Arguments to main in C

Question

I don t know what to do  I have a great understanding of C basics  Structures  file IO  strings  etc  Everything but CLA  For some reason I cant grasp the concept  Any suggestions  help  or advice  PS I am a linux user

User · Answer

Main is just like any other function and argc and argv are just like any other function arguments  the difference is that main is called from C Runtime and it passes the argument to main  But C Runtime is defined in c library and you cannot modify it  So if we do execute program on shell or through some IDE  we need a mechanism to pass the argument to main function so that your main function can behave differently on the runtime depending on your parameters  The parameters are argc   which gives the number of arguments and argv which is pointer to array of pointers  which holds the value as strings  this way you can pass any number of arguments without restricting it  it s the other way of implementing var args

User · Answer

The signature of main is   int main int argc  char   argv     argc refers to the number of command line arguments passed in  which includes the actual name of the program  as invoked by the user   argv contains the actual arguments  starting with index 1   Index 0 is the program name   So  if you ran your program like this     program hello world   Then    argc would be 3  argv 0  would be    program   argv 1  would be  hello   argv 2  would be  world

User · Answer

For parsing command line arguments on posix systems  the standard is to use the getopt   family of library routines to handle command line arguments   A good reference is the GNU getopt manual

User · Answer

Had made just a small change to  anthony code so we can get nicely formatted output with argument numbers and values  Somehow easier to read on output when you have multiple arguments    include  lt stdio h gt   int main int argc  char  argv          printf  The following arguments were passed to main    n        printf  argnum  t value  n        for  int i   0  i lt argc  i    printf   d  t  s  n   i  argv i        printf   n         return 0       And output is similar to   The following arguments were passed to main    0        D  Projects test vcpp bcppcomp1 Debug bcppcomp exe 1        -P 2        TestHostAttoshiba 3         http  tcp 4        local 5        80 6        MyNewArgument 7        200 124 211 235 8        type NewHost 9        test yes 10       result output

User · Answer

Imagine it this way    main   is also a function which is called by something else  like another FunctioN    the arguments to it is decided by the FunctioN   the second argument is an array of strings   the first argument is a number representing the number of strings   do something with the strings   Maybe a example program woluld help   int main int argc char  argv           printf  you entered in reverse order  n         while argc--                printf   s n  argv argc           return 0      it just prints everything you enter as args in reverse order but YOU should make new programs that do something more useful   compile it  as say hello  run it from the terminal with the arguments like    hello am i here   then try to modify it so that it tries to check if two strings are reverses of each other or not then you will need to check if argc parameter is exactly three if anything else print an error  if argc  3   3 because even the executables name string is on argc         printf  unexpected number of arguments n        return -1      then check if argv 2  is the reverse of argv 1  and print the result    hello asdf fdsa   should output  they are exact reverses of each other   the best example is a file copy program try it it s like cp  cp file1 file2  cp is the first argument  argv 0  not argv 1   and mostly you should ignore the first argument unless you need to reference or something  if you made the cp program you understood the main args really

User · Answer

Siamore  I keep seeing everyone using the command line to compile programs  I use x11 terminal from ide via code  blocks  a gnu gcc compiler on my linux box  I have never compiled a program from command line  So Siamore  if I want the programs name to be cp  do I initialize argv 0   cp   Cp being a string literal  And anything going to stdout goes on the command line    The example you gave me Siamore I understood  Even though the string you entered was a few words long  it was still only one arg  Because it was encased in double quotations  So arg 0   the prog name  is actually your string literal with a new line character   So I understand why you use if argc  3  print error  Because the prog name   argv 0  and there are 2 more args after that  and anymore an error has occured  What other reason would I use that  I really think that my lack of understanding about how to compile from the command line or terminal is my reason for lack understanding in this area   Siamore  you have helped me understand cla s much better  Still don t fully understand but I am not oblivious to the concept  I m gonna learn to compile from the terminal then re-read what you wrote  I bet  then I will fully understand  With a little more help from you lol   lt   Code that I have not written myself  but from my book     include  lt stdio h gt   int main int argc  char  argv          int i       printf  The following arguments were passed to main            for i 1  i lt argc  i    printf   s    argv i        printf   n         return 0       This is the output    anthony anthony   Documents C Programming CLA    CLA hey man The follow arguments were passed to main    hey man anthony anthony   Documents C Programming CLA    CLA hi how are you doing  The follow arguments were passed to main    hi how are you doing    So argv is a table of string literals  and argc is the number of them  Now argv 0  is  the name of the program  So if I type   CLA to run the program   CLA is argv 0   The above  program sets the command line to take an infinite amount of arguments  I can set them to  only take 3 or 4 if I wanted  Like one or your examples showed  Siamore    if argc  3  printf  Some error goes here    Thank you Siamore  couldn t have done it without you  thanks to the rest of the post for their time and effort also    PS in case there is a problem like this in the future   you never know lol the problem was because I was using the IDE  AKA Code  Blocks  If I were to run that program above it would print the path directory of the program  Example    Documents C CLA c it has to be ran from the terminal and compiled using the command line  gcc -o CLA main c and you must be in the directory of the file

[c] Arguments to main in C

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