in C.  Or you want
to use the Netpbm subroutine library in your program.  The Netpbm subroutine
library is a C-linkage library; the subroutines in it are not usable from a
Bourne Shell or Perl program.
.LP
A typical use of \fBpm_system()\fR is to place the contents of
some graphical image file in memory, run a Netpbm program against it,
and have what would ordinarily go into an output file in memory too,
for further processing.  To do that, you can use the memory buffer
Standard Input feeder and Standard Output accepter described below.
.LP
If your program uses the Netpbm subroutine library to read, write, and
manipulate images, you may have an image in an array of PAM tuples.  If you
want to manipulate that image with a Netpbm program (perhaps remap the
colors using \fBpnmremap\fR), you can use the pamtuple Standard Input
feeder and Standard Output acceptor described below.
.SH 2
Broken Pipe Behavior
.LP
.LP
When you set up a shell command to take input from a pipe, as
you do with \fBpm_system()\fR, you need to understand how pipes work with
respect to the programs at either end of the pipe agreeing to how much
data is to be transferred.  Here are some notes on that.
.LP
It is normal to read a pipe before the process on the other end has
written the data you hope to read, and it is normal to write to a pipe
before the process on the other end has tried to read your data.
Writes to a pipe can be buffered until the reading end requests the
data.  A process reading or writing a pipe can block until the other
end is ready.  Or a read or write can complete with an indication that
the other end is not ready at the moment and therefore no data, or
less data than was requested, was transferred.
.LP
The pipe is normally controlled by the writing end.  When you read
from a pipe, you keep reading until the program on the other end of
the pipe closes it, and then you get an end-of-file indication.  You then
normally close the reading end of the pipe, since it is no longer useful.
.LP
When you close the reading end of a pipe before getting the
end-of-file indication and the writer subsequently tries to write to
the pipe, that is an error condition for the writer.  In a typical
default Unix environment, that error causes the writer to receive a
SIGPIP signal and that signal causes the writer process to terminate
abnormally.  But if, alternatively, the writer has ordered that SIGPIPE
be blocked, ignored, or handled, the signal does not cause the death of
the writer.  Instead, the write operation simply completes with an error
indication.
.SH 2
Standard Feeders And Acceptors
.LP
.LP
You can supply anything you like as a Standard Input feeder or
Standard Output acceptor, but the Netpbm subroutine library comes with
a few that perform commonly needed functions.
.SH 3
Memory Buffer
.LP
.LP
These routines are for when you just want to treat an area of
memory as a file.  If the shell command would ordinarily read a 513
byte regular file from its Standard Input, you want it to take 513 bytes
from a certain address in your process' memory.  Whatever bytes the
shell command wants to write to its output file you want it to store at
another address in your process' memory.
.LP
The Standard Input feeder for this is called \fBpm_feed_from_memory\fR.
The Standard Output accepter is \fBpm_accept_to_memory\fR.
.LP
For both of these, the argument is the address of a \fBstruct
bufferDesc\fR, which is defined as follows:
.DS L
struct bufferDesc {
    unsigned int    size;
    unsigned char * buffer;
    unsigned int *  bytesTransferredP;
};
.DE
\fIsize\fR is the size of the memory buffer and \fIbuffer\fR is its
location in memory (address).  The Standard Input feeder will attempt
to feed the entire buffer to the shell command's Standard Input; the
Standard Output accepter will not accept any more data from the shell
command's Standard Output than will fit in the buffer.  Both return
the actual amount of data read or written, in bytes, at the location
identified by \fIbytesTransferredP\fR.  Unless
\fBbytesTransferredP\fR is NULL.
.LP
Because a process typically terminates abnormally when it is not
able to write everything to a pipe that it wanted to,
\fIbytesTransferredP\fR is not usually useful in the Standard Input feeder
case.
.SH 3
Pamtuple
.LP
.LP
These routines are for when you have images in memory in the data
structures used by the PAM family of subroutines in the Netpbm library --
i.e. struct PAM and an array of struct tuple.  With these routines, you
can run a Netpbm program against such an image just as you would against
the same image in a regular file.
.LP
The Standard Input feeder for this is called
\fBpm_feed_from_pamtuples\fR.  The Standard Output accepter is
\fBpm_accept_to_pamtuples\fR.
.LP
For both of these, the argument is the address of a \fBstruct
pamtuples\fR, which is defined as follows:
.DS L
struct pamtuples {
    struct pam * pamP;
    tuple ***    tuplesP;
};
.DE
.LP
For the Standard Input feeder, you supply a struct pam, valid up
through the \fItuple_type\fR member (except it doesn't matter what
the \fIfile\fR member is) and array of tuples.
.LP
For the Standard Output Accepter, you supply only space in memory
for the struct pam and the address of the tuple array.  The routine
fills in the struct pam up through the \fItuple_type\fR member
(except leaves the \fIfile\fR member undefined) and allocates space
for the tuple array with malloc().  You are responsible for freeing
that memory.
.SH 2
HISTORY
.LP
\fBpm_system()\fR was introduced in Netpbm 10.13 (January 2003).