Prepend variadic arguments and pass to another function - c

Question

I am having some trouble with a variadic printf-style function that I wrote, which works correctly when there are no variadic arguments but not when there are, e.g:
do_print("TestName", ""); works.
do_print("TestName", "Key1:%s\r\nKey2:%d\r\nKey3:%s", "Bob", 4, "Alice"); doesn't
I have done some debugging and I believe the issue is that the variadic arguments themselves are not getting processed as they should be. I looked at some examples and similar questions, and that has led me to believe that I'm misusing va_start and va_end in one of the functions. The format string that I'm generating is correct, but I'm getting a repeat for the first variadic arg of the name itself and then garbage values for the remaining variadic arguments:
Message:
Name:TestName
ID:42
Key1:TestName
Key2:-1
Key3:��-�*
(should be:)
Message:
Name:TestName
ID:42
Key1:Bob
Key2:4
Key3:Alice
My thought was perhaps that I should eliminate a va_start / va_end and just pass the ap in directly, although in certain cases I call the fmt_print function directly so I'm not sure what I would pass in for ap then. That's what I was playing with fmt_print2 for, but right now that just segfaults and is even worse than the original.
And yes, I do need vasprintf. The code here is printf-style, but ultimately I do need the entire message in a single buffer because I then pass it to the write function, not a printf-style function, so fmt_print does need to do the work of combining everything into a single buffer.
Here is a full minimal reproducible example, which I've been testing here: https://www.onlinegdb.com/online_c_compiler
#define _GNU_SOURCE /* needed for vasprintf */
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#pragma GCC diagnostic ignored "-Wsuggest-attribute=format"
static int fmt_print(const char *fmt, ...)
{
int res = 0;
int bytes = 0;
char *buf;
int len;
va_list ap;
va_start(ap, fmt);
if ((len = vasprintf(&buf, fmt, ap)) < 0) {
va_end(ap);
return -1;
}
va_end(ap);
printf("FORMAT STRING: %s\n", fmt);
printf("Message:\n%s[END]", buf);
return res;
}
static int fmt_print2(va_list ap, const char *fmt, ...)
{
int res = 0;
int bytes = 0;
char *buf;
int len;
//va_start(ap, fmt);
if ((len = vasprintf(&buf, fmt, ap)) < 0) {
va_end(ap);
return -1;
}
//va_end(ap);
printf("FORMAT STRING: %s\n", fmt);
printf("Message:\n%s[END]", buf);
return res;
}
#pragma GCC diagnostic pop
static void do_print(const char *name, const char *fmt, ...)
{
va_list ap;
char buf[200];
snprintf(buf, sizeof(buf), "%s%s%s", "Name:%s\r\nID:%d\r\n", fmt, "\r\n\r\n");
// send Name:%s\r\ID:%d\r\nKey1:%s\r\nKey2:%d\r\nKey3:%s fmt string + all args.
//fmt_print2(ap, buf, name, 42);
va_start(ap, fmt);
fmt_print(buf, name, 42);
va_end(ap);
}
int main()
{
do_print("TestName", "Key1:%s\r\nKey2:%d\r\nKey3:%s", "Bob", 4, "Alice");
// This is also valid:
fmt_print("Login", "Name:%s\r\nID:%d\r\nUsername:%s\r\nPassword:%s", "Login", 41, username, password);
return 0;
}
Could someone point me in the right direction of what approach is best suited for this?

