如何在 C 中连接用 sprintf 格式化的字符串

Question

I have multiple sentences which are being printed in the console currently. I have to gather them into a string.

A part of the code is:

#include <stdio.h>
#include<string.h>

int main()
{
    char buffer [100];
    sprintf (buffer, "%d plus %d is %d", 5, 3, 5+3);
    char *c=buffer;
    sprintf (buffer, "and %d minus %d is %d", 6, 3, 6-3);
    strcat(c, buffer);
    printf ("[%s]",c);
    return 0;
    return 0;
}

I tried to create formatted string using sprintf, but the result is wrong. The length of the sentences and their count are unlimited.

I want the output from the above code to be like:

[5 plus 3 is 8and 6 minus 3 is 3]

But it is:

[and 6 minus 3 is 3and 6 minus 3 is 3]

How do I need to concatenate them? Also, the length of the sentences and their count are unlimited. I have difficulties in using malloc and realloc. Can anyone help?

Answer 1

sprintf (buffer, "and %d minus %d is %d", 6, 3, 6-3); re-writes buffer instead of concatenating. The original result of the previous sprintf() is lost.

strcat(c, buffer); appends buffer to itself. As that overlaps, it is undefined behavior (UB). Do not do that.

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Instead use the return value of sprintf() (number of characters printed) to determine the offset printing.

int offset = sprintf (buffer, "%d plus %d is %d", 5, 3, 5+3);
offset += sprintf (buffer + offset, " and %d minus %d is %d", 6, 3, 6-3);
offset += sprintf (buffer + offset, " even more");
printf ("[%s]",buffer);

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Avoid strcat() as that requires code to iterate down the prior string, resulting in Schlemiel the Painter's Algorithm . Better to keep track of the string length and use that for the next sprintf() .

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Better code could use snprintf() to prevent buffer overrun.

// Pedantic example
char buffer [100];
size_t offset = 0;
size_t available = sizeof buffer;

int retval = snprintf (buffer, available, "%d plus %d is %d", 5, 3, 5+3);
if (retval < 0 || (unsigned) retval >= available) Handle_Overflow();
offset += retval;
available -= retval;

retval = snprintf (buffer + offset, available, " and %d minus %d is %d", 6, 3, 6-3);
if (retval < 0 || (unsigned) retval >= available) Handle_Overflow();
offset += retval;
available -= retval;

retval = snprintf (buffer + offset, available, " even more");
if (retval < 0 || (unsigned) retval >= available) Handle_Overflow();
offset += retval;
available -= retval;

printf ("[%s]",buffer);

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length of the sentences and their count are unlimited.

Usually a large buffer is sufficient as unlimited is not really unlimited , just potentially large. C strings are limited to SIZE_MAX .

Alternative: research non-C standard asprintf() .

Answer 2

Good program needs nice design.Simplify it.

#include <stdio.h>
#include <string.h>

int main()
{
    fprintf(stdout, "%d plus %d is %d", 5, 3, 5+3);
    fprintf (stdout, "and %d minus %d is %d", 6, 3, 6-3);
    return 0;
}

In order to deal with the unlimited problem, you can output to a file or other.