What is another alternative to strcat and strncat functions in C?

Question

This is the specific section of code where I am facing issues using both strcat() and strncat() functions to concatenate two strings.

The strcat() function is declared as char *strcat(char *dest, const char *src) and strncat() as char *strncat(char *dest, const char *src, size_t n), however both of them give issues when the second parameter is a single character from string, i.e., which does not end with '\0'. I need to concatenate the a character to a string.

So is there any alternative to these two functions or is there any way to make these functions work for me?

    char *cipher_str = (char *)malloc(sizeof(char) * 26);
    for (int j = 0; j < col; j++) {
        for (int i = 0; i < col; i++) {
            if (min == cipher[0][i] && (done[i] != 1)) {
                done[i] = 1;
                for (int k = 0; k < rows; k++)
                    strcat(cipher_str, cipher[i][k]);
                }
            }
            ...

For only appending a single character you can use `int l = strlen(s); s[l] = ch; s[l+1] = 0;` (provided that you have enough space allocated for s) — Ctx, Feb 27 '17 at 15:39
@PaulOgilvie Sorry for the missing piece of data, cipher is then intialized to '\0' before being processed and modified — Nishit Mengar, Feb 27 '17 at 15:45
Initialize the buffer to zero (using memset) and use memcpy instead of strcat. In general, the syntax is memcpy(buf+pos, &ch, 1); — aicastell, Feb 27 '17 at 15:49

score 6 · Answer 1 · answered Feb 27 '17 at 15:49

6

Th easiest way here is just to append the character "manually" to the string, for example:

    int m=0;

    ....
        cipher_str[m++]=cipher[i][k];
    ....
    cipher_str[m]='\0';

answered Feb 27 '17 at 15:49

Paul Ogilvie

25,048
4
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svtag · Answer 2 · 2017-02-28T00:17:07.520

2

Since you are using strcat to append 1 character, you can use this function.

void strcat(char* dest, char src)
{
    int size;
    for(size=0;dest[size]!='\0';++size);
    dest[size]=src;
    dest[size+1]='\0';
}

And you mentioned that you have '\0' at end of your 'cipher_str', i used it to determine length.

edited Feb 28 '17 at 00:17

answered Feb 27 '17 at 15:56

svtag

184
4
12

score 0 · Answer 3 · answered Jul 13 '17 at 16:31

What is another alternative to strcat and strncat functions in C?

In-line code can work well with size_t m like with @Paul Ogilvie answer.

As stand-alone functions:

char *strcat_c(char *s, char c) {
  size_t len = strlen(s);
  s[len++] = c;
  s[len] = '\0';
  return s;
}

char *strncat_c(char *s, char c, size_t n) {
  char sc[2] = { c, '\0' };
  return strncat(s, sc, n);
}

// or with C11 compound literals

char *strncat_c(char *s, char c, size_t n) {
  return strncat(s, (char [2])  { c, '\0' }, n);
}

score 0 · Answer 4 · answered Jul 13 '17 at 16:40

Convert the single char into a string with length 1, then use strcat()

#include <string.h>

void appendchar(char *x, char y) {
    char z[2];
    z[0] = y;
    z[1] = 0;
    strcat(x, z);
}

char data[1000] = "foo";
appendchar(data, 'b');
appendchar(data, 'a');
appendchar(data, 'r');
puts(data);

beware buffer overflows

score 0 · Answer 5 · answered Jul 13 '17 at 17:31

There are multiple problems with your code:

cipher_str is allocated with malloc(), so it is uninitialized. Passing it as the destination string for strcat() has undefined behavior as it does not contain a proper string.

Passing a char as second argument to strcat is incorrect. If you insist on using a function to append a single char, you could write:

char *cipher_str = calloc(26);
for (int j = 0; j < col; j++) {
    for (int i = 0; i < col; i++) {
        if (min == cipher[0][i] && done[i] != 1) {
            done[i] = 1;
            for (int k = 0; k < rows; k++)
                strncat(cipher_str, &cipher[i][k], 1);
            }
        }
        ...

But it is overkill to call a function for this simple task and you do not check if cipher_str is long enough.

Here is a much simpler solution with an index variable:

char *cipher_str = (char *)malloc(sizeof(char) * 26);
int ii = 0;
for (int j = 0; j < col; j++) {
    for (int i = 0; i < col; i++) {
        if (min == cipher[0][i] && (done[i] != 1)) {
            done[i] = 1;
            for (int k = 0; k < rows; k++) {
                if (ii < 25) {
                    cipher_str[ii++] = cipher[i][k];
                } else {
                    // cipher_str is too short for the job.
                }
            }
        }
        cipher_str[ii] = '\0';
        ...

What is another alternative to strcat and strncat functions in C?

5 Answers5