如何在运行时避免中止陷阱6错误使用strncat（）？

Question

Abort trap 6问题源于调用extra_info（）方法，它多次使用strncat（）。删除此功能不会在运行时产生错误。

据我所知：

中止陷阱：6是由于使用指向不存在的内存位置Abort trap: 6 in C Program的无效索引引起的。当需要释放可变内存时也可能发生这种情况。要避免这种情况，您可以在每次重复使用时使用多个变量或释放单个变量。但我感觉解决方案要简单得多。

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

char line[1001]; // The line supports up to a 1000 characters
char lines[11][1001]; // An array of lines (up to 10 lines where each line is a 1000 characters max)
char info[100]; // Holds extra info provided by user

char * extra_info(
        char string_1[],
        char string_2[],
        char string_3[],
        char string_4[],
        char string_5[]
    );

int main(){

    int 
    i, // Line number
    j; // Length of the line
    char result[100], text[100];
    FILE *file;

    strcpy(text, "String No."); // The default text

    file = fopen("test.txt", "w+"); // Open the file for reading and writing

    for(i = 0; i < 10; i++){ // Loop to create a line.

        if(i != 9){ // If the line is NOT at the 10th string

            sprintf(result, "%s%d, ", text, i); // Format the text and store it in result

        }
        else{

            sprintf(result, "%s%d ", text, i); // Format the text and store it in result            

        }

        extra_info(
            "st",
            "nd",
            "rd",
            "th",
            "th"
        );

        strncat(line, info, 100); // Append the extra info at the end of each line        

        printf("%s", result); // Display the result variable to the screen

        strncat(line, result, 15); // Concatenate all strings in one line

    }

    strncat(line, "\n\n", 2); // Add a new-line character at the end of each line

    for(j = 0; j < 10; j++){ // Now loop to change the line

        strcpy(lines[i], line); // Copy the line of text into each line of the array

        fputs(lines[i], file); // Put each line into the file        

    }

    fclose(file);  

}

char * extra_info( // Append user defined and predefined info at the end of a line
        char string_1[],
        char string_2[],
        char string_3[],
        char string_4[],
        char string_5[]
    ){
        char text[100]; // A variable to hold the text

        /* Append a default text into each strings 
        and concatenate them into one line */

        sprintf(text, " 1%s", string_1);
        strncat(line, text, 100);

        sprintf(text, ", 2%s", string_2);
        strncat(line, text, 100);

        sprintf(text, ", 3%s", string_3);
        strncat(line, text, 100);

        sprintf(text, ", 4%s", string_4);
        strncat(line, text, 100);

        sprintf(text, ", 5%s.", string_5);
        strncat(line, text, 100);

        strcpy(info, line); // Copies the line into the info global variable

        return line;

}

这段代码很好地使用GCC编译，但我偶然发现代码工作正常的情况，但是由于这个错误可能会破坏某些功能。这与以这种方式多次调用strncat（）有关，这使我认为会出现内存分配问题，但在尝试其他示例后，解决方案看起来要简单得多。任何有关这方面的帮助将不胜感激。提前致谢。

Answer 1

我在2018年3月编写了随附的代码，以满足自己在strncat()发生的问题，以便在我提交答案之前删除另一个问题。这只是重新定位代码。

strncat()功能（正如我在comment中所说的）邪恶和卑鄙。它也与strncpy()界面不一致 - 并且与你在其他任何地方遇到的任何东西都不同。读完之后，你会决定（幸运的话）你永远不应该使用strncat()。

TL;DR — Never use strncat()

C标准定义strncat()（和POSIX同意 - strncat()）

C11 §7.24.3.2 The strncat function

概要

#include <string.h>
char *strncat(char * restrict s1, const char * restrict s2, size_t n);

描述

strncat函数从n指向的数组到s2指向的字符串的末尾附加的s1字符（空字符和后面跟随的字符不会附加）不超过s2。 s1的初始字符会覆盖strncat末尾的空字符。终止空字符始终附加到结果.309）如果在重叠的对象之间进行复制，则行为未定义。

返回

s1函数返回s1的值。

309）因此，可以在strlen(s1)+n+1指向的数组中结束的最大字符数是strncat()。

脚注标识了与

char *source = …;

char target[100] = "";

strncat(target, source, sizeof(target));

