使用struct进行段错误

Question

I have been writing C for approximately a week, so bear with me. I'm getting a segmentation fault error in the method assemble_url and I dont know why. Here is my code:

/** Includes */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mysql/mysql.h>

/** Definitions */
#define MAX_OPTIONS 32
#define MAX_ARGS 32

/** Command option */
typedef struct {
    char *argname;
    char *value;
} command_option;

/** Command */
typedef struct {
    command_option options[MAX_OPTIONS];
} command_t;

/**
 * Prints the arguments passed in in a hashmap format (key => value)
 */
void populate_command(command_t *cmd,int argc,char *argv[])
{
    int i,j=0;

    /** Check to see if we have arguments. If argc is equal to 1 than there are no arguments besides the filename */
    if(argc>1)
    {

        /* Start at position 1, since argv[0] is the filename being called */
        for(i=1;i<argc;i++)
        {

            /* Head of argv array */
            char *arg = argv[i];

            /* Create a copy of head for traversal. */
            char *c = arg;

            /* traverse the char array, ensuring we arent dealing with NULL values(c!==NULL) and NULL pointers(*c!=='\0') */
            while(*c != '\0' && c != NULL)
            {

                /* Were only concerned with parsing the flags and obtaining the flag value. */
                if(strchr("-",*c))
                {

                    char *key = c; /* Key */
                    char *value = argv[i+1]; /* Value */

                    /* ensure we have a value existent for key */
                    if(strchr("-",*value))
                    {
                        /** user supplied a key with no value */
                        break;
                    }
                    command_option *option = &cmd->options[j];
                    option->argname = key;
                    option->value   = value;

                    j++;

                    /* Debug dump */
                    printf("arg %d: %s -> %s\n",i,option->argname,option->value);

                }/* end strchr */

                /** Increment the traversal array */
                c++;

            }/* end while loop */

        }/* end forloop */

    }/* endif */

}/** end print_args */

/**
 * Concatenates two strings and returns the resulting concatenated string
 */
char* concatstring(char *s1,char *s2)
{
    /* Allocate memory for *result. We add 1 to account for the extra byte to store the null character. Strlen accounts for all
     non-null bytes, so using strlen(s1) + strlen(s2) + 1 ensures that an overflow wont occur. An overflow occurs when
     the number of bytes being used (in our example, is the addition of strlen for s1 and s2) is more than the number of bytes
     allocated (in our example, the number of bytes allocated to *result)*/
    char *result = malloc(strlen(s1)+strlen(s2)+1);

    /*Copies the C string pointed by source(s1) into the array pointed by destination(result), including the terminating null character. */
    strcpy(result,s1);

    /* appends a copy of the source string(s2) to the destination string(result). The terminating null character in
     destination is overwritten by the first character of source, and a null-character is included at
     the end of the new string formed by the concatenation of both in destination.*/
    strcat(result,s2);

    /* return result */
    return result;

} /** end concatstring */

char* assemble_url(command_t *cmd,char *str)
{
    int i,opt_len = sizeof(&cmd->options);
    for(i=0;i<opt_len;i++)
    {
        command_option *option = &cmd->options[i];
        char *key = option->argname;
        char *value = option->value;
        if(i==0)
        {
            str = concatstring(str,key);
            str = concatstring(str,"=");
            str = concatstring(str,value);
        }
        else
        {
            str = concatstring(str,"&");
            str = concatstring(str,key);
            str = concatstring(str,"=");
            str = concatstring(str,value);
        }
    }
    return str;
}

What occurs in this program is the following:

1./ User types in program name and flag options,such as the following: program -test a -test2 b

2./ The program parses the command and populates a command struct with options. Each option has a flag(argname) and value associated to it(value)

3./ The program then tries to create a URL with those option keys and values, such as http://url/?test=a&test2=b

The program compiles but I'm still new to pointers and references (I think & is called a reference), so maybe that is why the program errors.

Any help is greatly appreciated! Also, if you see any probems or better way to handle assemble_url, please also let me know (I dont think its being handled in the best way, but like I said, I'm extremely new to C programming)

Thanks!

Answer 1

This initialization

int opt_len = sizeof(&cmd->options);

will give you the byte-size of a pointer &cmd->options . This makes no sense in the context of what you are trying to do.

If you want to determine the number of elements in an array (assuming it hasn't decayed to pointer), the proper technique would be

int opt_len = sizeof cmd->options / sizeof *cmd->options;

In this case opt_len will be initialized with MAX_OPTIONS value (meaning that you can just use MAX_OPTIONS in its place).

Answer 2

You have massive memory leaks from concatstring. Each invocation allocates a new buffer but you never bother to free them, and you loose the pointers so no chance of ever freeing that memory again.

This is not the cause of the segfault.

Answer 3

Because you are concatenating more than two strings it would be best if you allocated memory for all concatenations at once.

#include <stdarg.h>
#include <string>
char* stringBuilder(int count, ...)
{
    va_list ap, vacnt;
    int j;
    int len = 1;
    char* buffer;
    va_start(ap, count); 
#ifdef va_copy
        va_copy(vacnt,ap);
#else
    vacnt = ap;
#endif
    for(j=0; j<count; ++j){
        len+=strlen(va_arg(vacnt, char*)); 
    }
    va_end(vacnt);
    buffer = (char*) malloc(len * sizeof(char));
    buffer[0]=0;
    for (j=0;j<count;++j) {
       strcat (buffer, va_arg(ap, char*));
    }
    va_end(ap);
    return buffer;
 }

You can use it as such:

char* x = stringBuilder(4,"hello", " ", "world", "\n");
printf(x);
free(x);

Also note that both key and value must be URL-Encoded for the purposes of your application