A small C program can obtain declarations through included headers: #include <stdio.h> #include <stdlib.h> int main(void) { printf("hello\n"); return EXIT_SUCCESS; } The names printf, size_t, and EXIT_SUCCESS are declared elsewhere. A header file is a source file containing declarations and other material intended for inclusion before compilation. section 2.2 section 8.1.1
A header describes an interface; it does not create the declared feature. Repeating consistent declarations is usually harmless, but contradictory declarations in one scope are generally forbidden. Header contents are normally implementation details: consult platform documentation for semantics rather than trying to read barely readable system headers. section 2.2
1 #ifndef BRIAN_H
2 #define BRIAN_H 1
3 #include <time.h>
4
5 /** @file
6 ** @brief Following Brian the Jay
7 **/
8
9 typedef struct brian brian;
10 enum chap { sct, en, };
11 typedef enum chap chap;
12
13 struct brian {
14 struct timespec ts; /**< point in time */
15 unsigned counter; /**< wealth */
16 chap masterof; /**< occupation */
17 };
18
19 /**
20 ** @brief get the data for the next point in time
21 **/
22 brian brian_next(brian);
23
24 ...
25 #endif
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This header groups the declarations and type definitions needed to use a data type, includes a dependency, documents the interface, and prevents multiple inclusion with BRIAN_H. section 10.1
Header files commonly provide:
- Standard-library interfaces such as input/output through stdio.h, numerical and string facilities through stdlib and string.h, and time handling through time.h. section 8.8
- Project-specific interfaces, often grouping functions that treat one data type and including any headers needed to compile those declarations. section 8.1.1 section 10.1
- Include guards, which protect a header against multiple inclusion. section 10.1
Finding interface documentation
When printf is the feature of interest, search platform documentation rather than the header itself. The suggested command sequence is: apropos printf, man printf, or man 3 printf. section 2.2
