CGM_TOOLS(1NCARG)

🌸 NAME 🌸

okay so bestie, welcome to CGM_TOOLS — that's the NCAR GRAPHICS manual page, section 1NCARG, and it's literally the whole Computer Graphics Metafile operations toolkit 💅. Here's the whole squad of functions that live on this page:

• CGM_open, CGM_close, CGM_lseek, CGM_read, CGM_write, CGM_directory, CGM_freeDirectory, CGM_printDirectory, CGM_getInstr, CGM_flushGetInstr, CGM_putInstr, CGM_flushOutputInstr, CGM_initMetaEdit, CGM_termMetaEdit, CGM_copyFrames, CGM_deleteFrames, CGM_mergeFrames CGM_moveFrames, CGM_readFrames, CGM_valid, CGM_writeFile, CGM_writeFrames, CGM_appendFrames — Computer Graphics Metafile operations ✨

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🦋 SYNTAX 🦋

here's the tea, copied exactly so you don't have to squint at it:

#include <cgm_tools.h>

Cgm_fd CGM_open(metafile, size, flags, mode)
char *metafile;
unsigned size;
int flags;
int mode;

int CGM_close(cgm_fd)
Cgm_fd cgm_fd;

int CGM_lseek(cgm_fd, offset)
Cgm_fd cgm_fd;
int offset;

int CGM_read(cgm_fd, buf)
Cgm_fd cgm_fd;
unsigned char *buf;

int CGM_write(cgm_fd, buf)
Cgm_fd cgm_fd;
unsigned char *buf;

Directory *CGM_directory(cgm_fd)
Cgm_fd cgm_fd;

void CGM_freeDirectory(dir)
Directory *dir;

void CGM_printDirectory(dir)
Directory *dir;

int CGM_getInstr(cgm_fd, instr)
Cgm_fd cgm_fd;
Instr *instr;

void CGM_flushGetInstr(cgm_fd)
Cgm_fd cgm_fd;

int CGM_putInstr(cgm_fd, instr)
Cgm_fd cgm_fd;
Instr *instr;

int CGM_flushOutputInstr(cgm_fd)
Cgm_fd cgm_fd;

Directory *CGM_initMetaEdit (metafile, size)
char *metafile;
unsigned int size;

int CGM_termMetaEdit()

Directory *CGM_copyFrames(start, num, target )
unsigned int start;
int num;
unsigned int target;

Directory *CGM_deleteFrames(start, num)
unsigned int start,
num;

Directory *CGM_mergeFrames(bottom, top)
unsigned bottom, top;

Directory *CGM_moveFrames (start, num, target)
unsigned int start, num, target;

Directory *CGM_readFrames(metafile, start, num, target, size)
char *metafile;
unsigned int start;
int num;
unsigned int target, size;

int *CGM_validCGM(metafile)
char *metafile;

int CGM_writeFile(metafile)
char *metafile;

int CGM_writeFrames(metafile, start, num)
char *metafile;
unsigned start, num;

int CGM_appendFrames(metafile, start, num)
char *metafile;
unsigned start, num;

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💖 DESCRIPTION 💖

okay so first up, the cgm_fd argument is a valid file descriptor that got created for reading or writing — whichever is appropriate — by CGM_open. ✨

and the vibes check out like this:

• CGM_read, CGM_directory, CGM_getInstr and CGM_flushGetInstr need a file descriptor that's open for reading 👀
• CGM_write, CGM_getInstr,CGM_flushGetInstr and CGM_flushOutputInstr need a Cgm_fd open for writing ✍️
• CGM_close and CGM_lseek will accept any valid Cgm_fd, they're chill like that 🫶

The size argument is the CGM record size in bytes. For an NCAR CGM this value is 1440. not a vibe, just a fact 💅

buf is a pointer to user allocated memory of size size. This storage will be used for buffering input and output of CGM_read and CGM_write respectively.

The dir argument is a pointer to a Directory structure created with CGM_directory or CGM_initMetaEdit. dir is a private resource that should NOT be directly modified by the user. A set of convenience macros is provided for this purpose in cgm_tools.h.

The start, num and target arguments are used to address frame numbers in a metafile being edited with one of the commands: CGM_copyFrames, CGM_deleteFrames, CGM_readFrames, CGM_moveFrames, CGM_writeFrames and CGM_mergeFrames. The start argument is the first frame in a sequence of num frame(s) to perform the editing operation on. target is similar to start and is used by commands that require two frame addresses such as copy. Addressing begins at zero. 🌀

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🌟 CGM_open

This command is modeled after the unix open command. It will open a CGM for reading or writing as specified by the flags argument and return a Cgm_fd file descriptor. The flags and open parameters are passed directly on to the system open command. For a detailed explanation of these two arguments see open(2).

