Extern keyward on function in C Post: 302970932

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Top Forums Programming Extern keyward on function in C Post 302970932 by yifangt on Wednesday 13th of April 2016 02:43:53 PM

04-13-2016

Registered User

Extern keyward on function in C

I saw a header (.h) file with mixture of "regular" function declarations and other extern function declarations. As I was told all function declarations are implicitly external and the extern on functions declarations is superfluous. Here my focus is on function declaration, not variable yet.

Code:

int fq_read_length (char *data);
char *fa_count (char *start, int length);
extern int get_parainfo (char *full, Queue *head, char type);
extern char *fastq_relocate (char *data, int offset, int length);
extern char *jump (char *target, char type, float sampling_rate);
extern char *get_right_sp (char *start_point ,char type);
extern char *check_fmt (Queue *info, Queue *tail, char *start_point, char type);
int total_full_check (bloom * bl, char *p, int length, float tole_rate, F_set *File_head);
extern int fastq_read_check (char *begin, int length, char mode, bloom * bl, float tole_rate, F_set *File_head);
extern int fasta_read_check (char *begin, int length, char mode, bloom * bl, float tole_rate, F_set *File_head);
void isodate(char* buf);
int total_subscan (bloom *bl, F_set *File_head, char *begin, char *start_point, int read_length, int true_length, float tole_rate, char mode, char type);

Then why is extern on function declaration still used, and what is the benefit? Thanks!

Last edited by yifangt; 04-13-2016 at 04:54 PM..

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LEARN ABOUT MOJAVE

cc_md5_final

CC_MD5(3cc)							       LOCAL							       CC_MD5(3cc)

NAME

     CC_MD2_Init CC_MD2_Update CC_MD2_Final CC_MD2

     CC_MD4_Init CC_MD4_Update CC_MD4_Final CC_MD4

     CC_MD5_Init CC_MD5_Update CC_MD5_Final CC_MD5

     -- MD2, MD4, and MD5 hash functions

SYNOPSIS

     #include <CommonCrypto/CommonDigest.h>

     extern int
     CC_MD2_Init(CC_MD2_CTX *c);

     extern int
     CC_MD2_Update(CC_MD2_CTX *c, const void *data, CC_LONG len);

     extern int
     CC_MD2_Final(unsigned char *md, CC_MD2_CTX *c);

     extern unsigned char *
     CC_MD2(const void *data, CC_LONG len, unsigned char *md);

     extern int
     CC_MD4_Init(CC_MD4_CTX *c);

     extern int
     CC_MD4_Update(CC_MD4_CTX *c, const void *data, CC_LONG len);

     extern int
     CC_MD4_Final(unsigned char *md, CC_MD4_CTX *c);

     extern unsigned char *
     CC_MD4(const void *data, CC_LONG len, unsigned char *md);

     extern int
     CC_MD5_Init(CC_MD5_CTX *c);

     extern int
     CC_MD5_Update(CC_MD5_CTX *c, const void *data, CC_LONG len);

     extern int
     CC_MD5_Final(unsigned char *md, CC_MD5_CTX *c);

     extern unsigned char *
     CC_MD5(const void *data, CC_LONG len, unsigned char *md);

DESCRIPTION

     The following functions are used to produce an hash from data:

     CC_MD2_Init() initializes a CC_MD2_CTX structure.

     CC_MD2_Update() can be called repeatedly with chunks of the message to be hashed (len bytes at data).

     CC_MD2_Final() places the MD2 message digest in md, which must have space for CC_MD2_DIGEST_LENGTH == 16 bytes of output, and erases the
     CC_MD2_CTX.

     CC_MD2() computes the MD2 message digest of the len bytes at data and places it in md (which must have space for CC_MD2_DIGEST_LENGTH == 16
     bytes of output). It returns the md pointer.

     CC_MD4_Init(), CC_MD4_Update(), CC_MD4_Final(), CC_MD4(), CC_MD5_Init(), CC_MD5_Update(), CC_MD5_Final(), and CC_MD5() are analogous using an
     CC_MD4_CTX and CC_MD5_CTX structure.

NOTE

     MD2, MD4, and MD5 are recommended only for compatibility with existing applications. In new applications, SHA-256(or greater) should be pre-
     ferred.

RETURN VALUES

     All routines return 1 except for the one-shot routines ( CC_MD2() , etc.), which return the pointer passed in via the md parameter.

CONFORMING TO

     RFC 1319, RFC 1320, RFC 1321

SEE ALSO

     CC_crypto(3cc), CC_SHA(3cc), CCHmac(3cc), CCCryptor(3cc)

HISTORY

     These functions are available in OS X 10.4 and later.

     These functions provide similar functionality to the routines found in OpenSSL 0.9.6 and may use the same implementation.

BSD
								   April 5, 2007							       BSD

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