Editing C syntax (section)

===Global structure===
After preprocessing, at the highest level a [[C (programming language)|C]] [[computer program|program]] consists of a sequence of declarations at file scope. These may be partitioned into several separate source files, which may be compiled separately; the resulting object modules are then [[Linker (computing)|linked]] along with implementation-provided run-time support modules to produce an executable image.

The declarations introduce [[Function (programming)|functions]], [[Variable (programming)|variables]] and [[Data type|types]]. C functions are akin to the subroutines of [[Fortran]] or the procedures of [[Pascal (programming language)|Pascal]].

A ''definition'' is a special type of declaration. A variable definition sets aside storage and possibly initializes it, a function definition provides its body.

An implementation of C providing all of the standard library functions is called a ''hosted implementation''. Programs written for hosted implementations are required to define a special function called [[Entry point|{{code|main}}]], which is the first function called when a program begins executing.

Hosted implementations start program execution by invoking the {{code|main}} function, which must be defined following one of these prototypes (using different parameter names or spelling the types differently is allowed):

<syntaxhighlight lang=C>
int main() {...}
int main(void) {...}
int main(int argc, char *argv[]) {...}
int main(int argc, char **argv) {...} // char *argv[] and char **argv have the same type as function parameters
</syntaxhighlight>

The first two definitions are equivalent (and both are compatible with C++). It is probably up to individual preference which one is used (the current C standard contains two examples of {{code|main()}} and two of {{code|main(void)}}, but the draft C++ standard uses {{code|main()}}). The return value of {{code|main}} (which should be {{code|int}}) serves as ''termination status'' returned to the host environment.

The C standard defines return values {{code|0}} and {{code|EXIT_SUCCESS}} as indicating success and {{code|EXIT_FAILURE}} as indicating failure. ({{code|EXIT_SUCCESS}} and {{code|EXIT_FAILURE}} are defined in [[stdlib.h|{{code|<stdlib.h>}}]]). Other return values have implementation-defined meanings; for example, under [[Linux]] a program killed by a [[Signal (computing)|signal]] yields a return code of the numerical value of the signal plus 128.

A minimal correct C program consists of an empty {{code|main}} routine, taking no arguments and doing nothing:
<syntaxhighlight lang=C>
int main(void){}
</syntaxhighlight>

Because no <code>return</code> statement is present, <code>main</code> returns 0 on exit.<ref name="bk21st" /> (This is a special-case feature introduced in [[C99]] that applies only to <code>main</code>.)

The {{code|main}} function will usually call other functions to help it perform its job.

Some implementations are not hosted, usually because they are not intended to be used with an [[operating system]]. Such implementations are called ''free-standing'' in the C standard. A free-standing implementation is free to specify how it handles program startup; in particular it need not require a program to define a {{code|main}} function.

Functions may be written by the programmer or provided by existing libraries. Interfaces for the latter are usually declared by including header files—with the {{code|#include}} [[preprocessing directive]]—and the library objects are linked into the final executable image. Certain library functions, such as [[printf|{{code|printf}}]], are defined by the C standard; these are referred to as the [[ISO C standard library|standard library]] functions.

A function may return a value to caller (usually another C function, or the hosting environment for the function {{code|main}}). The {{code|printf}} function mentioned above returns how many characters were printed, but this value is often ignored.