CODEWITHSUNDEEP
arrayscstringmacroschar
user16059715
user16059715

Reputation:

C invalid operands to binary + (have ‘char *’ and ‘char *’)?

In C I have:

filedata->string_table + (X)->sh_name

Now I want to add the following string next to it: "new"

I tried:

filedata->string_table + (X)->sh_name + "new"

but this won't compile and I get:

readelf.c:6807:83: error: invalid operands to binary + (have ‘char *’ and ‘char *’)
  filedata->string_table + (X)->sh_name : filedata->string_table + (X)->sh_name+"new")
                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~      ^

How to fix this?

Upvotes: 0

Views: 2173

Answers (4)

John Bode
John Bode

Reputation: 123458

Remember that C doesn't have an actual string type - strings are stored in arrays of char, and C doesn't define the + or = operators for array types.

You can append strings to a character buffer using strcat or strncat:

char foo[] = "This is ";
char bar[] = "a test";
char target[ sizeof foo + sizeof bar ];

strcpy( target, foo ); // copies contents of foo to target buffer
strcat( target, bar ); // appends contents of bar to target buffer

The target buffer must be large enough to accommodate the final string including the terminator. In this example, we took the sizes (not lengths!) of foo and bar which are 9 and 7 respectively (including the string terminators). Note that this would not work if they were declared as

char *foo = "This is ";
char *bar = "a test";

because sizeof foo would only give us the size of the pointer object, not the size of the string to which it points. In that case you'd have to get the size at runtime using strlen. If you're using a version that supports VLAs (C99 or later), you could simply do

char *foo = "This is ";
char *bar = "a test";

char target[ strlen( foo ) + strlen( bar ) + 1];

If not, you'd have to allocate your buffer dynamically:

char *target = malloc( strlen( foo ) + strlen( bar ) + 1 );

and you'd have to remember to free it when you're done.

If you have a mix of string and non-string types, you can use sprintf:

char *name = "blah";
int number = 42;

sprintf( target, "%s-%d", name, number ); // writes "blah-42" to target buffer

Again, the target buffer must be large enough to store the resulting string plus the terminator.

Upvotes: 1

0___________
0___________

Reputation: 67476

(X)->sh_name+"new" + in C does not concatenate the strings. It tries to add pointer to pointer which is invalid.

You need to use function strcat or strncat

Sorry These functions affect the strings but I want a new one –

it is so simple:

char *strcatAlloc(const char * restrict s1, const char * restrict s2)
{
    size_t s1len;
    char *newstring;

    if(s1 && s2)
    {
        s1len = strlen(s1);
        newstring = malloc(s1len + strlen(s2) + 1);
        if(newstring)
        {
            strcpy(newstring, s1);
            strcpy(newstring + s1len, s2);
        }
    }
    return newstring;
}

int main(void)
{
    // you should check if the function fails. Omitted for the example clarity
    printf("`%s`\n", strcatAlloc("hello ", "world"));
    printf("`%s`\n", strcatAlloc("", "world"));
    printf("`%s`\n", strcatAlloc("hello ", ""));
    printf("`%s`\n", strcatAlloc("", ""));

    //you should free allocated space
}

or version where you can pass your buffer (it will allocate memory if that buffer is NULL)

char *strcatAlloc(const char * restrict s1, const char * restrict s2, char *buff)
{
    size_t s1len;

    if(s1 && s2)
    {
        s1len = strlen(s1);
        if(!buff) buff = malloc(s1len + strlen(s2) + 1);
        if(buff)
        {
            strcpy(buff, s1);
            strcpy(buff + s1len, s2);
        }
    }
    return buff;
}

int main(void)
{

    char s[100];
    // you should check if the function fails. Omitted for the example
     printf("`%s`\n", strcatAlloc("hello ", "world", NULL));
    printf("`%s`\n", strcatAlloc("", "world", NULL));
    printf("`%s`\n", strcatAlloc("hello ", "", s));  //you can pass your own large enough buffer 
    printf("`%s`\n", strcatAlloc("", "", NULL));

    //you should free allocaated space
}

Upvotes: 1

Arcaniaco
Arcaniaco

Reputation: 420

You cannot concatenate strings or char using the operator +. Use strncat() or snprintf().

Upvotes: 1

MikeCAT
MikeCAT

Reputation: 75062

Use proper way to concatenate strings.

If you want to add the result to existing buffer, use strcat() to concatenate strings.

char buffer[1024000]; /* enough size */
strcpy(buffer, filedata->string_table + (X)->sh_name);
strcat(buffer, "new");

You also can use snprintf() to put the result of concatenation in a buffer.

char buffer[1024000]; /* enough size */
snprintf(buffer, sizeof(buffer), "%s%s",
    filedata->string_table + (X)->sh_name, "new");

To print the result to the standard output, use two %s format specifier to print the two strings in a row.

printf("%s%s",
    filedata->string_table + (X)->sh_name, "new");

If the string "new" is fixed, also you can do like this:

printf("%snew",
    filedata->string_table + (X)->sh_name);

Upvotes: 1

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