🌟 CGM_close

Delete a file descriptor. The inverse of CGM_open. See close(2).

🌟 CGM_read

CGM_read attempts to read size bytes from the object referenced through the descriptor cgm_fd. size is set at the creation of cgm_fd by CGM_open. CGM_read returns the number of bytes successfully read. A zero is returned on EOF and a negative number implies an error occurred. The unix system call read is called by CGM_read. See read(2).

🌟 CGM_write

Attempts to write a single record of size bytes from buf from the object referenced by cgm_edit where size is the record size parameter provided at the creation of cgm_fd. write returns the number of bytes successfully written. A negative return number implies an error occurred. The unix system call write is called by CGM_write. See write(2).

🌟 CGM_lseek

Advance the file pointer of cgm_fd to offset bytes. Upon successful completion the current file pointer offset is returned. A negative return value is an error. The unix system call lseek is called by CGM_lseek. See lseek(2).

🌟 CGM_directory

Create a table of contents for the metafile referenced by cgm_fd. Return a pointer to this table of type Directory. The contents of the directory include number of metafiles, number of frames, record offset for each frame, frame length in records, optional frame description and metafile status. These fields are meant to be read only and should only be referenced by the convenience macros provided in cgm_tools.h. A NULL pointer is returned on failure. 😭

🌟 CGM_freeDirectory

Free memory allocated to a directory created by CGM_directory or CGM_initMetaEdit.

🌟 CGM_printDirectory

Print the contents of a directory pointed to by dir to the standard output.

🌟 CGM_getInstr,

Fetch the next instruction in file referenced by cgm_edit and convert it into a usable format pointed to by instr. CGM_getInstr provides an interface to the metafile for extracting CGM elements. The user need not be concerned with the binary format of the metafile. The fields of the Instr are as described in cgm_tools.h.

👑 heads up bestie, the maximum allowable data length returned in a single invocation is 32760 bytes. The CGM standard allows upto 32767 bytes to be stored in a single instruction. But 32767 is not a nice number to work with. Should the data length of a CGM instruction exceed 32760 bytes, indicated by the boolean more flag, the next invocation of CGM_getInstr will return the remaining data up to the same limit, etc.

CGMgetInstr requires a valid Cgm_fd open for reading. For a description on CGM see the ANSI standard.

🌟 CGM_flushGetInstr

Flush the input buffer used by CGM_getInstr. CGM_getInstr buffers the contents of the CGM and only performs actual reads as necessary. If the user desires other then sequential read access to a CGM it becomes necessary to flush the input buffer before reading from a new location.

🌟 CGM_putInstr

The analog to CGM_getInstr. This function buffers CGM instructions to be written to a CGM referenced by cgm_fd. Again the user need not be concerned with the binary format of the file. Writes are performed sequentially in record size size as specified during the creation of cgm_fd.

The same data length constraints that are placed on CGM_getInstr hold for CGM_putInstr. If the user wants to output instructions with a data length greater than 32760 bytes then the data must be broken up into blocks no greater than this size. The user must also set the boolean more flag in the Instr. cgm_fd must be a valid file descriptor open for writing. For a description of the fields of the Instr see the file cgm_tools.h.

🌟 CGM_flushOutputInstr

Flush the output buffer used by CGM_putInstr for the file referenced by cgm_fd.

💀 okay this one matters: it is necessary to explicitly flush the output buffer used by CGM_putInstr before the file is closed or any random access is performed. Otherwise not all CGM elements will actually get written.

🌟 CGM_initMetaEdit

Initialize a metafile for editing. This is the initialization routine for the higher level editing routines contained in this package: CGM_copyFrames, CGM_deleteFrames, CGM_readFrames, CGM_moveFrames, CGM_writeFile, CGM_writeFrames, and CGM_mergeFrames.

These routines only work on one metafile at a time (the one named in CGM_initMetaEdit. Invoking this routine for a second time without explicitly saving any changes will have the effect of loading a new file and discarding all changes made in the previous file. 😭

CGM_initMetaEdit and all proceeding editing functions that make changes to the file return a pointer to a Directory as a convenience that allows the user to examine the state of the file. The contents of the directory are private and should NOT be changed by the user. A set of macros is provided in cgm_tools.h to be used for retrieving the directory's contents.

