13.7.1 Simple C program (gcc)

A simple example to compile example.c with a library libm into an executable run_example:

     $ cat > example.c << EOF
     #include <stdio.h>
     #include <math.h>
     #include <string.h>
     
     int main(int argc, char **argv, char **envp){
             double x;
             char y[11];
             x=sqrt(argc+7.5);
             strncpy(y, argv[0], 10); /* prevent buffer overflow */
             y[10] = '\0'; /* fill to make sure string ends with '\0' */
             printf("%5i, %5.3f, %10s, %10s\n", argc, x, y, argv[1]);
             return 0;
     }
     EOF
     $ gcc -Wall -g -o run_example example.c -lm
     $ ./run_example
             1, 2.915, ./run_exam,     (null)
     $ ./run_example 1234567890qwerty
             2, 3.082, ./run_exam, 1234567890qwerty

Here, -lm is needed to link library libm for sqrt(). The actual library is in /lib/ with filename libm.so.6, which is a symlink to libm-2.1.3.so.

Look at the last parameter in the output text. There are more than 10 characters even though %10s is specified.

The use of pointer memory operation functions without boundary checks, such as sprintf and strcpy, is deprecated to prevent buffer overflow exploits that leverage the above overrun effects. Instead, use snprintf and strncpy.