c - C 中的字符串连接

Question

所以我得到了这个函数，如果没有空格，它可以在“/”C 前面放置一个空格。它很好地分割了字符串，但当我尝试将字符串连接在一起时，我收到一个错误，可能是内存冲突。请帮帮我。

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

char* substr(const char *pstr, int start, int numchars) {
    char* pnew = malloc(numchars + 1);
    strncpy(pnew, pstr + start, numchars);
    pnew[numchars] = '\0';
    return pnew;
}

char* fixString(char str[]) {
    char* position;
    char* newString = "";
    char* finalString;

    int oldPosition = 0;
    printf("Original str: %s\n", str);
    printf("Original length: %d\n\n", strlen(str));

    position = strchr(str, '/');
    while (position != NULL) {
        int charPosition = position - str;

        printf("String position: %d->%d\n", oldPosition, charPosition);
        newString = substr(str, oldPosition, charPosition - oldPosition);
        oldPosition = charPosition;
        if (charPosition > 0 && str[charPosition - 1] != ' ') {
            printf("Previous char: %c\n", str[charPosition - 1]);
            newString = strcat(newString, " ");
        }

        printf("String: |%s|\n", newString);
        if (strlen(newString) > 0) {
            finalString[0] = strcat(finalString, newString);
        }
        printf("------------\n");
        position = strchr(position + 1, '/');
    }
    char* lastString = substr(str, oldPosition, strlen(str));
    finalString = strcat(finalString, lastString);
    printf("lastString: %s\n\n", lastString);
    return finalString;
}

int main() {
    char* testString = "/Filter /FlateDecode/Length 7108/Subtype /Type1C";
    printf("%s", fixString(testString));

    return 0;
}

Answer 1

您永远不会分配目标缓冲区。 finalString 变量未初始化为任何内容。

这也不是您的代码的唯一问题。您似乎将 char * 视为某种智能字符串类型，但它只不过是指向内存位置的指针。例如，这个:

newString = strcat(newString, " ");

不会连接两个字符串并返回连接结果。它将一个空格附加到 newString 指向的字符缓冲区上并返回相同的缓冲区。分配给 newString 是无害的，但会产生误导。

// It is the callers responsibility to free the returned string.
char *fixString(char *str) {
    int len;
    char *s;
    char *dest;
    int after_space;

    // First pass, figure out the size of the output.
    len = 0;
    after_space = 0;
    for (s = str; *s; s++) {
        len += 1 + (!after_space && *s == '/');
        after_space = *s == ' ';
    }
    dest = malloc(len + 1);

    s = dest;
    after_space = 0;
    while(*str) {
        if (!after_space && *str == '/') *s++ = ' ';
        after_space = (*s++ = *str++) == ' ';
    }
    return dest;
}