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缘由：

最近在Linux环境下用C语言实现一些功能，在完成工作任务的同时学习、收藏了一些经典的代码片段/链接，想到以后可能会用到，所以在功能实现之后又重新梳理了一下相关内容并整理至此，方便以后碰到类似需求的时候能快速上手。

正文：

1.用getline函数循环读取文件内容

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>

int main(void) {
    FILE *fp;
    char *line = NULL;
    size_t len = 0;
    ssize_t read;

   fp = fopen("/etc/motd", "r");
    if (fp == NULL)
        exit(EXIT_FAILURE);

   while ((read = getline(&line, &len, fp)) != -1) {
        printf("Retrieved line of length %zu :'%s'\n", read, line); /* 在line中包含了一个换行符'\n'*/
    }

   free(line);
   exit(EXIT_SUCCESS);
}

另外还有fgets函数，一般用于固定格式(每行长度)文件的读取，你可以根据具体情况决定是否选用：

#include <stdio.h>
#include <stdlib.h>

int main (void) {
    char buf[256];
    while (fgets (buf, sizeof(buf), stdin)) {
        printf("line: %s", buf);
    }
    if (ferror(stdin)) {
        fprintf(stderr,"Oops, error reading stdin\n");
        abort();
    }
    return 0;
}

• http://stackoverflow.com/questions/13555097/linux-c-read-file-line-by-line
• http://stackoverflow.com/questions/3501338/c-read-file-line-by-line
• http://linux.die.net/man/3/getline
• http://rosettacode.org/wiki/Read_a_file_line_by_line#C

2.如何判断字符串指针为空/指向的内容为空

首先检查该字符串指针是否为NULL，然后看该字符串指针指向内容的第一个字符是否为「\0」来进行判断：

char *c = "";
if ((c != NULL) && (c[0] == '\0')) {
    printf("c is empty\n");
}

• http://stackoverflow.com/questions/7970617/how-can-i-check-if-char-variable-points-to-empty-string

3.用sscanf函数进行字符串截取

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main() {
    char src_ip[16];
    char * ssh_client = NULL;
    if ((getenv ("SSH_CLIENT"))) {
        ssh_client = getenv ("SSH_CLIENT");
        printf("SSH_CLIENT = %s\n", ssh_client);
    }

    if ( ssh_client != NULL ) {
        printf("SSH_CLIENT = %s\n", ssh_client);
        sscanf(ssh_client, "%15[^ ]", src_ip);  // 取 ssh_client 变量指向的字符串中从开头到第一个空白字符之间的最多15个字符存入 src_ip 变量中
        printf("SRC_IP\t= %s\n", src_ip);
    } else {
        printf("SSH_CLIENT = null\n");
    }
    return 0;
}

• http://stackoverflow.com/questions/3478636/extract-a-substring-in-c
• https://linux.die.net/man/3/sscanf
• http://stackoverflow.com/questions/4214314/get-a-substring-of-a-char
• http://stackoverflow.com/questions/2114377/strings-in-c-how-to-get-substring
• http://randu.org/tutorials/c/strings.php

4.用strncpy函数进行字符串复制

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(){
    char * ssh_client = (char *)NULL;
    char src_ip[16] = "";
    if ( ssh_client != NULL ) {
        sscanf(ssh_client, "%15[^ ]", src_ip);
    } else {
        // src_ip = "null";
        strncpy(src_ip, "null", 15);//这里需要用strncpy函数进行字符串数组赋值，上面那行的操作是错误的！
    }
    printf("src_ip = %s\n", src_ip);
    return 0;
}

5.将 字符串指针 转换成 字符串数组

• http://stackoverflow.com/questions/1508838/how-to-copy-char-str-to-char-c-in-c
• http://stackoverflow.com/questions/13553752/convert-char-pointer-to-unsigned-char-array

6.将 字符串数组 转换成 字符串指针

• http://stackoverflow.com/questions/9627962/is-it-possible-to-convert-char-to-char-in-c

7.字符串数组的初始化

• http://stackoverflow.com/questions/18688971/c-char-array-initialization
• http://stackoverflow.com/questions/963911/how-to-initialize-char-array-from-a-string

……不定期更新……

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