Answer 1

As I said before in a comment, you can't do what you are trying to do the way you are trying to do it. The snprintf() in do_print() is trying to set a format string that requires two separate va_list values to process it. You will have to keep the two lots of formatting separate. And then, if necessary, combine the two strings.
This code works. It contains a lot of diagnostic printing which you can remove, but it showed me where things were going wrong. It includes the trailing \r\n\r\n as a separate operation.
/* SO 7274-3429 */
#define _GNU_SOURCE /* needed for vasprintf */
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static int fmt_print(const char *fmt1, va_list ap1, const char *fmt2, ...)
{
char *buf1;
char *buf2;
int len1;
int len2;
va_list ap2;
char *buffer;
printf("%s(): fmt1 = [[%s]]\n", __func__, fmt1);
len1 = vasprintf(&buf1, fmt1, ap1);
printf("%s(): buf1 = [[%s]]\n", __func__, buf1);
printf("%s(): fmt2 = [[%s]]\n", __func__, fmt2);
va_start(ap2, fmt2);
len2 = vasprintf(&buf2, fmt2, ap2);
va_end(ap2);
printf("%s(): buf2 = [[%s]]\n", __func__, buf2);
if (len1 < 0 || len2 < 0)
return -1;
printf("%s(): Format strings:\n[[%s]]\n[[%s]]\n", __func__, fmt1, fmt2);
printf("%s(): Message strings:\n[[%s]]\n[[%s]]\n[END]\n", __func__, buf1, buf2);
buffer = malloc(len1 + len2 + sizeof("\r\n\r\n"));
if (buffer == 0)
{
free(buf1);
free(buf2);
return -1;
}
strcpy(buffer, buf2);
strcpy(buffer + len2, buf1);
strcpy(buffer + len2 + len1, "\r\n\r\n");
printf("Full buffer: [[%s]]\n", buffer);
free(buf1);
free(buf2);
free(buffer);
return 0;
}
static void do_print(const char *name, const char *fmt, ...)
{
va_list ap;
char buf[] = "Name:%s\r\nID:%d\r\n";
va_start(ap, fmt);
fmt_print(fmt, ap, buf, name, 42);
va_end(ap);
}
int main(void)
{
do_print("TestName", "Key1:%s\r\nKey2:%d\r\nKey3:%s", "Bob", 4, "Alice");
return 0;
}
The output from it is:
fmt_print(): fmt1 = [[Key1:%s
Key2:%d
Key3:%s]]
fmt_print(): buf1 = [[Key1:Bob
Key2:4
Key3:Alice]]
fmt_print(): fmt2 = [[Name:%s
ID:%d
]]
fmt_print(): buf2 = [[Name:TestName
ID:42
]]
fmt_print(): Format strings:
[[Key1:%s
Key2:%d
Key3:%s]]
[[Name:%s
ID:%d
]]
fmt_print(): Message strings:
[[Key1:Bob
Key2:4
Key3:Alice]]
[[Name:TestName
ID:42
]]
[END]
Full buffer: [[Name:TestName
ID:42
Key1:Bob
Key2:4
Key3:Alice
]]
Note that this code frees the memory allocated by vasprintf().
Since you try to compile with -Wmissing-format-attribute, you will still get errors from a verbatim copy of this code. It will compile cleanly with your options if you add:
#if !defined(PRINTFLIKE)
#if defined(__GNUC__)
#define PRINTFLIKE(n, m) __attribute__((format(printf, n, m)))
#else
#define PRINTFLIKE(n, m) /* If only */
#endif /* __GNUC__ */
#endif /* PRINTFLIKE */
and change the function definition lines to:
static int PRINTFLIKE(3, 4) fmt_print(const char *fmt1, va_list ap1, const char *fmt2, ...)
and
static void PRINTFLIKE(2, 3) do_print(const char *name, const char *fmt, ...)
and in do_print(), modify the call to fmt_print() to take a literal format (eliminating the variable buf):
fmt_print(fmt, ap, "Name:%s\r\nID:%d\r\n", name, 42);
A lot of the time, the 'non-literal format' check is helpful and sensible, but there are times when it is counter-productive.

Answer 2

You simply forgot to allocate memory for your fmt_print function.
static int fmt_print2(const char *fmt, va_list ap);
static int fmt_print(const char *fmt, ...)
{
int res = 0;
int bytes = 0;
char *buf;
int len;
va_list ap;
va_start(ap, fmt);
if ((len = vsnprintf(NULL, 0, fmt, ap)) < 0) {
va_end(ap);
return -1;
}
buf = malloc(len + 1);
printf("LEN = %d\n", len);
va_start(ap, fmt);
if(buf)
{
vsnprintf(buf, len, fmt, ap);
printf("FORMAT STRING: %s\n", fmt);
printf("Message:\n%s[END]", buf);
}
va_end(ap);
return res;
}
static int fmt_print1(const char *fmt, ...)
{
int res = 0;
int bytes = 0;
char *buf;
int len;
va_list ap;
va_start(ap, fmt);
fmt_print2(fmt, ap);
va_end(ap);
return res;
}
static int fmt_print2(const char *fmt, va_list ap)
{
int res = 0;
int bytes = 0;
char *buf;
int len;
va_list copy;
va_copy(copy, ap);
if ((len = vsnprintf(NULL, 0, fmt, ap)) < 0) {
va_end(ap);
return -1;
}
buf = malloc(len + 1);
if(buf)
{
vsnprintf(buf, len, fmt, copy);
printf("FORMAT STRING: %s\n", fmt);
printf("Message:\n%s[END]", buf);
}
free(buf);
return res;
}
int main()
{
fmt_print("Key1:%s\r\nKey2:%d\r\nKey3:%s", "Bob", 4, "Alice");
fmt_print("Key1:%s\r\nKey2:%d\r\nKey3:%s", "AAAA", 8, "BBBB");
}