sprintf in C (pointer to an array of char elements)

Question

I am a newbie in C, and I want to do something like

    uint8_t message[12];

    message[0] = 0x00;
    message[1] = 0x00;
    message[2] = 0x00;
    message[3] = 0x00;
    message[4] = 0x00;
    message[5] = 0x00;
    message[6] = 0x00;
    message[7] = 0x00;
    message[8] = 0x00;
    message[9] = 0x00;
    message[10] = 0x00;
    message[11] = 0x00;

sprintf(_smess,"AT$SS=%02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x\n",
                            message[0], message[1], message[2], message[3],
                            message[4], message[5], message[6], message[7],
                            message[8], message[9], message[10], message[11]);

But I don't know how to declare the variable _smess, I've tried with char *_smess or uint8_t _smess[12] but I got an error anyway

if a declare char _smess[43]; then I got this error:

Multiple markers at this line
    - incompatible implicit declaration of built-in function 'sprintf' [enabled by 
     default]
    - implicit declaration of function 'sprintf' [-Wimplicit-function-declaration]
    - each undeclared identifier is reported only once for each function it 
     appears in
    - '_smess' undeclared (first use in this function)

with uint8_t message[12] = { 0 }; you don't need the following 12 lines that initialize each array element. — Borgleader
– Borgleader, Commented Aug 7, 2015 at 17:29
@NuñitodelaCalzada it should be char _smess[43]; as answered below - it needs a nul terminator. — Weather Vane
– Weather Vane, Commented Aug 7, 2015 at 17:35
implicit declaration of function 'sprintf' means that you haven't included the correct header. (#include <stdio.h>) — rici
– rici, Commented Aug 7, 2015 at 17:36
@WeatherVane I've just edited along the lines of the answer's OP who stated "make sure to leave room ...". Originally it was 50, I just did some optimization to make the answer more concise. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Aug 7, 2015 at 17:39
@WeatherVane That might be coming up implementation dependent. Safest portable way is to ask for results of sprintf() with a NULL pointer passed beforehand, and allocate the buffer after that (may be using VLA Compiler extension). — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Aug 7, 2015 at 17:46

dbush · Accepted Answer · 2015-08-07 17:36:13Z

1

You know how big the string is based on the format string passed to sprintf (i.e. 42 characters), so declare a char array of at least that size, making sure to leave room for the NUL ('\0') terminator:

char _smess[43];

Edit:

The error "incompatible implicit declaration of built-in function 'sprintf'" is because you didn't #include <stdio.h> at the top of your file.

The "'_smess' undeclared" error is most likely because you didn't define it before it was used. It needs to appear before the sprintf call.

edited Aug 7, 2015 at 17:36

answered Aug 7, 2015 at 17:29

dbush

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1 Comment

Serge Ballesta Over a year ago

snprintf should be preferred when it exists - throw away buffer overflow risks...

Kumar L · Accepted Answer · 2015-08-07 18:37:58Z

0

int sprintf( char* buffer, const char* format, ... );

each character we have to allocate memory... In your case "AT$SS=" required 6 bytes,
%02x required 2bytes empty space between each charcter required 1byte Total bytes required is 6 + 24 + 11 =41 bytes. Allocate memory by static variable char _smess[41] or dynamic allocation char *_smess = new char[41]; unsigned char will not accepted

answered Aug 7, 2015 at 18:37

Kumar L

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