Netpbm subroutine library:Netpbmssubroutineolibrary: pm_system()subroutine(3)Name
pm_system - run a Netpbm program with program input and output
Synopsis
#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);
Example
This simple example converts a PNM image on Standard Input to a JFIF
(JPEG) image on Standard Output. In this case, pm_system() is doing no
more than system() would do.
pm_system(NULL, NULL, NULL, NULL, "pnmtojpeg");
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:
#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);
This example reads an image into libnetpbm PAM structures, then bright‐
ens it, then writes it out, to illustrate use of pm_system with PAM
structures.
#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);
DESCRIPTION
This library function is part of Netpbm(1).
pm_system() is a lot like the standard C library system() subroutine.
It runs a shell and has that shell execute a shell command that you
specify. But pm_system() gives you more control over the Standard
Input and Standard Output of that shell command than system(). sys‐
tem() passes to the shell command as Standard Input and Output whatever
is the Standard Input and Output of the process that calls system().
But with pm_system(), 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.
Your Standard Input feeder subroutine can generate the stream in limit‐
less ways. pm_system() gives it a file descriptor of a pipe to which
to write the stream it generates. pm_system() hooks up the other end
of that pipe to the shell command's Standard Input.
Likewise, your Standard Output accepter subroutine can do anything it
wants with the stream it gets. pm_system() gives it a file descriptor
of a pipe from which to read the stream. pm_system() hooks up the
other end of that pipe to the shell command's Standard Output.
The argument stdinFeeder is a function pointer that identifies your
Standard Input feeder subroutine. pm_system() runs it in a child
process and waits for that process to terminate (and accepts its com‐
pletion status) before returning. feederParm is the argument that
pm_system() passes to the subroutine; it is opaque to pm_system().
If you pass stdinFeeder = NULL, pm_system() simply passes your current
Standard Input stream to the shell command (as system() would do), and
does not create a child process.
The argument stdoutAccepter is a function pointer that identifies your
Standard Output accepter subroutine. pm_system() calls it in the cur‐
rent process. accepterParm is an argument analogous to feederParm.
If you pass stdoutAccepter = NULL, pm_system() simply passes your cur‐
rent Standard Output stream to the shell command (as system() would do.
The argument shellCommand 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:
ppmbrighten -v 100 | pamdepth 255 | pamscale .5
pm_system() creates a child process to run the shell and waits for that
process to terminate (and accepts its completion status) before return‐
ing.
Applications
The point of pm_system() is to allow you write a C program that uses
other programs internally, as a shell script would. This is particu‐
larly desirable with Netpbm, because Netpbm consists of a lot of pro‐
grams that perform basic graphic manipulations and you'd like to be
able to build a program that does a more sophisticated graphic manipu‐
lation by calling the more basic Netpbm programs. These building block
programs typically take input from Standard Input and write output to
Standard Output.
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 pro‐
gram. The Netpbm subroutine library is a C-linkage library; the sub‐
routines in it are not usable from a Bourne Shell or Perl program.
A typical use of pm_system() 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 pro‐
cessing. To do that, you can use the memory buffer Standard Input
feeder and Standard Output accepter described below.
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 pnmremap), you can use the pamtuple Standard Input
feeder and Standard Output acceptor described below.
Broken Pipe Behavior
When you set up a shell command to take input from a pipe, as you do
with pm_system(), 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.
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.
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 nor‐
mally close the reading end of the pipe, since it is no longer useful.
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 envi‐
ronment, 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.
Standard Feeders And Acceptors
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.
Memory Buffer
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 cer‐
tain 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.
The Standard Input feeder for this is called pm_feed_from_memory. The
Standard Output accepter is pm_accept_to_memory.
For both of these, the argument is the address of a struct bufferDesc,
which is defined as follows:
struct bufferDesc {
unsigned int size;
unsigned char * buffer;
unsigned int * bytesTransferredP;
};
size is the size of the memory buffer and buffer 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 Out‐
put 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
bytesTransferredP. Unless bytesTransferredP is NULL.
Because a process typically terminates abnormally when it is not able
to write everything to a pipe that it wanted to, bytesTransferredP is
not usually useful in the Standard Input feeder case.
Pamtuple
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.
The Standard Input feeder for this is called pm_feed_from_pamtuples.
The Standard Output accepter is pm_accept_to_pamtuples.
For both of these, the argument is the address of a struct pamtuples,
which is defined as follows:
struct pamtuples {
struct pam * pamP;
tuple *** tuplesP;
};
For the Standard Input feeder, you supply a struct pam, valid up
through the tuple_type member (except it doesn't matter what the file
member is) and array of tuples.
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 tuple_type member (except leaves the
file member undefined) and allocates space for the tuple array with
malloc(). You are responsible for freeing that memory.
HISTORYpm_system() was introduced in Netpbm 10.13 (January 2003).
netpbm documentation Netpbm2subroutine0library: pm_system()subroutine(3)
