Print man contents as help prompt of a C program - c

Question

I plan to use pandoc to generate the man files from markdown for a very small C program.
I do not want to repeat myself and type the help prompt (command [ -h | --help ]) into the source code of the program.
Question
Q: Could I permanently include man content from a file at compile time of a C program?
OR
Q: Is there a way in C to print the contents of the installed man page to stdout?
[WIP] Solution
Based on #KamilCuk's answer here's what i am doing:
In terminal:
file command.1
# OUTPUT --> command.1: troff or preprocessor input, ASCII text
xxd -i command.1 > command.hex
Then in main.c
#include "command.hex"
printf("%s", command_1);
This is almost exactly what I am looking for except
I need the hexed troff to be parsed before printing. Or parsed before hexing. I am not sure which to do / how to do that.
No, I do not want to call man and pipe the output.
I would prefer using a library + a macro to generate the hex but I am not sure how it's done.
Clarifications
What I mean by help prompt:
The output of git --help which prints to stdout
NOT the output of git --help branch which opens the man for git-branch
However, please do share the latter too. It isn't what I want: my manual is very short; but I would be glad to learn an alternative if not the exact one I seek.
Considerations I have noted:
man pages are not always installed to the same location
man pages may not be installed in some systems; or there may be no man program

Answer 1

Could I permanently include man content from a file at compile time?
You can include any content of any file at compile time. The simplest is use xxd -i and compile the output.
Is there a way in C to print the contents of the installed man page?
You can troff output from man -k
You can get output from man by replacing pager, along MANPAGER=cat man.
In C: setenv("MANPAGER", "cat"); execp("man", (char*[]){"man", "something", 0}); or just system("MANPAGER=cat man some_command").
man pages are not always installed to the same location
And system administractors can use /etc/man_db.conf to configure the behavior of man command.
man pages may not be installed in some systems; or there may be no man program
So it seems users of such systems do not care about man pages, so there's no point in supporting such cases.
I believe your intention is to render a man page from your program, where you have a static troff input saved and want to use man to display it. You could save the input to a file, save it and pass the filename to man - man will parse troff and render it. Or you could the usual pipe()+fork()+exec() and from one process pass troff data to stdin to man - - the - will make man read troff input from stdin.

Answer 2

Is there a way in C to print the contents of the installed man page to stdout?
You could invoke the mancommand by using a pipe stream . Function popen is what you are looking for. More info here.
This is an fgets() solution, though you might want to be careful, in case you want to read more characters.
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
FILE *fp = NULL;
char buffer[1000];
fp = popen("man man","r");
if (fp == NULL)
{
/* error running the command */
exit(EXIT_FAILURE);
}
while (fgets(buffer,sizeof(buffer),fp))
{
puts(buffer);
}
fclose(fp);
return 0;
}
getline() solution, in case you don't know how many bytes you are going to read.
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
FILE *fp = NULL;
char *buffer = NULL;
size_t len = 0U;
ssize_t nread = 0;
fp = popen("man man","r");
if (fp == NULL)
{
/* error running the command */
exit(EXIT_FAILURE);
}
/* getline will set errno uppon failure, you might want to check that too */
while ((nread = getline(&buffer,&len,fp)) != -1)
{
buffer[nread] = 0; /* NULL terminate your buffer */
puts(buffer);
}
free(buffer);
fclose(fp);
return 0;
}
More on getline() here.

Answer 3

The man program is written in C and is open source. It is more complex than you believe, since it deals with terminal (and their width). See also termios(3) and tty(4). Be aware of ANSI escape codes and the ncurses library. Take into account that in 2021 UTF-8 is used everywhere, and that some man pages could contain § or ° characters. Be also aware of internationalization and localization issues, see locale(7). In France, a lot of Linux systems are installed with man pages in French.
So you could get the source code of the man-db package, study it, and incorporate it inside your program.
My opinion is that it would be silly to do so.
You could instead read Advanced Linux Programming, study various syscalls(2), and use fork(2), execve(2), waitpid(2) to run the existing and installed /usr/bin/man program (whose existence could be tested with test(1) or access(2) or stat(2) in C code; but if it does not exist your execve(2) will fail...)
Regarding display of --help, you could use (if so allowed) GNU libc program argument parsing facilities, and take inspiration from the source code of existing free software programs, such as GNU coreutils.
Of course, your build infrastructure (e.g. your Makefile if you build your software with GNU make) could run programs to share data and textual descriptions. You could (if so allowed) use open source tools to generate some C code (e.g. #included stuff) at build time, such as GNU m4, GNU gawk, GPP, etc..
Take also inspiration from the source code of git. It is open source (and useful for you): you can download its source code and study it.