抄来的代码

今天(2010-12-28)初创这个帖子，其实也不是真正的撷英，只是随着水平的提高，越来越能接近这个意思吧  ^__^  

anyway，开始第一个小代码段吧

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1、后台进程的标准写法：

// http://blog.chinaunix.net/u/15940/showart_90807.html


#include <stdio.h>
#include <unistd.h> // fork();



int main (void)
{
    pid_t pid;

    pid = fork();

    if (pid>0) {
        printf("daemon on duty!\n");
        return 0;
    }
    else if (pid<0)
    {
        printf("Can't fork!\n");
        return -1;
    }

    for (;;)
    {
        printf("I am the daemon!\n");
        sleep(3);
        /* do something your own here */
    }

}

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2、关于MD5

从这里摘来的一些代码：


MD5的全称是Message-Digest Algorithm 5（信息-摘要算法），相关的标准是RFC1321,...

md5的头文件和代码

/* ******* md5.h ******* */

#ifndef MD5_H
#define MD5_H
 
#ifdef __alpha
typedef unsigned int uint32;
#else
typedef unsigned long uint32;
#endif
 
struct MD5Context
{
    uint32 buf[4]; /* state (ABCD) */
    uint32 bits[2]; /* number of bits, modulo 2^64 (lsb first) */
    unsigned char in[64]; /* input buffer */
};


void MD5Init (struct MD5Context *context);
void MD5Update (struct MD5Context *context, unsigned char const *buf, unsigned len);
void MD5Final (unsigned char digest[16], struct MD5Context *context);
void MD5Transform (uint32 buf[4], uint32 const in[16]);
 
/*
 * This is needed to make RSAREF happy on some MS-DOS compilers.
 */
typedef struct MD5Context MD5_CTX;
 
#endif /* !MD5_H */

/* ******* md5.c ******* */

/*
 * This code implements the MD5 message-digest algorithm.
 * The algorithm is due to Ron Rivest. This code was
 * written by Colin Plumb in 1993, no copyright is claimed.
 * This code is in the public domain; do with it what you wish.
 *
 * Equivalent code is available from RSA Data Security, Inc.
 * This code has been tested against that, and is equivalent,
 * except that you don't need to include two pages of legalese
 * with every copy.
 *
 * To compute the message digest of a chunk of bytes, declare an
 * MD5Context structure, pass it to MD5Init, call MD5Update as
 * needed on buffers full of bytes, and then call MD5Final, which
 * will fill a supplied 16-byte array with the digest.
 */

#include <string.h> /* for memcpy() */
#include "md5.h"
 
#ifndef HIGHFIRST
#define byteReverse(buf, len) /* Nothing */
#else
void byteReverse(unsigned char *buf, unsigned longs);
 
#ifndef ASM_MD5
/*
 * Note: this code is harmless on little-endian machines.
 */
void byteReverse(unsigned char *buf, unsigned longs)
{
    uint32 t;
    do {
    t = (uint32) ((unsigned) buf[3] << 8 | buf[2]) << 16 |
        ((unsigned) buf[1] << 8 | buf[0]);
    *(uint32 *) buf = t;
    buf += 4;
    } while (--longs);
}
#endif
#endif
 
/*
 * Start MD5 accumulation. Set bit count to 0 and buffer to mysterious
 * initialization constants.
 */
void MD5Init(struct MD5Context *ctx)
{
    ctx->buf[0] = 0x67452301;
    ctx->buf[1] = 0xefcdab89;
    ctx->buf[2] = 0x98badcfe;
    ctx->buf[3] = 0x10325476;
 
    ctx->bits[0] = 0;
    ctx->bits[1] = 0;
}
 
/*
 * Update context to reflect the concatenation of another buffer full
 * of bytes.
 */
void MD5Update(struct MD5Context *ctx, unsigned char const *buf, unsigned len)
{
    uint32 t;
 
    /* Update bitcount */
 
    t = ctx->bits[0];
    if ((ctx->bits[0] = t + ((uint32) len << 3)) < t)
        ctx->bits[1]++; /* Carry from low to high */
    ctx->bits[1] += len >> 29;
 
    t = (t >> 3) & 0x3f; /* Bytes already in shsInfo->data */
 
    /* Handle any leading odd-sized chunks */
 
    if (t)
    {
        unsigned char *p = (unsigned char *) ctx->in + t;
     
        t = 64 - t;
        if (len < t)
        {
            memcpy(p, buf, len);
            return;
        }
        memcpy(p, buf, t);
        byteReverse(ctx->in, 16);
        MD5Transform(ctx->buf, (uint32 *) ctx->in);
        buf += t;
        len -= t;
    }
    /* Process data in 64-byte chunks */
 
    while (len >= 64)
    {
        memcpy(ctx->in, buf, 64);
        byteReverse(ctx->in, 16);
        MD5Transform(ctx->buf, (uint32 *) ctx->in);
        buf += 64;
        len -= 64;
    }
 
