C 字符串定义未知长度

Question

I need to store data (Of previously unknown format/size) inside a string for processing later (To be stored in an XML file)

How do I do this?

As you can see, the code below will generate a segfault.

char * type;
char * output;

for (i=0; i< 10; i++){ 

if(strcmp(GTK_OBJECT_TYPE_NAME(g_hash_table_lookup(widgetbuffer,allocate[i])), "GtkAdjustment") == 0){
    type = "spin";
    sprintf(output, "%f", gtk_adjustment_get_value(GTK_ADJUSTMENT(g_hash_table_lookup(widgetbuffer,allocate[i]))));

}else if(strcmp(GTK_OBJECT_TYPE_NAME(g_hash_table_lookup(widgetbuffer,allocate[i])), "GtkCheckButton") == 0){
    type = "check";
    sprintf(output, "%d", gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(g_hash_table_lookup(widgetbuffer,allocate[i]))));

}else if(strcmp(GTK_OBJECT_TYPE_NAME(g_hash_table_lookup(widgetbuffer,allocate[i])), "GSList") == 0){
    type = "radio"; // Loop through grouped buttons and find active one
    sprintf(output, "%d", g_slist_position(g_hash_table_lookup(widgetbuffer,allocate[i]),
                g_slist_find_custom(g_hash_table_lookup(widgetbuffer,allocate[i]),
                    NULL, (GCompareFunc) searchRadio)));

}else if(strcmp(GTK_OBJECT_TYPE_NAME(g_hash_table_lookup(widgetbuffer,allocate[i])), "GtkComboBox") == 0){
    type = "combo";
    sprintf(output, "%d", gtk_combo_box_get_active(GTK_COMBO_BOX(g_hash_table_lookup(widgetbuffer,allocate[i]))));

}else if(strcmp(GTK_OBJECT_TYPE_NAME(g_hash_table_lookup(widgetbuffer,allocate[i])), "GtkEntry") == 0){
    type = "entry";
    output = (char *) gtk_entry_get_text(GTK_ENTRY(g_hash_table_lookup(widgetbuffer,allocate[i])));

}
[...]

Answer 1

In general, to "store data of an unknown size" in a C string, you have two choices:

1. Allocate a buffer large enough to hold any expected size (which means you would truncate the data if it exceeds that size), or

2. Dynamically allocate a buffer (using malloc() ) that is large enough to hold the data. Don't forget to free() it, too.

Your code is using an uninitialised pointer output and that is why it is segfaulting. You will need to do one of the above.

Answer 2

You are not allocating output and that is precisely why you are getting segmentation faults. As it is in the code provided, output has not been initialized. Your compiler should warn you of this.

If you know a safe maximum size, you can simply allocate it on the stack:

char output[512];

...if the maximum size was 512 bytes. Otherwise, you can look at malloc to allocate memory off the heap.

Answer 3

If your platform has snprintf or similar (most do), then you want something like this:

int n = snprintf( NULL, 0, "%s is %d", somestring, someinteger );
char * p = malloc( n + 1 );
sprintf( p, "%s is %d", somestring, someinteger );

The first call to snprintf returns how many chars would be needed to store the formatted output, but doesn't actually do any formatting. Then you allocate the space needed and do the real formatting.

Answer 4

In addition to pre-allocate a big enough buffer or to truncate the output to size:

1. Pre-process the generation (no writes, just count the lengthes) and allocate a suitable buffer for the second (really write) pass
2. Write to 'unlimited' storage: a file