最大的陷阱 - 您无法安全使用：

strncat()

这与在C代码中采用数组大小​​参数1的大多数其他函数所发生的情况相反。

要安全地使用target，你应该知道：

• sizeof(target)
• strlen(target) - 或者，对于动态分配的空间，分配的长度
• source - 你必须知道目标字符串中已有的长度
• strlen(source)
• strncat(target, source, sizeof(target) - strlen(target) - 1); - 如果您担心源字符串是否被截断;如果你不在乎，就不需要了

有了这些信息，您可以使用：

strlen(target)

但是，这样做会有点愚蠢;如果你知道strncat()，你可以避免让strncat(target + strlen(target), source, sizeof(target) - strlen(target) - 1); 再次找到它：

strncat()

请注意，与strncpy()不同，

size_t t_size = sizeof(target);
size_t t_length = strlen(target);
strncpy(target + t_length, source, t_size - t_length - 1);
target[t_size - 1] = '\0';

保证无效终止。这意味着您可以使用：

strncat()

如果源字符串太长而无法附加到目标，则可以保证得到相同的结果。

演示代码

说明strncat()方面的多个程序。请注意，在macOS上，<string.h>中有一个__builtin___strncat_chk的宏定义，它调用了一个不同的函数 - strncat() - 它验证了-Wmissing-prototypes -Wstrict-prototypes的用法。对于命令行的紧凑性，我删除了我通常使用的两个警告编译器选项 - strncat19.c - 但这不会影响任何编译。

strncat()

这证明了

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

int main(void)
{
    char spare1[16] = "abc";
    char buffer[16] = "";
    char spare2[16] = "xyz";
    strncat(buffer, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", sizeof(buffer) - 1);
    printf("%zu: [%s]\n", strlen(buffer), buffer);
    printf("spare1 [%s]\n", spare1);
    printf("spare2 [%s]\n", spare2);
    return 0;
}

的一个安全用途：

clang

它编译得很干净（Apple的Apple LLVM version 10.0.0 (clang-1000.11.45.5)来自XCode 10.1（

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror strncat19.c -o strncat19
$ ./strncat19
15: [ABCDEFGHIJKLMNO]
spare1 [abc]
spare2 [xyz]
$

）和GCC 8.2.0，即使设置了严格的警告：

strncat29.c

strncat19.c

这与sizeof(buffer)类似，但（a）允许您指定要在命令行上复制的字符串，并且（b）错误地使用sizeof(buffer) - 1而不是

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

int main(int argc, char **argv)
{
    const char *data = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    if (argc == 2)
        data = argv[1];
    char spare1[16] = "abc";
    char buffer[16] = "";
    char spare2[16] = "xyz";
    strncat(buffer, data, sizeof(buffer));
    printf("%zu: [%s]\n", strlen(buffer), buffer);
    printf("spare1 [%s]\n", spare1);
    printf("spare2 [%s]\n", spare2);
    return 0;
}

作为长度。

$ clang -O3 -g -std=c11 -Wall -Wextra -Werror strncat29.c -o strncat29  
strncat29.c:12:27: error: the value of the size argument in 'strncat' is too large, might lead to a buffer
      overflow [-Werror,-Wstrncat-size]
    strncat(buffer, data, sizeof(buffer));
                          ^~~~~~~~~~~~~~
strncat29.c:12:27: note: change the argument to be the free space in the destination buffer minus the terminating null byte
    strncat(buffer, data, sizeof(buffer));
                          ^~~~~~~~~~~~~~
                          sizeof(buffer) - strlen(buffer) - 1
1 error generated.
$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror strncat29.c -o strncat29  
In file included from /usr/include/string.h:190,
                 from strncat29.c:2:
strncat29.c: In function ‘main’:
strncat29.c:12:5: error: ‘__builtin___strncat_chk’ specified bound 16 equals destination size [-Werror=stringop-overflow=]
     strncat(buffer, data, sizeof(buffer));
     ^~~~~~~
cc1: all warnings being treated as errors
$

