c - 当我用printf删除不相关的句子时，我的程序将无法正常运行

Question

事实上，我已经快完成练习5-13了，但是出现了一个很奇怪的问题，我找不到我犯了什么错误:当我删除代码的第 13 行时，即printf("%d\n",argc);我的程序无法正常运行，并且遇到一个问题:当我输入 ./tail -10

#include <stdio.h>
#define MAXSIZE 1000

/* prints the last linesof its input,and the n's default value is 10 */
int main(int argc, char *argv[])
{
    int atoi(char*);
    int readlines(char *lineptr[], int maxlines);/* read the input to the lineptr */
    int n = 10;
    int i;
    char *lineptr[MAXSIZE];/* store the the point of input lines */
    int lines;
    printf("%d\n",argc);
    while(--argc > 0){
            if((*++argv)[0] == '-') {
                    n = atoi(++*argv);
                    printf("%d\n",n);
            }
            else {
                    printf("Error input\n");
                    return -1;
            }
    }
    lines = readlines(lineptr,MAXSIZE);// lines is the the actual lines of input */
    if(lines > n) /* print ten the last n lines */
            for(i = lines - n;i <= lines -1;i++)
                    printf("%s\n",lineptr[i]);
    else
            for(i=0;i < lines;i++)  /* print all the input */
                    printf("%s\n",lineptr[i]);
    return 0;
}

readline.c

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

#define MAXLEN 1000     /* max length of any input line */

int getLine(char *, int);
char *alloc(int);
/* readlines: read input lines */
int readlines(char *lineptr[], int maxlines)
{
    int len,nlines;
    char *p,line[MAXLEN];

    while((len = getLine(line,MAXLEN)) > 0)
            if(nlines >= maxlines || (p = alloc(len)) == NULL)
                    return -1;
            else {
                    line[len-1] = '\0';
                    strcpy(p,line);
                    lineptr[nlines++] = p;
            }
    return nlines;
}

int getLine(char *s, int lim)
{
    int i;
    int c;
    for(i = 0;i<lim-1 && (c=getchar())!=EOF && c != '\n';i++)
            *s++ = c;
    if(c=='\n') {
            *s++ = c;
            i++;
    }
    *s = '\0';
    return i;
}

分配.c

#define ALLOCSIZE 10000

static char allocbuf[ALLOCSIZE];
static char *allocp = allocbuf;

char *alloc(int n)
{
    if(allocbuf + ALLOCSIZE - allocp >=n) {
            allocp += n;
            return allocp -n;
    }
    else
            return 0;
}

atoi.c

#include <ctype.h>

int atoi(char *p)
{
    int sign,n;
    int c;
    while(isspace(*p))
            p++;
    if((c = (*p)) == '+' || c == '-')
                    p++;
    sign = c == '-' ? -1 : 1;
    for(n = 0;isdigit(c=*p);p++)
            n = n * 10 + (c - '0');
    return n * sign;
}

Answer 1

您在 readLines 中有一个未初始化的变量:

int len,nlines;       // nlines not initialized
char *p,line[MAXLEN];

while((len = getLine(line,MAXLEN)) > 0)
        // nlines used here
        if(nlines >= maxlines || (p = alloc(len)) == NULL)

未初始化的局部变量具有未定义的内容，因此尝试使用该值会导致 undefined behavior .

声明时需要设置nlines=0。

我能够通过 valgrind 快速找到这个问题，这给了我以下输出:

==8219== Conditional jump or move depends on uninitialised value(s)
==8219==    at 0x40076A: readlines (x1.c:48)
==8219==    by 0x4006CE: main (x1.c:27)
==8219==
==8219== Use of uninitialised value of size 8
==8219==    at 0x4007C2: readlines (x1.c:53)
==8219==    by 0x4006CE: main (x1.c:27)
==8219==