将char数组分配给char *

Question

I'm trying to write a function that prefixes a string with its length. I can't seem to assign a char[] to a char *. Mysteriously, if I print out some debugging code before the assignment, it works.

char *prefixMsgWLength(char *msg){
  char *msgWLength;
  int msgLength = strlen(msg);

  if (msgLength == 0){
    msgWLength = "2|";
  }
  else{

    int nDigits = floor(log10(abs(msgLength))) + 1;
    int nDigits2 = floor(log10(abs(msgLength + nDigits + 1))) + 1;

    if (nDigits2 > nDigits){
      nDigits = nDigits2;
    }

    msgLength += nDigits + 1;

    char prefix[msgLength];
    sprintf(prefix, "%d|", msgLength);

    strcat(prefix, msg);
    // if I uncomment the below, msgWLength is returned correctly
    // printf("msg: %s\n", prefix);
    msgWLength = prefix;
  }
  return msgWLength;
}

Answer 1

The problem in your code is

 msgWLength = prefix;

here, you're assigning the address of a local variable ( prefix ) to the pointer and you try to return it.

Once the function finishes execution, the local variables will go out of scope and the returned pointer will be invalid.

You need to make prefix as a pointer and allocate memory dynamically, if you want it to retain it's existence after returning from the function.

Answer 2

String reallocation to the exact length can be very cumbersome in C. You'd probably be much better off just using a sufficiently large buffer. Here, I use limits.h to determine the size of a line buffer according to the system ( LINE_MAX ):

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

int main()
{
    /* Our message */
    char const msg[] = "Hello, world!";
    /* Buffer to hold the result */
    char buffer[LINE_MAX];

    /* Prefix msg with length */
    snprintf(buffer, LINE_MAX, "%lu|%s", strlen(msg)+1, msg);

    /* Print result */
    printf("%s\n", buffer);

    return 0;
}