C中的字符串串联

Question

So I got this function which is able to put a space infront of a "/" C if there is no space. And it cuts up the string fine but I get an error, possibly a memory violation when I try to concat the string together. Please give me a hand.

#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

You never allocate a target buffer. The finalString variable isn't initialised to anything.

That's not the only problem with your code, either. You seem to be treating the char * as some kind of smart string type, but it is nothing more than a pointer into a memory location. For instance, this:

newString = strcat(newString, " ");

doesn't concatenate two strings and return the concatenated result. It appends a space onto the char buffer newString points to and returns the same buffer. Assigning to newString is harmless, but misleading.

// 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;
}

Answer 2

You need to allocate some memory for finalString .

Try doing it like this:

#define BUFFER_SIZE 1024
char* finalString = malloc(BUFFER_SIZE);
// ...
strncat(finalString, "something", 1024);

Remember to call free() on the pointer when you don't need it anymore (not in your function - after the function returns, somewhere in client code, whenever the result's not needed).

Answer 3

You never malloc memory for your newString and finalString pointers. strcat expects that the destination pointer you give it has enough space to store the strings.

Answer 4

You might also have to check for the size after malloc() and realloc() if necessary. Welcome to the beautiful world of memory management in C :)