Adding erf files

21st_century_c/first_compile/README.md

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+# Compilation
+
+## Things you should know
+
+- There is an implicit -lc passed to the compiler for the linker to link the standard libc.
+- Adding the -g flag adds symbols for debugging. Without it gdb can't give you variable or function names.
+ - There is some overhead in size of the execuable when using this command but it is worth the ability debug
+- Add the standard `-std=gnu11` you are targeting to the compile command.
+- `-Wall` adds compiler warnings. 
+- `Werror` will actually treat warnings as errors preventing compilation. Trust the compiler. If an error or
+  warning occurs fix it.
+
+## Paths
+
+The linker will normally just look for the header and object files in the file system hierarchy's usual places.
+In the `erf.c` program we can explicitly pass the math and the libc libraries to the linker like so:
+
+```
+gcc erf.c -o erf lm -g -Wall -O3
+```
+
+If we have a library in a non-standard location you have to tell the compiler where the header files are 
+and you have to tell linker where it is located. 
+	
+```
+gcc -I/usr/local/include use_useful.c -o use_useful -L/usr/local/lib -luseful
+```
+
+- `-I` Adds the given path to the include search path that the compiler searches for header files.
+- `-L` Adds the library (object code) to the Linker's search path.
+- `-l` Says to link the `useful` library
+
+### Order matters in dependencies
+
+
+If you have `specific.o` depend on `lbroad` which depends on `lgeneral` then you need:
+```
+gcc specific.o -lbroad -lgeneral
+```
+
+Any other order will probably fail.
+
+### Hunting for libraries
+
+`pkg-config` addresses many problems by creating a repository of flags and locations that packages self-report
+as being necessary for compilation.
+
+```
+pkg-config --libs gsl libxml-2.0 
+pkg-config --cflags gsl libxml-2.0
+
+> -lgsl -lgslcblas -lm -lxml2
+> -I/usr/include/libxml2 
+```
+
+
+If these libraries are installed it will produce exactly the flags needed to compile using GSL and LibXML2. This
+example command also demonstrates how GSL is dependent on the GNU Basic Linear Algebra Subprograms which in turn
+is dependent on the math.c library (`-lm`).
+
+Since we don't see a `-L` we know that all the libraries are located in the typical `lib` locations, but the `-I`
+tells use that the header files for `libxml2` are in a different location.
+
+When you want to use `pkg-config` to evaluate the at the shell we can use backticks or `$()`(command substituation).
+The `$(...)` is the more common (modern) syntax, but both are valid. 
+
+```
+gcc `pkg-config --cflags --libs gsl libxml2` -o specific specific.c
+```
+This first evaluates the backticks which is then used as input to the command producing:
+```
+gcc -I/usr/include/libxml2 -lgsl -lgslcblas -lm -lxml2 -o specific specific.c 
+```
+
+As you can probably tell the more libraries your program begins using, the more locations we need to include/link to.
+
+### Path Environment Variables
+You can also set the path for the C compiler and Linker with environmental variables. They tend to be different on each OS,
+so you need to read the `man` for your compiler.
+
+1. `man gcc`
+2. Search for pattern: `/ENVIRONEMT`
+
+

21st_century_c/first_compile/erf.c

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+#include <math.h> //erf, sqrt
+#include <stdio.h> //printf
+
+int main(){
+   printf("The integral of a Normal(0,1) distribution "
+          "between -1.96 and 1.96 is: %g\n", erf(1.96*sqrt(1/2.)));
+
+   return 0;
+}
+