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The system starts a C program by calling the function main
. It
is up to you to write a function named main
—otherwise, you
won’t even be able to link your program without errors.
In ISO C you can define main
either to take no arguments, or to
take two arguments that represent the command line arguments to the
program, like this:
int main (int argc, char *argv[])
The command line arguments are the whitespace-separated tokens given in
the shell command used to invoke the program; thus, in ‘cat foo
bar’, the arguments are ‘foo’ and ‘bar’. The only way a
program can look at its command line arguments is via the arguments of
main
. If main
doesn’t take arguments, then you cannot get
at the command line.
The value of the argc argument is the number of command line
arguments. The argv argument is a vector of C strings; its
elements are the individual command line argument strings. The file
name of the program being run is also included in the vector as the
first element; the value of argc counts this element. A null
pointer always follows the last element: argv[argc]
is this null pointer.
For the command ‘cat foo bar’, argc is 3 and argv has
three elements, "cat"
, "foo"
and "bar"
.
In Unix systems you can define main
a third way, using three arguments:
int main (int argc, char *argv[], char *envp[])
The first two arguments are just the same. The third argument
envp gives the program’s environment; it is the same as the value
of environ
. See Environment Variables. POSIX.1 does not
allow this three-argument form, so to be portable it is best to write
main
to take two arguments, and use the value of environ
.
• Argument Syntax: | By convention, options start with a hyphen. | |
• Parsing Program Arguments: | Ways to parse program options and arguments. |
Next: Environment Variables, Up: Program Basics [Contents][Index]