I am trying to add integers to a string. When I debug my code everything works perfectly fine, but when i run it normally two unwanted characters are being printed at the beginning of the string. How do I avoid this?

    int number_of_ints = 5;
    char s[number_of_ints*2];
    char suffix[4];
    for(int i = 1; i <= number_of_ints; i  ){
        snprintf(suffix, number_of_ints, "%d*", i);
        strncat(s, suffix, 2);
        printf("%s\n", s);
    }

This is the output when building and running the code normally.

`û1*
`û1*2*
`û1*2*3*
`û1*2*3*4*
`û1*2*3*4*5*

CodePudding user response：

Strings in C are a sequence of nonzero bytes followed by a terminating byte with a value of zero (the null-terminating byte, '\0').

You must size your buffer to have an additional space, to guarantee room for this null-terminating byte.

You must additionally make sure the contents of the buffer contain a valid string. You can do this by setting the first element of the buffer to the null-terminating byte, creating a zero-length string.

Failing to initialize the contents of your buffer means it will contain indeterminate values, and passing such a buffer to a function expecting a valid string will invoke Undefined Behavior.

The second argument to snprintf should be, at most, the size of the destination buffer.

Finally, consider using size_t when applicable, as it is the appropriate type for dealing with memory sizes (e.g., sizing variable-length arrays; the sizeof operator resolves to this type; snprintf expects this type as its second argument).

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

int main(void) {
    size_t number_of_ints = 5;
    char s[number_of_ints * 2   1];
    char suffix[4];

    s[0] = '\0';

    for (size_t i = 1; i <= number_of_ints; i  ) {
        snprintf(suffix, sizeof suffix, "%zu*", i);
        strncat(s, suffix, 2);
        printf("%s\n", s);
    }
}

CodePudding user response：

You have a few issues with initialization and size of your buffers.

• The size of your buffer is too short by 1 byte that is needed for 0-termination. Adding the last number causes buffer overrun and undefined behaviour.
• In addition the calculated size is only sufficient as long as number_of_ints<10 because it only allows for single digit numbers.
• That buffer is not initialized and us very likely not holding an empty string. Accessing it (via strcat etc.) invokes undefined behaviour.
• The size you provide to snprintf is not related to the size of the buffer.

You should apply these changes:

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

int main(void)
{
    int number_of_ints = 5;
    char s[number_of_ints*2 1];
    char suffix[4];
    s[0] = 0;
    for(int i = 1; i <= number_of_ints; i  ){
        snprintf(suffix, sizeof(suffix), "%d*", i);
        strncat(s, suffix, 2);
        printf("%s\n", s);
    }

CodePudding user response：

You can try this code:

#include <iostream>
#include <string.h>
using namespace std;
int main(void){
    int number_of_ints = 5;
    char s[number_of_ints*2];
    char suffix[4];
    for(int i = 1; i <= number_of_ints; i  ){
        snprintf(suffix, number_of_ints, "%d*", i);
        printf("");
        strncat(s, suffix, 2);
        printf("%s\n", s);
    }
    system("PAUSE");
    return 0;
}