How to add a null character to the end of a string?

I start of with a

memcpy(g->db_cmd,l->db.param_value.val,l->db.param_value开发者_如何学Go.len);

which contains the value "function" however I want a null character to be appended like "function'\0'" I've tried a

memcpy(&g->db_cmd[l->db.param_value.len],0,1);

This crashes the program. I've tried memset also

 memset(&g->db_cmd[l->db.param_value.len],0,1);

This doesnt work. Any idea?

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g->db_cmd[l->db.param_value.len] = 0;

assuming you have allocated space for that.

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First off, C (and C++) is not dynamic like you know it from Java, C#, PHP and others. When you are presented with a string in C, the string is pretty much static in length.

To make the answer simpler, lets redefine your variables:

• g->db_cmd will be called dest,
• l->db.param_value.val will be called src, and
• l->db.param_value.len will be called len.

You should allocate a new string of size len plus one (for the extra null).

Allocate a new dest:

dest = calloc(sizeof(char), len + 1);

calloc allocates an array of chars as long as len plus one. After calloc() has allocated the array it fills it with nulls (or \0) thus you automatically will have a null appended to your deststring.

Next, copy the src to dest with strncpy:

 strncpy(dest, src, len);

To convert this back to your variable names:

g->db_cmd = calloc(sizeof(char), l->db.param_value.len + 1);
strncpy(g->db_cmd, l->db.param_value.val, l->db.param_value.len);

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If you want string-copying semantics, why not use a string-copying function?

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Strings are by default null-terminated.
If you want a to ad an extra NULL at the end you can write "String\0"
or db_cmd[len]='\0'

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If the source you're copying from also contains a NULL terminated string use

memcpy( g->db_cmd, l->db.param_value.val, l->db.param_value.len + 1 );

Otherwise you'll have to add the terminator yourself.

g->db_cmd[l->db.param_value.len] = '\0';

Of course, you need to ensure that the destination has enough room for this character.

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memcpy takes two pointers, and an integer. In the lines that you say are crashing, you pass it a pointer, and two integers. The code cannot dereference the second argument (0).
If you really really want to use memcpy, you have to have a pointer to a zero

char zero = 0;
memcpy(&g->db_cmd[l->db.param_value.len], &zero , 1);

But I would really suggest pyroscope's answer. It's faster, and clearer.