此代码无法使用严格的警告选项进行编译：

-Werror

即使没有请求警告，GCC也会发出警告，但由于缺少

$ gcc -o strncat29 strncat29.c
In file included from /usr/include/string.h:190,
                 from strncat29.c:2:
strncat29.c: In function ‘main’:
strncat29.c:12:5: warning: ‘__builtin___strncat_chk’ specified bound 16 equals destination size [-Wstringop-overflow=]
     strncat(buffer, data, sizeof(buffer));
     ^~~~~~~
$ ./strncat29
Abort trap: 6
$ ./strncat29 ZYXWVUTSRQPONMK
15: [ZYXWVUTSRQPONMK]
spare1 [abc]
spare2 [xyz]
$ ./strncat29 ZYXWVUTSRQPONMKL
Abort trap: 6
$

选项，它会产生一个可执行文件：

__builtin__strncat_chk

这就是strncat97.c的功能。

strncat()

此代码还采用可选的字符串参数;它还要注意命令行上是否有另一个参数，如果是，它会直接调用

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

/*
** Demonstrating that strncat() should not be given sizeof(buffer) as
** the size, even if the string is empty to start with.  The use of
** (strncat) inhibits the macro expansion on macOS; the code behaves
** differently when the __strncat_chk function (on High Sierra or
** earlier - it's __builtin__strncat_chk on Mojave) is called instead.
** You get an abort 6 (but no other useful message) when the buffer
** length is too long.
*/

int main(int argc, char **argv)
{
    const char *data = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    if (argc >= 2)
        data = argv[1];
    char spare1[16] = "abc";
    char buffer[16] = "";
    char spare2[16] = "xyz";
    size_t len = (argc == 2) ? sizeof(buffer) : sizeof(buffer) - 1;
    if (argc < 3)
        strncat(buffer, data, len);
    else
        (strncat)(buffer, data, len);
    printf("buffer %2zu: [%s]\n", strlen(buffer), buffer);
    printf("spare1 %2zu: [%s]\n", strlen(spare1), spare1);
    printf("spare2 %2zu: [%s]\n", strlen(spare2), spare2);
    return 0;
}

函数，而不是让宏首先检查它：

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror strncat97.c -o strncat97  
strncat97.c: In function ‘main’:
strncat97.c:26:9: error: ‘strncat’ output truncated copying 15 bytes from a string of length 26 [-Werror=stringop-truncation]
         (strncat)(buffer, data, len);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
$ clang -O3 -g -std=c11 -Wall -Wextra -Werror strncat97.c -o strncat97  
$

现在编译器会产生不同的结果：

$ ./strncat97
0x7ffee7506420: buffer 15: [ABCDEFGHIJKLMNO]
0x7ffee7506430: spare1  3: [abc]
0x7ffee7506410: spare2  3: [xyz]
$ ./strncat97 ABCDEFGHIJKLMNOP
Abort trap: 6
$ ./strncat97 ABCDEFGHIJKLMNO
0x7ffeea141410: buffer 15: [ABCDEFGHIJKLMNO]
0x7ffeea141420: spare1  3: [abc]
0x7ffeea141400: spare2  3: [xyz]
$

这证明了使用多个编译器的优势 - 不同的编译器有时会检测到不同的问题。这段代码很乱，试图使用不同数量的选项来做多件事。它足以表明：

strncat37.c

getopt()

这是上述节目的全能，全舞蹈版本，通过SOQ进行选项处理。它还使用我的错误报告例程;它们的代码可以在GitHub上的stderr.c（Stack Overflow Questions）存储库中找到stderr.h子目录中的文件src/libsoq和

#include "stderr.h"
#include <stdio.h>
#include <string.h>
#include <unistd.h>

/*
** Demonstrating that strncat() should not be given sizeof(buffer) as
** the size, even if the string is empty to start with.  The use of
** (strncat) inhibits the macro expansion on macOS; the code behaves
** differently when the __strncat_chk function (on High Sierra or
** earlier - it's __builtin__strncat_chk on Mojave) is called instead.
** You get an abort 6 (but no other useful message) when the buffer
** length is too long.
*/

static const char optstr[] = "fhlmsV";
static const char usestr[] = "[-fhlmsV] [string]";
static const char hlpstr[] =
    "  -f  Function is called directly\n"
    "  -h  Print this help message and exit\n"
    "  -l  Long buffer length -- sizeof(buffer)\n"
    "  -m  Macro cover for the function is used (default)\n"
    "  -s  Short buffer length -- sizeof(buffer)-1 (default)\n"
    "  -V  Print version information and exit\n"
    ;