🌈 note: no changes are actually made to the edit file unless it is explicitly overwritten with either CGM_writeFile or CGM_writeFrames.

🌟 CGM_termMetaEdit

Terminate the editing session started with CGM_initMetaEdit. This routine should be called after any editing changes have been saved, if desired to save them, and before exiting the editing session. CGM_termMetaEdit frees valuable resources. 🫶

🌟 CGM_copyFrames

Copy num frames beginning with start to the frame addressed by target. If target is already occupied then the source frames are inserted in its place while the target frame, and all proceeding frames, are advanced. CGM_copy operates on the file initialized by CGM_initMetaEdit (the edit file). On successful completion a pointer to the current directory is returned. On error a NULL pointer is returned.

🌟 CGM_deleteFrames

Delete num frames from the edit file starting with frame start. On successful completion a pointer to the current directory is returned. On error a NULL pointer is returned.

💅 CGM_mergeFrames

Okay so picture two frames, right? You've got one at bottom and one at top. This function overwrites whatever's chillin' in the frame addressed bottom with the union of the frame at location bottom and the frame at location top. ✨

And like… what does that actually do? It's the same effect as just drawing the top frame right on top of the bottom frame. That's it. That's the tea. 🫖

Bestie, real talk: this is not a union in the true sense of the word. The manpage said it, I'm saying it, no cap. Don't come for me.

On a successful glow-up, a pointer to the current directory gets returned. 💖 On error? A NULL pointer. 💀

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🦋 CGM_moveFrames

Moving day!! 🚚 This one moves a block of num frames from the edit file, starting with frame start, over to the position occupied by frame target.

Success = pointer to the current directory comes back to you. 🎀 Error = NULL pointer, sad girl hours. 😭

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🌸 CGM_readFrames

Reads num frames from metafile file, starting with frame start, and inserts them at address target in the edit file. Very "importing your whole aesthetic into a new save file" energy. 💾✨

Successfully done? Pointer to the current directory, returned. 🫶 Error? You already know — NULL pointer. 🙈

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👑 CGM_validCGM

The vibe check function, literally. 💅 It determines whether a file is a valid NCAR CGM or not.

It does a few simple diagnostics, just trying to figure out if the given file is in the NCAR CGM format. That's the whole effort.

⚠️ Heads up though: the tests performed are not rigorous, and it's conceivable that the information retrieved is incorrect. So like… trust but verify, bestie. 🔍

The return values, memorise them:

• 1 — it IS a valid NCAR CGM. slay. 🎉
• 0 — it is NOT a NCAR CGM. 💀
• -1 — an error occurred, and the global variable errno is set accordingly. 🚨

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🍓 CGM_writeFile

Writes the entire contents of the current edit file to file. All of it. The whole thing. No crumbs left. 🧚

CGM_writeFile returns the integer one on success and a negative number on failure.

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⭐ CGM_writeFrames

Writes a block of num frames, starting with frame start, to file. The source frames come from the edit file. 💫

📝 Note, and this one's important: CGM frames are contained in a wrapper made up of CGM delimiter elements. The file created by CGM_writeFrames will use the wrapper provided by the current edit file. So if a file foo contains n frames that get read into an editing session with a file goo, and then those same frames get written out to a file zoid, zoid may or may not be the same as the original foo. 🙃 Not a bug, just the wrapper situation. iykyk.

CGM_writeFrames returns the integer one on success and a negative number on failure.

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🌈 CGM_appendFrames

Appends a block of num frames, starting with frame start, to file. Adding to the collection, not replacing it. 💖

Two conditions, bestie: file must already exist, and it must be a valid NCAR CGM. No exceptions. 🚫

CGM_appendFrames returns the integer one on success and a negative number on failure.

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✨ SEE ALSO ✨

• ANSI X3.122 Computer Graphics Metafile for the Storage and Transfer of Picture Description Information. 📚

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💀 BUGS 💀

• CGMs with more the one metafile stored in the are not guaranteed to work. (Yes, that's the sentence. We keep it real here. 🫠)
• Should not have to explicitly flush the output buffer for CGM_getInstr. This should be handled automatically when the file is closed. 😤

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🎀 COPYRIGHT 🎀

Copyright (C) 1987-2009 University Corporation for Atmospheric Research

The use of this Software is governed by a License Agreement. 📜

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okay that's the whole page, you made it, I'm so proud of you 🥹💜 this lil doc is from the NCARG crew and it's dated January 1993 — vintage but make it iconic. go forth and CGM, bestie!! 💅✨