How do I read a string entered by the user in C?

user_358193 0
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I want to read the name entered by my user using C programmes.

For this I wrote:

char name[20];

printf("Enter name: ");
gets(name);

But using gets is not good, so what is a better way?

stdin c
8 Answers

Using scanf removing any blank spaces before the string is typed and limiting the amount of characters to be read:

#define SIZE 100

....

char str[SIZE];

scanf(" %99[^\n]", str);

/* Or even you can do it like this */

scanf(" %99[a-zA-Z0-9 ]", str);

If you do not limit the amount of characters to be read with scanf it can be as dangerous as gets

Admin1

ANSI C unknown maxinum length solution

Just copy from Johannes Schaub's https://stackoverflow.com/a/314422/895245

Don't forget to free the returned pointer once you're done with it.

char * getline(void) {
    char * line = malloc(100), * linep = line;
    size_t lenmax = 100, len = lenmax;
    int c;
    
    if(line == NULL)
        return NULL;

    for(;;) {
        c = fgetc(stdin);
        if(c == EOF)
            break;

        if(--len == 0) {
            len = lenmax;
            char * linen = realloc(linep, lenmax *= 2);

            if(linen == NULL) {
                free(linep);
                return NULL;
            }
            line = linen + (line - linep);
            linep = linen;
        }

        if((*line++ = c) == '\n')
            break;
    }
    *line = '\0';
    return linep;
}

This code uses malloc to allocate 100 chars. Then it fetches char by char from the user. If the user reaches 101 chars, it doubles the buffer with realloc to 200. When 201 is reached, it doubles again to 400 and so on until memory blows.

The reason we double rather say, just adding 100 extra every time, is that increasing the size of a buffer with realloc can lead to a copy of the old buffer, which is a potentially expensive operation.

Arrays must be contiguous in memory because we wan to be able to random access them efficiently by memory address. Therefore if we had in RAM:

content     buffer[0] | buffer[1] | ... | buffer[99] | empty | empty | int i
RAM address 1000      | 1001      |     | 1100       | 1101  | 1102  | 1103

we wouldn't be able to just increase the size of buffer, as it would overwrite our int i. So realloc would need to find another location in memory that has 200 free bytes, and then copy the old 100 bytes there and free the 100 old bytes.

By doubling rather than adding, we quickly reach the order of magnitude of the current string size, since exponentials grow really fast, so only a reasonable number of copies is done.

user_8952450
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