int main(int argc, char **argv)
{
    err_setarg0(argv[0]);

    int f_flag = 0;
    int l_flag = 0;
    int opt;

    while ((opt = getopt(argc, argv, optstr)) != -1)
    {
        switch (opt)
        {
        case 'f':
            f_flag = 1;
            break;
        case 'h':
            err_help(usestr, hlpstr);
            /*NOTREACHED*/
        case 'l':
            l_flag = 1;
            break;
        case 'm':
            f_flag = 0;
            break;
        case 's':
            l_flag = 0;
            break;
        case 'V':
            err_version(err_getarg0(), &"@(#)$Revision$ ($Date$)"[4]);
            /*NOTREACHED*/
        default:
            err_usage(usestr);
            /*NOTREACHED*/
        }
    }

    if (optind < argc - 1)
        err_usage(usestr);

    const char *data = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    if (optind != argc)
        data = argv[optind];
    char spare1[16] = "abc";
    char buffer[16] = "";
    char spare2[16] = "xyz";
    size_t len = l_flag ? sizeof(buffer) : sizeof(buffer) - 1;

    printf("Specified length: %zu\n", len);
    printf("Copied string: [%s]\n", data);
    printf("Copied %s\n", f_flag ? "using strncat() function directly"
                                 : "using strncat() macro");

    if (f_flag)
        (strncat)(buffer, data, len);
    else
        strncat(buffer, data, len);

    printf("%p: buffer %2zu: [%s]\n", (void *)buffer, strlen(buffer), buffer);
    printf("%p: spare1 %2zu: [%s]\n", (void *)spare1, strlen(spare1), spare1);
    printf("%p: spare2 %2zu: [%s]\n", (void *)spare2, strlen(spare2), spare2);
    return 0;
}

。

-Werror

和以前一样，Clang和GCC对代码的可接受性有不同的看法（而

$ clang -O3 -g -I./inc -std=c11 -Wall -Wextra -Werror strncat37.c -o strncat37 -L./lib  -lsoq 
$ gcc -O3 -g -I./inc -std=c11 -Wall -Wextra -Werror strncat37.c -o strncat37 -L./lib  -lsoq 
strncat37.c: In function ‘main’:
strncat37.c:80:9: error: ‘strncat’ output may be truncated copying between 15 and 16 bytes from a string of length 26 [-Werror=stringop-truncation]
         (strncat)(buffer, data, len);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
$

意味着来自GCC的警告被视为错误）：

$ ./strncat37 -h
Usage: strncat37 [-fhlmsV] [string]
  -f  Function is called directly
  -h  Print this help message and exit
  -l  Long buffer length -- sizeof(buffer)
  -m  Macro cover for the function is used (default)
  -s  Short buffer length -- sizeof(buffer)-1 (default)
  -V  Print version information and exit

$ ./strncat37
Specified length: 15
Copied string: [ABCDEFGHIJKLMNOPQRSTUVWXYZ]
Copied using strncat() macro
0x7ffedff4e400: buffer 15: [ABCDEFGHIJKLMNO]
0x7ffedff4e410: spare1  3: [abc]
0x7ffedff4e3f0: spare2  3: [xyz]
$ ./strncat37 -m -s
Specified length: 15
Copied string: [ABCDEFGHIJKLMNOPQRSTUVWXYZ]
Copied using strncat() macro
0x7ffeeaf043f0: buffer 15: [ABCDEFGHIJKLMNO]
0x7ffeeaf04400: spare1  3: [abc]
0x7ffeeaf043e0: spare2  3: [xyz]
$ ./strncat37 -m -l
Specified length: 16
Copied string: [ABCDEFGHIJKLMNOPQRSTUVWXYZ]
Copied using strncat() macro
Abort trap: 6
$ ./strncat37 -f -s
Specified length: 15
Copied string: [ABCDEFGHIJKLMNOPQRSTUVWXYZ]
Copied using strncat() function directly
0x7ffeef0913f0: buffer 15: [ABCDEFGHIJKLMNO]
0x7ffeef091400: spare1  3: [abc]
0x7ffeef0913e0: spare2  3: [xyz]
$ ./strncat37 -f -l
Specified length: 16
Copied string: [ABCDEFGHIJKLMNOPQRSTUVWXYZ]
Copied using strncat() function directly
0x7ffeed8d33f0: buffer 16: [ABCDEFGHIJKLMNOP]
0x7ffeed8d3400: spare1  0: []
0x7ffeed8d33e0: spare2  3: [xyz]
$

