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Reading command line parameters
(4 answers)
Closed 8 years ago.
I was doing my programs Python, so not much familiar with C.
I am doing a program on creating a binary tree in C. I am using an "insert" function created by me . The goal is that for typing "insert " in command line, the function should work. But I have not much idea regarding how to get and parse command line arguments in C. Can anyone help?
http://www.cprogramming.com/tutorial/c/lesson14.html
int main( int argc, char *argv[])
This should be the declaration of your main function. argc is the amount of arguments. argv[] is an array that contains each command-line argument as a string, the program name is argv[0] so the first argument will be argv[1]. I'm not a C programmer, so this might not be good info, I highly recommend checking out the link.
Use a library to handle the low-level details, such as getopt. The code is much more involved than with Python's argparse or getopt, but is similar in concept. (the wikipedia article to which I linked contains example C code using getopt)
Command line arguments are passed during run time.
You have to specify the number of arguments and also a char pointer to point to these arguments. This is done in main() syntax itself.
Void main(int argc, char* argv)
In order to compile and generate an executable in CC compiler,
cc -o exec_name program_name.c
In order to run,
exec_name arg1 arg2.........
It is to be noted that the exec_name is also considered as an argument
Related
I want to compare the different elements of a command-line argument. It would be entered all together resulting in the string being found at argv[1]. However, I am not sure how to compare the elements and individual characters as I am looking for repetitions.
If I compared [2] to [3] in the string, there would be nothing there as only 1 string is entered in the command line argument and I need to compare the characters found within that string argv[1]. I am unable to include spaces so I wouldn't be able to compare argv[2] to argv[1].
Read Modern C and see this C reference.
If you code for Linux, read also documentation of GNU libc, in particular the section on parsing program arguments.
You could also use strcmp(3) to compare your program argument to some fixed constant string.
The program arguments are given to your main function, traditionally defined as int main(int argc, char**argv). They are always strings (of different addresses), and on POSIX you are practically certain that argc>0, argv[0] is a non-empty string (somehow the name of the program), all argv[i] with i >= 0 and i < argc are non-null, and argv[argc] is NULL.
You could code inside your main something like
if (argc>1 && !strcmp(argv[1], "foo")) {
/// handle `foo` as first program argument
}
I would recommend to study for inspiration the source code of GNU findutils or of GNU make.
I don't recommend modifying program arguments with e.g. some code like strcpy(argv[1], "bar"). It is not portable, perhaps forbidden, and certainly unreadable and brittle.
If you are on Linux, see also proc(5) about /proc/self/cmdline
I'm making a university project which is supposed to read a table from stdin, apply some changes to it and print to stdout. Here's how the program should be run:
./main [delimiter] [function] <file1.txt >file2.txt
[delimiter] is the character that will divide the cells in the resulting table, defined in the body;
[function] is the function that will be performed to modify rows or columns, defined in the body.
So my question is, how can I read the [delimiter] and [function] from the terminal so that I can use them accordingly in the body of the program?
A C program generally has a main function with a signature like:
int main (int argc, char *argv[])
where argc is an integer that tells you how many things are in the array, argv, and argv is an array of arguments starting with the name of the program (at index 0) and including all the options and parameters that you specify when you invoke the program. Since parsing arguments is something that so many programs have to do, there are various libraries that simplify the task. You can find a number of them listed in the question Parsing command-line arguments in C?.
Parsing the arguments yourself really isn't difficult, though, especially if your program expects the arguments in a particular sequence. Just loop over the entries in argv and read the strings.
This question already has answers here:
How do command line arguments work?
(3 answers)
Closed 5 years ago.
I am trying to understand how command line arguments work in details.
This is what I think happens:
When you compile a source code that contains the main() function in C, the generated object file will be linked with the CRT, and the entry point for the program will be the _start() function (which exists in the CRT), and _start() will call main().
Now when you run your program and pass it some command line arguments, the command line arguments will be passed to the _start() function, and then _start() will re-pass the command line arguments to main().
Am I correct?
Am I correct?
Yes and no:
The _start() function is not a C function but an assembler function. The reason for this is that the CPU is not in a "state" which is required by C programs so the _start() function also has to set up the CPU for executing C code.
One difference between the "state" required by C programs and the "state" of the CPU when _start() is called is the way arguments (here: command line arguments) are stored.
Under Linux (at least 32 bit - I don't know about 64 bit) you actually have an array that later represents argv. _start() has to calculate the location of argv and then pass the calculated value to main().
Under Windows there is a function that returns the entire command line as pointer to a single string (const char *)! The _start() function has to call that function and then to split the string into parts that will later become argv...
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How to execute a program from file descriptor?
(2 answers)
Closed 5 years ago.
Is there any way to run a program and/or file from within a running c program using a file pointer, or would one have to use the normal file name and system/fork etc? If it matters, writing this on linux.
Essentially, I'm asking if something like this is possible:
FILE * fp=fopen("somefile")
//dostuff
run the file pointed to by fp here (with fp, not with the filename)
Is there any way to run a program and/or file from within a running c
program using a file pointer
It is not really clear what you are asking so I'll try to show you some of the way you can do it.
Maybe you're talking about execve():
int execve(const char *filename, char *const argv[], char *const envp[]);
where execve() executes the program pointed to by filename and argv is an array of argument strings passed to the new program. Similarly there is fexecve() that works the same as execve() with the difference that the file to be executed is specified via a file descriptor, fd, rather than via a pathname.
Or, you can use system() for running some program. For example:
int result = system("./myProgram 2 3");
This will wait for myProgram to be executed, then will get the exitcode.
Last but not least, you can use fork() a system call used to create processes.
My question is about the exec family of C system calls. How do I predict what type of input the program I am executing is looking for since the exec family varies between a list of strings arg1, arg2, arg3 ... and a 2d char array for its parameters. Can most Linux command line programs handle both forms of input?
for example:
int execl(const char *path, const char *arg0, ... /*, (char *)0 */);
int execv(const char *path, char *const argv[]);
What would happen if I executed ls:
execl('/bin/ls', 'ls', NULL);
versus:
execv('/bin/ls', lsArgsArray);
Presumably ls can handle both forms of input, but my real question is generic about other utilities in general, is it safe to assume that you can invoke them either way? If it is not safe to invoke with either method how would something like bash distinguish which to use?
Neither the executed program nor the kernel see any difference whatever one you call: It is both converted to the same system call by your C library. The executed program comes one additional step behind the kernel, thus has even less chance to see any difference.
For the exact kernel interface, look in the sources of either the kernel or your platform C library.
The shell (bash/ash/csh/sh/...) is not in any way priviliged. Still, going directly to the API call allows you to pass a bogus program name.