exp() math.c i c not working if in a loop - embedded

Question

I'm currently working on a pid regulator for at school project.
Since the corona is has shut down the school we can't get any hardware to test it with.
So i want to simulate the PID regulator on a microprocessor (PSoC5LP).
So i'm implementing a function that returns a step respons of the chosen dc motor.
When i'm finding the output of the transferfunction "(-3.47*exp(-6.36*s)+3.47))" i get the right result if i define the value of "s" manually in the code.
But when i set s to be incremented in the code "undefined reference to "exp".
The problem seems to be that i can't run exp in a loop while incrementing it.
tried it in another project, where the only thing that happens is that a loop goes 10 times and prints out the exp() with the nr of times the loop have run (does not work)
float step_respons(){
double s = 0.1;
snprintf(outpurBuffer, sizeof(outpurBuffer), "%f \r\n", (-3.47*exp(-6.36*s)+3.47));
pc_uart_PutString(outpurBuffer);
snprintf(outpurBuffer, sizeof(outpurBuffer), "%f \r\n", (-3.47*exp(-6.36*1)+3.47));
pc_uart_PutString(outpurBuffer);
//These works just fine
pc_uart_PutString("Loop:\r\n");
for(s = 0.1; s < 2; s++){
snprintf(outpurBuffer, sizeof(outpurBuffer), "%f %f \r\n", s,(-3.47*exp(-6.36*s)+3.47));
pc_uart_PutString(outpurBuffer);
//this does not work (if "s" is changed with a number it works fine.)
};
return 0;
};
So it works if i choose S manually in the loop, but if it gets incremented it stops.
I've tried to use another variable and increment that on and it still does not work
for(s = 0.1; s < 2; s++){
snprintf(outpurBuffer, sizeof(outpurBuffer), "%f %f \r\n", s,(-3.47*exp(-6.36*s)+3.47));
pc_uart_PutString(outpurBuffer);
};
does not work
for(s = 0.1; s < 2; s++){
snprintf(outpurBuffer, sizeof(outpurBuffer), "%f %f \r\n", s,(-3.47*exp(-6.36*1)+3.47));
pc_uart_PutString(outpurBuffer);
};
does work but no incrementation of the result is done
As you can se the only changes done in the loop is the "s" have been defined a value
double temp = 0.5;
for(s = 0.1; s < 2; s++){
snprintf(outpurBuffer, sizeof(outpurBuffer), "%f %f \r\n", s,(-3.47*exp(-6.36*temp)+3.47));
pc_uart_PutString(outpurBuffer);
};
this works
double temp = 0.5;
for(s = 0.1; s < 2; s++){
snprintf(outpurBuffer, sizeof(outpurBuffer), "%f %f \r\n", s,(-3.47*exp(-6.36*temp)+3.47));
pc_uart_PutString(outpurBuffer);
temp += 0.5;
};
does not
This is the error: Build error: undefined reference to 'exp'
The log from the compiler output
--------------- Build Started: 04/28/2020 01:34:42 Project: Plotterkode, Configuration: ARM GCC 5.4-2016-q2-update Debug ---------------
The code generation step is up to date.
The compile step is up to date, no work needs to be done.
