创建动态字符串时的额外字节

Question

I have written a program to take input and dynamically allocate memory using realloc() , however there seems to be an error because if I print the final string char by char I seem to have 2 empty bytes at the end, I am sure this is going to be something silly but I have spent some time trying to discover the cause and have failed so hope to learn something here.

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

int main(void) {
    int count = 0;
    char *str;
    char tmp;

    str = malloc(sizeof(char));

    while (tmp != '\n') {
        tmp = getchar();

        str = realloc(str, (count + 1) * sizeof(char));

        *(str + count) = tmp;

        count += 1;

    }

    *(str + count) = '\0';

    puts(str);

    // This is just to try and see what was happening
    for (int i = 0; i <= count; i++)
        printf("str[%d] = %c\n", i, str[i]);

    free(str);
    str = NULL;

    return EXIT_SUCCESS;

}

Answer 1

This loop should look at least like

for(int i = 0; i < count; i++)
                ^^^
    printf("str[%d] = %c\n", i, str[i]);

Or it would be better to write

for(int i = 0; str[i]; i++)
    printf("str[%d] = %c\n", i, str[i]);

Or

int i = 0;
for ( char *p = str; *p; ++p )
    printf( "str[%d] = %c\n", i++, *p );

And change these statements

while(tmp != '\n') {
tmp = getchar();

to

while ( ( tmp = getchar() ) != EOF && tmp != '\n' )

Also it would be more safe instead of this statement

str = realloc(str, (count + 0x01) * sizeof(char));

to write

char *p = realloc(str, (count + 0x01) * sizeof(char));
if ( !p ) break;
else str = p;

Answer 2

Four things to mention here.

1. while(tmp != '\\n') is reading uninitialised automatic local variable value without initialization. It invokes undefined behaviour .

2. str = realloc(str, (count + 0x01) * sizeof(char)); is very bad, if realloc() fails, you'll lose the actual pointer, too. Always use a temporary pointer to hold the return value from realloc() and after proper error check, assign it back to the main pointer.

3. sizeof(char) is guaranteed to be 1 . You don't need to use as a multiplier. It's redundant.

4. The for loop condition, should be i < count otherwise, you'll run into off-by-one error. C uses 0 based indexing.

That said,

• You should always check for the success of the return vale of realloc() and family of functions for success before using the returned pointer.
• getchar() returns an int . You should change the type of tmp to int tmp = 0;

Answer 3

Apart from the uninitialized variable access, the two “empty characters” are:

1. a newline, since you check for \\n before you read and store the next character, and

2. a character in uninitialized memory since you're incorrectly looping while i <= count and not while i < count .

Use a for (;;) (infinite loop) and check if (tmp == '\\n') { break; } if (tmp == '\\n') { break; } immediately after getchar() to avoid both the uninitialized variable access and trailing newline.