C STANDARD I/O LIBRARY

For example, the following code, according to the standard, produces undefined behavior (specifically, because the parameters passed to the standard strcpy() function should not overlap).

\r
include \r
include \r
\r
int main (void)\r
{\r
    char *s = "Hello World!\
";\r
\r
    strcpy (s, s+1); /* remove first character from string s --\r
                        s = s+1;\r
                        is probably what the programmer wanted */\r
    return 0;\r
}\r

"Undefined behavior" means that the resulting program can do anything, including (accidentally) working as the programmer intended it to, producing incorrect output, crashing horribly every time it is run, crashing only under certain obscure conditions, etc. The canonical expression among experienced C programmers is that "demons may fly out of your nose" (usually abbreviated to "nasal demons")—i.e., anything can happen.

Some compilers do not adhere to either of the standards in their default mode, which leads to many programs being written which will only compile with a certain version of a certain compiler on a certain platform. Any program written only in standard C will compile unchanged on any platform which has a conforming C implementation.

Although C is usually termed a high level language, this is only in comparison to assembly language; it is significantly lower-level than most other programming languages. In particular, it is up to the programmer to manage the contents of computer memory. C provides no facilities for array bounds checking or automatic garbage collection. C has sometimes been termed "portable assembly language".

Manual memory management provides the programmer with greater leeway in tuning the performance of a program, which is particularly important for programs such as device drivers. However, it also makes it easy to accidentally create code with bugs stemming from erroneous memory operations, such as buffer overflows. Tools have been created to help programmers avoid these errors, including libraries for performing array bounds checking and garbage collection, and the lint source code checker. Intentional exploitation of programs written in C containing potential buffer overruns is often used to break computer security, either manually or by viruses and worms.

Some of the perceived shortcomings of C have been addressed by newer programming languages derived from C. The Cyclone programming language has features to guard against erroneous memory operations. C++ and Objective C provide constructs designed to aid object-oriented programming. Java and C# add object-oriented programming constructs as well as a higher level of abstraction, such as automatic memory management.

K&R C is often considered the most basic part of the language that is necessary for a C compiler to support. Since not all of the currently-used compilers have been updated to fully support ANSI C fully, and reasonably well-written K&R C code is also legal ANSI C, K&R C is considered the lowest common denominator that programmers should stick to when maximum portability is desired. For example, the bootstrapping version of the GCC compiler, xgcc, is written in K&R C. This is because many of the platforms supported by GCC did not have an ANSI C compiler when GCC was written, just one supporting K&R C.

However, ANSI C is now supported by almost all the widely used compilers. Most of the C code being written nowadays uses language features that go beyond the original K&R specification.

The ANSI C standard, with a few minor modifications, was adopted as ISO standard number ISO 9899. The first ISO edition of this document was published in 1990 (ISO 9899:1990.)

  if () 
     
  else 
     

The switch statement causes control to be transferred to one of several statements depending on the value of an expression, which must have integral type. The substatement controlled by a switch is typically compound. Any statement within the substatement may be labeled with one or more case labels, which consist of the keyword case followed by a constant expression and then a colon (:). No two of the case constants associated with the same switch may have the same value. There may be at most one default label associated with a switch; control passes to the default label if none of the case labels are equal to the expression in the parentheses following switch. Switches may be nested; a case or default label is associated with the smallest switch that contains it. Switch statements can "fall-through", that is, when one case section has completed its execution, statements will continue to be executed downward until a break statement is encountered. This may prove useful in certain circumstances, newer programming languages forbid case statements to "fall-through". In the below example, if is reached, the statements are executed and nothing more inside the braces. However if is reached, both and are executed since there is no break to separate the two case statements.

  switch () {
     case  :
        
     case  :
        
        break;
     default :
        
  }

Iteration statements

C has three forms of iteration statement:

  do 
     
  while ();

  while ()
     

  for ( ;  ; )
     

If all three expressions are present in a for, the statement

  for (e1; e2; e3)
     s;

  e1;
  while (e2) {
     s;
     e3;
  }

A continue statement may appear only within an iteration statement and causes control to pass to the loop-continuation portion of the smallest enclosing such statement. That is, within each of the statements

  while (expression) {
     /* ... */
     cont: ;
  }

  do {
     /* ... */
     cont: ;
  } while (expression);

  for (optional-expr; optexp2; optexp3) {
     /* ... */
     cont: ;
  }   

The break statement is used to get out of a for loop, while loop, do loop, or switch statement. Control passes to the statement following the terminated statement.

A function returns to its caller by the return statement. When return is followed by an expression, the value is returned to the caller of the function. Flowing off the end of the function is equivalent to a return with no expression. In either case, the returned value is undefined.

Data declaration

a[0][0]	a[0][1]	a[0][2]	a[0][3]
a[1][0]	a[1][1]	a[1][2]	a[1][3]
a[2][0]	a[2][1]	a[2][2]	a[2][3]

Another operator, the & (ampersand), called the address-of operator, returns the address of variable, array, or function. Thus, given the following

  int i, *pi; /* int and pointer to int */
  pi = &i;

• strcoll(s1, s2) - compare two strings according to a locale-specific collating sequence
• strcspn(s1, s2) - returns the index of the first character in s1 that matches any character in s2
• strerror(err) - returns a string with an error message corresponding to the code in err
• strpbrk(s1, s2) - returns a pointer to the first character in s1 that matches any character in s2 or a null pointer if not found
• strspn(s1, s2) - returns the index of the first character in s1 that matches no character in s2
• strstr(st, subst) - returns a pointer to the first occurrence of the string subst in st or a null pointer if no such substring exists.
• strtok(s1, s2) - returns a pointer to a token within s1 delimited by the characters in s2.
• strxfrm(s1, s2, n) - transforms s2 into s1 using locale-specific rules

File Input / Output

• stdin standard input
• stdout standard output
• stderr standard error

The following example demonstrates how a filter program is typically structured:

\r
include \r
\r
int main()\r
{\r
   int c;\r
\r
   while (( c = getchar()) != EOF ) {\r
         /* do various things \r
            to the characters */\r
\r
          if (anErrorOccurs) {\r
              fputs("an error eee occurred\
", stderr);\r
              break;\r
          }\r
\r
         /* ... */\r
         putchar(c);\r
         /* ... */\r
\r
    }\r
    return 0;\r
}\r

(Note: there is no guarantee that the individual strings are contiguous.)

The individual values of the parameters may be accessed with argv[1], argv[2], and argv[3].

See C library, C standard library (ANSI C standard library), GNU Compiler Collection.

External links