c-lab

Exploring the C language.

My Toolbox

The string.h header contains useful string functions.
strstr(a, b) will return the address of string b in string a.
strcpy() copies one string to another.
strcmp() compares two strings.
strcat() concatenates two strings.
strlen() finds the length of a string.
strchr() finds the location of a character inside a string.
An array of strings is an array of arrays.
You create an array of arrays using char strings [...][...]
C functions like printf() and scanf() use the Standard Output and Standard Input to communicate.
The Standard Output goes to the display by default.
The Standard Input reads from the keyboard by default.
The Standard Error is a separate output intended for error messages.
You can change where the Standard Input, Output, and Error are connected to using redirection.
You can print to the Standard Error using fprintf(stderr,...).
Command-line arguments are passed to main() as an array of string pointers.
You can create custom data streams with fopen(“filename”, mode).
The mode can be “w” to write, “r” to read, or “a” to append.
The getopt() function makes it easier to read command- line options.

The printf() function sends data to the Standard Output.
The Standard Output goes to the display by default.
You can redirect the Standard Output to a file by using > on the command line.
scanf() reads data from the Standard Input.
The Standard Input reads data from the keyboard by default.
You can redirect the Standard Input to read a file by using < on the command line.
The Standard Error is reserved for outputting error messages.

There are two versions of the main() function—one with command-line arguments, and one without.
Command-line arguments are passed to main() as an argument count and an array of pointers to the argument strings.
Command-line options are command-line arguments prefixed with “-”.
The getopt() function helps you deal with command-line options.
You define valid options by passing a string to getopt() like ae:.
A “:” (colon) following an option in the string means that the option takes an additional argument.
getopt() will record the options argument using the optarg variable.
After you have read all of the options, you should skip past them using the optind variable.

If the compiler finds a call to a function it hasn’t heard of, it will assume the function returns an int.
So if you try to call a function before you define it, there can be problems.
Function declarations tell the compiler what your functions will look like before you define them.
If function declarations appear at the top of your source code, the compiler won’t get confused about return types.
Function declarations are often put into header files.
You can tell the compiler to read the contents of a header file using #include.
The compiler will treat included code the same as code that is typed into the source file.
You can redirect the Standard Error using 2>.
You can connect the Standard Output of one process to the Standard Input of another process using a pipe.