运行时：

-m -l

默认行为也是正确的行为;程序不会崩溃，也不会产生意想不到的副作用。当使用宏运行并且指定的长度太长（-f -l）时，程序崩溃。当使用函数运行并且长度太长（spare1）时，程序会在buffer结束后添加null来覆盖数组scanf()的第一个字节，并显示16个字节的数据而不是15个字节。

---

1当你使用%31s或类似的时候，sizeof(string) - 1有一个例外;指定的数字是可以存储在字符串中的非空字符数;它会在读取其他31个字符后添加一个空字节。因此，可以安全使用的最大尺寸是strncatXX.c。

您可以在SOQ子目录中的GitHub上的src/so-5405-4423（Stack Overflow Questions）存储库中找到int main(){的代码。

---

Analysis of Code from Question

从问题中获取代码并将int main(void){更改为-Werror，因为我的默认编译选项会产生错误（如果我不使用main()则会发出警告）非原型return 0;，并在main()末尾添加

$ gcc -O3 -g -std=c11 -Wall -Wextra -Werror -Wmissing-prototypes -Wstrict-prototypes so-5405-4423-v1.c -o so-5405-4423-v1 
In file included from /opt/gcc/v8.2.0/lib/gcc/x86_64-apple-darwin17.7.0/8.2.0/include-fixed/stdio.h:425,
                 from so-5405-4423-v1.c:1:
so-5405-4423-v1.c: In function ‘main’:
so-5405-4423-v1.c:32:29: error: ‘%d’ directive writing between 1 and 2 bytes into a region of size between 1 and 100 [-Werror=format-overflow=]
             sprintf(result, "%s%d, ", text, i); // Format the text and store it in result
                             ^~~~~~~~
so-5405-4423-v1.c:32:29: note: directive argument in the range [0, 10]
so-5405-4423-v1.c:32:13: note: ‘__builtin___sprintf_chk’ output between 4 and 104 bytes into a destination of size 100
             sprintf(result, "%s%d, ", text, i); // Format the text and store it in result
             ^~~~~~~
so-5405-4423-v1.c:37:29: error: ‘ ’ directive writing 1 byte into a region of size between 0 and 99 [-Werror=format-overflow=]
             sprintf(result, "%s%d ", text, i); // Format the text and store it in result
                             ^~~~~~~
so-5405-4423-v1.c:37:13: note: ‘__builtin___sprintf_chk’ output between 3 and 102 bytes into a destination of size 100
             sprintf(result, "%s%d ", text, i); // Format the text and store it in result
             ^~~~~~~
cc1: all warnings being treated as errors
$

，这是什么在运行macOS 10.14.2 Mojave的Mac上，左边给出了我在GCC 8.2.0上编译的这些错误：

text

编译器指出", "是一个可以包含0到99个字符的字符串，因此理论上它可以在与数字和" "（或"String No."进行一次迭代）连接时导致溢出。它被初始化为text这一事实意味着没有溢出风险，但你可以通过使用较短的长度来减轻20 - 比如说100而不是-Werror。

我承认这个在GCC中相对较新的警告并不总是像所有那样有用（这是代码正常的情况，但警告仍然出现）。我通常会解决这个问题，如果只是因为它目前显示我的默认选项而且代码没有使用clang的任何警告进行编译，而且如果没有这种级别的保护，我还没准备好。我不使用-Weverything的-Wall选项raw;它产生的警告肯定适得其反（至少是AFAIAC）。但是，我反驳了对我不起作用的“一切”选项。如果-Wextra或

for(j = 0; j < 10; j++){ // Now loop to change the line

    strcpy(lines[i], line); // Copy the line of text into each line of the array

    fputs(lines[i], file); // Put each line into the file        

}   

选项太痛苦，出于某种原因，我会反击它，但要小心翼翼。我会检查疼痛程度，并旨在处理任何症状。

你也有循环：

i

不幸的是，当这个循环运行时，10等于lines，它超出了数组j的界限。这可能导致崩溃。据推测，该指数应该是i而不是so-5405-4423-v2.c。

这是您的代码的检测版本（

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

char line[1001];
char lines[11][1001];
char info[100];

