I need all the C code for the following program. Design and test a
function that searches the string specified by the first function
parameter for the first occurrence of a character specified by the
second function parameter.
Have the function return a pointer to the character if successful, and
a null if the character is not found in the string. ( This duplicates
the way that the library strchr() function works.) Test the function
in a complete program that uses a loop to provide input values for
feeding to the function.

Thank you for your question.

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I have to warn you that the search() function does use some C idioms
that look a little concise and mysterious at first . But one gets used
to them.

The main thing to remember here is that the string argument is a
pointer, and incrementing the pointer in the search() function does
NOT change the value of the pointer in the calling function.

I am enclosing the code for the C function you requested below. You
can also find this function at:

http://www.rbnn.com/google/search.c 

since sometimes code excerpts do not render well when cut-and-pasted.

When writing test code, a natural question is: how do we know the test
code works? Fortunately, my original version of this program had a bug
(much to my chagrin) and the test code did indeed print out a number
of nasty error messages.

Here is a sample script:
----SAMPLE SCRIPT----
% gcc search.c
% a.exe
Testing string: one
Testing string: two
Testing string: three
Testing string: 
Testing string: x
Testing string: ZZZz
Testing string:    X
Success

-----CODE FOR search.c-----
#include <stdlib.h>

/* This function takes a string as its first argument and a char as
its second.
If the first argument is NULL then NULL is returned.
Otherwise if the second argument occurs in the first argument, a
pointer to the first occurrence is returned,
otherwise NULL is returned.

The terminating 0 is considered part of the string when doing this
comparison.

*/

char * search (char * string, char sought){

  /*Check if string is NULL and if so do not do search.*/
  if (string==NULL)return NULL;

  /*Keep incrementing the string pointer until we hit a 0 character*/
  while(*string)
    if (*string==sought)return string;
    else ++string;

  /*Now string points to the terminating NULL, and we have check if
this is the char sought*/
  if (*string==sought) return string;

  /*We didn't find the char*/
  return NULL;
}

/*Test the function search() 
This function just loops through an array of strings and each char
from 0 to 126, and compares the results against strchr() .
It prints a message if it finds a mismatch.
*/

int main(int argc, char**argv){
  char*strings[]={"one","two","three","","x","ZZZz","   X"};
  int numberstrings=7;
  int error=0;
  int i=0;
  char * string;
  char c; //the character to test against
  for (i=0;i<numberstrings;i++){
    string=strings[i];
    printf("Testing string: %s\n",string);
    for (c=0;c<127;++c)
      if (search(string,c)!=strchr(string,c)){
	error=1;
	printf("Failure at %c for string: %s\n",c,string);
      }
  }
  if (error)
    printf("Failure\n");
  else
    printf("Success\n");
}

-----SEARCH STRATEGY----
I used the site:

"Notes on C" at http://www.phys.ufl.edu/~rlliu/comp/cnotes.html to
check some C syntax.

I used search keywords "C specification stdlib" to check the semantics
of strchr .