    /* Handle any remaining bytes of data. */
 
    memcpy(ctx->in, buf, len);
}
 
/*
 * Final wrapup - pad to 64-byte boundary with the bit pattern
 * 1 0* (64-bit count of bits processed, MSB-first)
 */
void MD5Final(unsigned char digest[16], struct MD5Context *ctx)
{
    unsigned count;
    unsigned char *p;
 
    /* Compute number of bytes mod 64 */
    count = (ctx->bits[0] >> 3) & 0x3F;
 
    /* Set the first char of padding to 0x80. This is safe since there is
       always at least one byte free */
    p = ctx->in + count;
    *p++ = 0x80;
 
    /* Bytes of padding needed to make 64 bytes */
    count = 64 - 1 - count;
 
    /* Pad out to 56 mod 64 */
    if (count < 8)
    {
        /* Two lots of padding: Pad the first block to 64 bytes */
        memset(p, 0, count);
        byteReverse(ctx->in, 16);
        MD5Transform(ctx->buf, (uint32 *) ctx->in);
     
        /* Now fill the next block with 56 bytes */
        memset(ctx->in, 0, 56);
    }
    else
    {
        /* Pad block to 56 bytes */
        memset(p, 0, count - 8);
    }
    byteReverse(ctx->in, 14);
 
    /* Append length in bits and transform */
    ((uint32 *) ctx->in)[14] = ctx->bits[0];
    ((uint32 *) ctx->in)[15] = ctx->bits[1];
 
    MD5Transform(ctx->buf, (uint32 *) ctx->in);
    byteReverse((unsigned char *) ctx->buf, 4);
    memcpy(digest, ctx->buf, 16);
    memset(ctx, 0, sizeof(ctx)); /* In case it's sensitive */
}
 
#ifndef ASM_MD5
 
 
/* The four core functions - F1 is optimized somewhat */
 
 
/* #define F1(x, y, z) (x & y | ~x & z) */
#define F1(x, y, z) (z ^ (x & (y ^ z)))
#define F2(x, y, z) F1(z, x, y)
#define F3(x, y, z) (x ^ y ^ z)
#define F4(x, y, z) (y ^ (x | ~z))
 
 
 
/* This is the central step in the MD5 algorithm. */
#define MD5STEP(f, w, x, y, z, data, s) \
    ( w += f(x, y, z) + data, w = w<<s | w>>(32-s), w += x )
 
 
 
/*
 * The core of the MD5 algorithm, this alters an existing MD5 hash to
 * reflect the addition of 16 longwords of new data. MD5Update blocks
 * the data and converts bytes into longwords for this routine.
 */
void MD5Transform(uint32 buf[4], uint32 const in[16])
{
    register uint32 a, b, c, d;
 
    a = buf[0];
    b = buf[1];
    c = buf[2];
    d = buf[3];
 
    MD5STEP(F1, a, b, c, d, in[0] + 0xd76aa478, 7);
    MD5STEP(F1, d, a, b, c, in[1] + 0xe8c7b756, 12);
    MD5STEP(F1, c, d, a, b, in[2] + 0x242070db, 17);
    MD5STEP(F1, b, c, d, a, in[3] + 0xc1bdceee, 22);
    MD5STEP(F1, a, b, c, d, in[4] + 0xf57c0faf, 7);
    MD5STEP(F1, d, a, b, c, in[5] + 0x4787c62a, 12);
    MD5STEP(F1, c, d, a, b, in[6] + 0xa8304613, 17);
    MD5STEP(F1, b, c, d, a, in[7] + 0xfd469501, 22);
    MD5STEP(F1, a, b, c, d, in[8] + 0x698098d8, 7);
    MD5STEP(F1, d, a, b, c, in[9] + 0x8b44f7af, 12);
    MD5STEP(F1, c, d, a, b, in[10] + 0xffff5bb1, 17);
    MD5STEP(F1, b, c, d, a, in[11] + 0x895cd7be, 22);
    MD5STEP(F1, a, b, c, d, in[12] + 0x6b901122, 7);
    MD5STEP(F1, d, a, b, c, in[13] + 0xfd987193, 12);
    MD5STEP(F1, c, d, a, b, in[14] + 0xa679438e, 17);
    MD5STEP(F1, b, c, d, a, in[15] + 0x49b40821, 22);
 