char *extra_info(char string_1[], char string_2[], char string_3[],
                 char string_4[], char string_5[]);

int main(void)
{
    char result[100], text[20];
    const char filename[] = "test.txt";
    FILE *file;

    strcpy(text, "String No.");

    file = fopen(filename, "w+");
    if (file == NULL)
    {
        fprintf(stderr, "Failed to open file '%s' for writing/update\n", filename);
        return 1;
    }

    for (int i = 0; i < 10; i++)
    {
        if (i != 9)
            sprintf(result, "%s%d, ", text, i);
        else
            sprintf(result, "%s%d ", text, i);

        fprintf(stderr, "Iteration %d:\n", i);
        fprintf(stderr, "1 result (%4zu): [%s]\n", strlen(result), result);
        fprintf(stderr, "1 line   (%4zu): [%s]\n", strlen(line), line);
        extra_info("st", "nd", "rd", "th", "th");
        fprintf(stderr, "2 line   (%4zu): [%s]\n", strlen(line), line);
        fprintf(stderr, "1 info   (%4zu): [%s]\n", strlen(info), info);
        strncat(line, info, 100);
        fprintf(stderr, "3 line   (%4zu): [%s]\n", strlen(line), line);
        printf("%s", result);
        strncat(line, result, 15);
        fprintf(stderr, "3 line   (%4zu): [%s]\n", strlen(line), line);
    }

    fprintf(stderr, "4 line   (%4zu): [%s]\n", strlen(line), line);
    strncat(line, "\n\n", 2);

    for (int j = 0; j < 10; j++)
    {
        strcpy(lines[j], line);
        fputs(lines[j], file);
    }

    fclose(file);

    return 0;
}

char *extra_info(char string_1[], char string_2[], char string_3[],
                 char string_4[], char string_5[])
{
    char text[100];

    sprintf(text, " 1%s", string_1);
    fprintf(stderr, "EI 1: add (%zu) [%s] to (%zu) [%s]\n", strlen(string_1), string_1, strlen(line), line);
    strncat(line, text, 100);

    sprintf(text, ", 2%s", string_2);
    fprintf(stderr, "EI 2: add (%zu) [%s] to (%zu) [%s]\n", strlen(string_2), string_2, strlen(line), line);
    strncat(line, text, 100);

    sprintf(text, ", 3%s", string_3);
    fprintf(stderr, "EI 3: add (%zu) [%s] to (%zu) [%s]\n", strlen(string_3), string_3, strlen(line), line);
    strncat(line, text, 100);

    sprintf(text, ", 4%s", string_4);
    fprintf(stderr, "EI 4: add (%zu) [%s] to (%zu) [%s]\n", strlen(string_4), string_4, strlen(line), line);
    strncat(line, text, 100);

    sprintf(text, ", 5%s.", string_5);
    fprintf(stderr, "EI 5: add (%zu) [%s] to (%zu) [%s]\n", strlen(string_5), string_5, strlen(line), line);
    strncat(line, text, 100);

    fprintf(stderr, "EI 6: copy (%zu) [%s] to info\n", strlen(line), line);
    strcpy(info, line);

    return line;
}

）：

Iteration 0:
1 result (  13): [String No.0, ]
1 line   (   0): []
EI 1: add (2) [st] to (0) []
EI 2: add (2) [nd] to (4) [ 1st]
EI 3: add (2) [rd] to (9) [ 1st, 2nd]
EI 4: add (2) [th] to (14) [ 1st, 2nd, 3rd]
EI 5: add (2) [th] to (19) [ 1st, 2nd, 3rd, 4th]
EI 6: copy (25) [ 1st, 2nd, 3rd, 4th, 5th.] to info
2 line   (  25): [ 1st, 2nd, 3rd, 4th, 5th.]
1 info   (  25): [ 1st, 2nd, 3rd, 4th, 5th.]
3 line   (  50): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.]
3 line   (  63): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0, ]
Iteration 1:
1 result (  13): [String No.1, ]
1 line   (  63): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0, ]
EI 1: add (2) [st] to (63) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0, ]
EI 2: add (2) [nd] to (67) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st]
EI 3: add (2) [rd] to (72) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd]
EI 4: add (2) [th] to (77) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd]
EI 5: add (2) [th] to (82) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th]
EI 6: copy (88) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.] to info
2 line   (  88): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.]
1 info   (  88): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.]
3 line   ( 176): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.]
3 line   ( 189): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1, ]
Iteration 2:
1 result (  13): [String No.2, ]
1 line   ( 189): [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1, ]
EI 1: add (2) [st] to (189) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1, ]
EI 2: add (2) [nd] to (193) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1,  1st]
EI 3: add (2) [rd] to (198) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1,  1st, 2nd]
EI 4: add (2) [th] to (203) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1,  1st, 2nd, 3rd]
EI 5: add (2) [th] to (208) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1,  1st, 2nd, 3rd, 4th]
EI 6: copy (214) [ 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th. 1st, 2nd, 3rd, 4th, 5th.String No.0,  1st, 2nd, 3rd, 4th, 5th.String No.1,  1st, 2nd, 3rd, 4th, 5th.] to info
String No.0, String No.1, Abort trap: 6