arm-none-eabi-ar.exe -rs .\CortexM3\ARM_GCC_541\Debug\Plotterkode.a .\CortexM3\ARM_GCC_541\Debug\cy_em_eeprom.o .\CortexM3\ARM_GCC_541\Debug\CyDmac.o .\CortexM3\ARM_GCC_541\Debug\CyFlash.o .\CortexM3\ARM_GCC_541\Debug\CyLib.o .\CortexM3\ARM_GCC_541\Debug\cyPm.o .\CortexM3\ARM_GCC_541\Debug\CySpc.o .\CortexM3\ARM_GCC_541\Debug\cyutils.o .\CortexM3\ARM_GCC_541\Debug\pc_uart.o .\CortexM3\ARM_GCC_541\Debug\pc_uart_PM.o .\CortexM3\ARM_GCC_541\Debug\pc_uart_INT.o .\CortexM3\ARM_GCC_541\Debug\pc_uart_BOOT.o .\CortexM3\ARM_GCC_541\Debug\pc_uart_IntClock.o .\CortexM3\ARM_GCC_541\Debug\isr_pc_uart.o .\CortexM3\ARM_GCC_541\Debug\Pin_adc_input.o .\CortexM3\ARM_GCC_541\Debug\MISO.o .\CortexM3\ARM_GCC_541\Debug\MOSI.o .\CortexM3\ARM_GCC_541\Debug\SCLK.o .\CortexM3\ARM_GCC_541\Debug\pot_adc_sar.o .\CortexM3\ARM_GCC_541\Debug\pot_adc_sar_INT.o .\CortexM3\ARM_GCC_541\Debug\pot_adc_sar_PM.o .\CortexM3\ARM_GCC_541\Debug\ui_spi.o .\CortexM3\ARM_GCC_541\Debug\ui_spi_PM.o .\CortexM3\ARM_GCC_541\Debug\ui_spi_INT.o .\CortexM3\ARM_GCC_541\Debug\pot_adc_sar_IRQ.o .\CortexM3\ARM_GCC_541\Debug\pot_adc_sar_theACLK.o .\CortexM3\ARM_GCC_541\Debug\ui_spi_IntClock.o .\CortexM3\ARM_GCC_541\Debug\motor_pwm.o .\CortexM3\ARM_GCC_541\Debug\motor_pwm_PM.o .\CortexM3\ARM_GCC_541\Debug\Clock.o .\CortexM3\ARM_GCC_541\Debug\pin_pwm_x.o .\CortexM3\ARM_GCC_541\Debug\pin_pwm_y.o .\CortexM3\ARM_GCC_541\Debug\pin_enable.o .\CortexM3\ARM_GCC_541\Debug\pin_border.o .\CortexM3\ARM_GCC_541\Debug\isr_border.o .\CortexM3\ARM_GCC_541\Debug\pin_led.o .\CortexM3\ARM_GCC_541\Debug\isr_goal_left.o .\CortexM3\ARM_GCC_541\Debug\pin_goal_right.o .\CortexM3\ARM_GCC_541\Debug\pin_goal_left.o .\CortexM3\ARM_GCC_541\Debug\isr_goal_right.o .\CortexM3\ARM_GCC_541\Debug\isr_ui_spi.o .\CortexM3\ARM_GCC_541\Debug\AMux.o .\CortexM3\ARM_GCC_541\Debug\CyBootAsmGnu.o
arm-none-eabi-ar.exe: creating .\CortexM3\ARM_GCC_541\Debug\Plotterkode.a
arm-none-eabi-gcc.exe -Wl,--start-group -o C:\Users\Vikto\Desktop\plottercode\Plotterkode.cydsn\CortexM3\ARM_GCC_541\Debug\Plotterkode.elf .\CortexM3\ARM_GCC_541\Debug\main.o .\CortexM3\ARM_GCC_541\Debug\cyfitter_cfg.o .\CortexM3\ARM_GCC_541\Debug\cymetadata.o .\CortexM3\ARM_GCC_541\Debug\Cm3Start.o .\CortexM3\ARM_GCC_541\Debug\Plotterkode.a "C:\Program Files (x86)\Cypress\PSoC Creator\4.2\PSoC Creator\psoc\content\cycomponentlibrary\CyComponentLibrary.cylib\CortexM3\ARM_GCC_541\Debug\CyComponentLibrary.a" -mcpu=cortex-m3 -mthumb -L Generated_Source\PSoC5 -Wl,-Map,.\CortexM3\ARM_GCC_541\Debug/Plotterkode.map -T Generated_Source\PSoC5\cm3gcc.ld -specs=nano.specs -u _printf_float -Wl,--gc-sections -u_printf_float -g -ffunction-sections -Og -ffat-lto-objects -Wl,--end-group
.\CortexM3\ARM_GCC_541\Debug\main.o: In function step_respons':
C:\Users\Vikto\Desktop\plottercode\Plotterkode.cydsn/./plotter_position.h:125: undefined reference toexp'
collect2.exe: error: ld returned 1 exit status
The command 'arm-none-eabi-gcc.exe' failed with exit code '1'.
--------------- Build Failed: 04/28/2020 01:34:42 ---------------

Answer 1

You are likely not linking the library that includes the exp() function.
It works in the case where you're passing it a literal (or a value the compiler can deduce) because the compiler is computing the value and putting that in the code.
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
I believe exp() is included in 'libm', so adding -Wl,-lm (or however else you specify libm in your build system) should fix it.
And if it is already there, try putting it at the end of the list.