    MD5STEP(F2, a, b, c, d, in[1] + 0xf61e2562, 5);
    MD5STEP(F2, d, a, b, c, in[6] + 0xc040b340, 9);
    MD5STEP(F2, c, d, a, b, in[11] + 0x265e5a51, 14);
    MD5STEP(F2, b, c, d, a, in[0] + 0xe9b6c7aa, 20);
    MD5STEP(F2, a, b, c, d, in[5] + 0xd62f105d, 5);
    MD5STEP(F2, d, a, b, c, in[10] + 0x02441453, 9);
    MD5STEP(F2, c, d, a, b, in[15] + 0xd8a1e681, 14);
    MD5STEP(F2, b, c, d, a, in[4] + 0xe7d3fbc8, 20);
    MD5STEP(F2, a, b, c, d, in[9] + 0x21e1cde6, 5);
    MD5STEP(F2, d, a, b, c, in[14] + 0xc33707d6, 9);
    MD5STEP(F2, c, d, a, b, in[3] + 0xf4d50d87, 14);
    MD5STEP(F2, b, c, d, a, in[8] + 0x455a14ed, 20);
    MD5STEP(F2, a, b, c, d, in[13] + 0xa9e3e905, 5);
    MD5STEP(F2, d, a, b, c, in[2] + 0xfcefa3f8, 9);
    MD5STEP(F2, c, d, a, b, in[7] + 0x676f02d9, 14);
    MD5STEP(F2, b, c, d, a, in[12] + 0x8d2a4c8a, 20);
 
    MD5STEP(F3, a, b, c, d, in[5] + 0xfffa3942, 4);
    MD5STEP(F3, d, a, b, c, in[8] + 0x8771f681, 11);
    MD5STEP(F3, c, d, a, b, in[11] + 0x6d9d6122, 16);
    MD5STEP(F3, b, c, d, a, in[14] + 0xfde5380c, 23);
    MD5STEP(F3, a, b, c, d, in[1] + 0xa4beea44, 4);
    MD5STEP(F3, d, a, b, c, in[4] + 0x4bdecfa9, 11);
    MD5STEP(F3, c, d, a, b, in[7] + 0xf6bb4b60, 16);
    MD5STEP(F3, b, c, d, a, in[10] + 0xbebfbc70, 23);
    MD5STEP(F3, a, b, c, d, in[13] + 0x289b7ec6, 4);
    MD5STEP(F3, d, a, b, c, in[0] + 0xeaa127fa, 11);
    MD5STEP(F3, c, d, a, b, in[3] + 0xd4ef3085, 16);
    MD5STEP(F3, b, c, d, a, in[6] + 0x04881d05, 23);
    MD5STEP(F3, a, b, c, d, in[9] + 0xd9d4d039, 4);
    MD5STEP(F3, d, a, b, c, in[12] + 0xe6db99e5, 11);
    MD5STEP(F3, c, d, a, b, in[15] + 0x1fa27cf8, 16);
    MD5STEP(F3, b, c, d, a, in[2] + 0xc4ac5665, 23);
 
    MD5STEP(F4, a, b, c, d, in[0] + 0xf4292244, 6);
    MD5STEP(F4, d, a, b, c, in[7] + 0x432aff97, 10);
    MD5STEP(F4, c, d, a, b, in[14] + 0xab9423a7, 15);
    MD5STEP(F4, b, c, d, a, in[5] + 0xfc93a039, 21);
    MD5STEP(F4, a, b, c, d, in[12] + 0x655b59c3, 6);
    MD5STEP(F4, d, a, b, c, in[3] + 0x8f0ccc92, 10);
    MD5STEP(F4, c, d, a, b, in[10] + 0xffeff47d, 15);
    MD5STEP(F4, b, c, d, a, in[1] + 0x85845dd1, 21);
    MD5STEP(F4, a, b, c, d, in[8] + 0x6fa87e4f, 6);
    MD5STEP(F4, d, a, b, c, in[15] + 0xfe2ce6e0, 10);
    MD5STEP(F4, c, d, a, b, in[6] + 0xa3014314, 15);
    MD5STEP(F4, b, c, d, a, in[13] + 0x4e0811a1, 21);
    MD5STEP(F4, a, b, c, d, in[4] + 0xf7537e82, 6);
    MD5STEP(F4, d, a, b, c, in[11] + 0xbd3af235, 10);
    MD5STEP(F4, c, d, a, b, in[2] + 0x2ad7d2bb, 15);
    MD5STEP(F4, b, c, d, a, in[9] + 0xeb86d391, 21);
 
    buf[0] += a;
    buf[1] += b;
    buf[2] += c;
    buf[3] += d;
}
 
#endif