运行时，它产生的输出类似于：

line

当您观察到214个字节从info（大到足以容纳该字符串）复制到lldb（不是 - 它只有100个字节长）时，随之而来的崩溃并不是很令人惊讶。尚不完全清楚所期望的行为是什么。

在我的Mac上，__strcpy_chk调试器报告了extra_info()中的崩溃; AFAICT，它位于

frame #6: 0x00007fff681bbe84 libsystem_c.dylib`__strcpy_chk + 83
frame #7: 0x00000001000017cc so-5405-4423-v2`extra_info(string_1=<unavailable>, string_2=<unavailable>, string_3="rd", string_4="th", string_5="th") at so-5405-4423-v2.c:86

末尾突出显示的行中：

strncat()

因此，虽然显然不是strncat()导致崩溃，但使用strncat()的方式显然不正确 - IMO，它是不正确的，但视图可能不同。我仍然支持我的基本结论：不要使用

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

char line[1001]; // The line supports up to a 1000 characters
char lines[11][1001]; // An array of lines (up to 10 lines where each line is a 1000 characters max)
char info[100]; // Holds extra info provided by user

char * extra_info(
        char string_1[],
        char string_2[],
        char string_3[],
        char string_4[],
        char string_5[]
    );

int main(){

    int 
    i, // Line number
    j; // Length of the line
    char result[100], text[100];
    FILE *file;

    strcpy(text, "String No."); // The default text

    file = fopen("test.txt", "w+"); // Open the file for reading and writing

    for(i = 0; i < 10; i++){ // Loop to create a line.

        if(i != 9){ // If the line is NOT at the 10th string

            sprintf(result, "%s%d, ", text, i); // Format the text and store it in result

        }
        else{

            sprintf(result, "%s%d ", text, i); // Format the text and store it in result            

        }

        strncat(line, info, 100); // Append the extra info at the end of each line        

        strncat(line, result, 15); // Concatenate all strings in one line

    }

    extra_info(
        "st",
        "nd",
        "rd",
        "th",
        "th"
    );    

    strncat(line, "\n\n", 2); // Add a new-line character at the end of each line

    for(j = 0; j < 10; j++){ // Now loop to change the line

        strcpy(lines[i], line); // Copy the line of text into each line of the array

        fputs(lines[i], file); // Put each line into the file        

    }

    fclose(file);  

}

char * extra_info( // Append user defined and predefined info at the end of a line
        char string_1[],
        char string_2[],
        char string_3[],
        char string_4[],
        char string_5[]
    ){
        char text[100]; // A variable to hold the text

        /* Append a default text into each strings 
        and concatenate them into one line */

        sprintf(text, " 1%s", string_1);
        strncat(line, text, 100);

        sprintf(text, ", 2%s", string_2);
        strncat(line, text, 100);

        sprintf(text, ", 3%s", string_3);
        strncat(line, text, 100);

        sprintf(text, ", 4%s", string_4);
        strncat(line, text, 100);

        sprintf(text, ", 5%s.", string_5);
        strncat(line, text, 100);

        return line;

}

。

Answer 2

解决方案很简单，因为我感觉到，在C中没有任何邪恶，恶毒或愤世嫉俗。第一个strcpy（）没有必要发生，第二个extra_info（）被错放了它的目的，第三个即使我要使用strcpy（）参数也要被交换。因此错误中止陷阱6：

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