standard c library for escaping a string.

Question

Is there a standard C library function to escape C-strings?

For example, if I had the C string:

char example[] = "first line\nsecond line: \"inner quotes\"";

And I wanted to print

"first line\nsecond line: \"inner quotes\""

Is there a library function that will do that transformation for me? Rolling my own just seems a little silly.

Bonus points if I can give it a length to escape (so it stops before or beyond the \0).

Answer 1

No standard C function, but not too hard to roll your own

nothing too pretty but :-

void escape_str(char *dest, char *src)
{
     *dest = 0;
     while(*src)
     {
         switch(*src)
         {
             case '\n' : strcat(dest++, "\\n"); break;
             case '\"' : strcat(dest++, "\\\""); break;
             default:  *dest = *src;
         }
         *src++;
         *dest++;
         *dest = 0;                     
     }
}

Answer 2

while(*src++)
{
  if(*src == '\\' || *src == '\"' || *src == '\'')
    *dest++ = '\\';

  *dest++ = *src++;
}

Answer 3

If you were writing GPL stuff you might use http://git.savannah.gnu.org/gitweb/?p=gnulib.git;a=blob;f=lib/quotearg.c;hb=HEAD

Answer 4

You just mentioned that you wanted to print the string.

char example[] = "first line\nsecond line: \"inner quotes\"";
size_t len = strlen(example);
size_t i;

static const char *simple = "\\\'\"";
static const char *complex = "\a\b\f\n\r\t\v";
static const char *complexMap = "abfnrtv";

for (i = 0; i < length; i++)
{
    char *p;
    if (strchr(simple, example[i]))
    {
        putchar('\\');
        putchar(example[i]);
    }
    else if ((p = strchr(complex, example[i]))
    {
        size_t idx = p - complex;
        putchar('\\');
        putchar(complexMap[idx]);
    }
    else if (isprint(example[i]))
    {
        putchar(example[i]);
    }
    else
    {
        printf("\\%03o", example[i]);
    }
}

Answer 5

There is no standard C library function for this.

When you use the declaration

char example[] = "first line\nsecond line: \"inner quotes\"";

the escape sequences will be interpreted and replaced by the compiler. You will have to "un-interpret" the characters that C escapes. Here's a quick-n-dirty example:

void print_unescaped(char* pStr, int len) {
    char* ptr = pStr;
    int   i = 0;
    if (pStr == NULL || len == 0) return;
    while (ptr != '\0' && i < len) {
        switch (*ptr) {
            case '\0': printf("\\0");     break;
            case '\a': printf("\\a");     break;
            case '\b': printf("\\b");     break;
            case '\f': printf("\\f");     break;
            case '\n': printf("\\n");     break;
            case '\r': printf("\\r");     break;
            case '\t': printf("\\t");     break;
            case '\v': printf("\\v");     break;
            case '\\': printf("\\\\");    break;
            case '\?': printf("\\\?");    break;
            case '\'': printf("\\\'");    break;
            case '\"': printf("\\\"");    break;
            default  : printf("%c",*ptr); break;
        }
        ++i;
        ++ptr;
    }
}

Answer 6

#include <string.h>    
/* int c_quote(const char* src, char* dest, int maxlen)
 *
 * Quotes the string given so that it will be parseable by a c compiler.
 * Return the number of chars copied to the resulting string (including any nulls)
 *
 * if dest is NULL, no copying is performed, but the number of chars required to 
 * copy will be returned.
 *
 * maxlen characters are copied. If maxlen is negative, 
 * strlen is used to find the length of the source string, and the whole string
 * including the NULL-terminator is copied.
 *
 * Note that this function will not null-terminate the string in dest.
 * If the string in src is not null-terminated, or maxlen is specified to not
 * include the whole src, remember to null-terminate dest afterwards.
 *
 */
int c_quote(const char* src, char* dest, int maxlen) {
    int count = 0;
    if(maxlen < 0) {
        maxlen = strlen(src)+1;      /* add 1 for NULL-terminator */
    }

    while(src  && maxlen > 0) {
        switch(*src) {

            /* these normal, printable chars just need a slash appended */
            case '\\':
            case '\"':
            case '\'':
                if(dest) {
                    *dest++ = '\\';
                    *dest++ = *src;
                }
                count += 2;
                break; 

            /* newlines/tabs and unprintable characters need a special code.
             * Use the macro CASE_CHAR defined below.
             * The first arg for the macro is the char to compare to,
             * the 2nd arg is the char to put in the result string, after the '\' */
#define CASE_CHAR(c, d) case c:\
    if(dest) {\
        *dest++ = '\\'; *dest++ = (d);\
        }\
count += 2;\
break;
            /* --------------  */
            CASE_CHAR('\n', 'n');
            CASE_CHAR('\t', 't');
            CASE_CHAR('\b', 'b');
            /*  ------------- */

#undef CASE_CHAR


            /* by default, just copy the char over */
            default:
                if(dest) {
                    *dest++ = *src;
                }
                count++;
        }

        ++src;
        --maxlen;
    }
    return count;
}