Programming Forums > Application Development > Assembly

Arnvidr · Mar 20th, 2005, 11:16 AM

Consider this bit of code which is part of a kinda itoa procedure, only the string is reversed:

#get a number from a parameter, this bit is identical to functional bits getting chars and strings
movl (%esi), %eax
addl $4, %esi

#uses the %ecx-register for the string, adds a zero at the beginning for later reversal purposes
movl $0, %ecx
incl %ecx

#the actual procedure
loop:
cmpl $0, (%eax)
je done
cdq #is this really necessary? doesn't seem to make any difference atm
movl $10, %ebx # the divisor
idivl %ebx
addl $48, %edx
movb (%edx), %bl
movb %bl, (%ecx)
incl %ecx
jmp loop

ignoring the fact that it doesn't handle negative integers at the moment, I can't for the life of me see why this would be segfaulting...

Arnvidr · Mar 20th, 2005, 12:18 PM

I've found that (at least the first) reason for the segfault is this line:

(Toggle Plain Text)

movl   (%esi), %ecx

The %esi register contains the parameters for the whole program, and this line is identical to functioning procedures for getting both chars and strings, so I gather there must be some other method of handling ints?

Mar 20th, 2005, 11:16 AM	#1
Arnvidr Newbie Join Date: Mar 2005 Posts: 2 Rep Power: 0	x86 assembly (segfault) Consider this bit of code which is part of a kinda itoa procedure, only the string is reversed: (Toggle Plain Text) #get a number from a parameter, this bit is identical to functional bits getting chars and strings movl (%esi), %eax addl $4, %esi #uses the %ecx-register for the string, adds a zero at the beginning for later reversal purposes movl $0, %ecx incl %ecx #the actual procedure loop: cmpl $0, (%eax) je done cdq #is this really necessary? doesn't seem to make any difference atm movl $10, %ebx # the divisor idivl %ebx addl $48, %edx movb (%edx), %bl movb %bl, (%ecx) incl %ecx jmp loop #get a number from a parameter, this bit is identical to functional bits getting chars and strings movl (%esi), %eax addl $4, %esi #uses the %ecx-register for the string, adds a zero at the beginning for later reversal purposes movl $0, %ecx incl %ecx #the actual procedure loop: cmpl $0, (%eax) je done cdq #is this really necessary? doesn't seem to make any difference atm movl $10, %ebx # the divisor idivl %ebx addl $48, %edx movb (%edx), %bl movb %bl, (%ecx) incl %ecx jmp loop ignoring the fact that it doesn't handle negative integers at the moment, I can't for the life of me see why this would be segfaulting...

Mar 20th, 2005, 12:18 PM	#2
Arnvidr Newbie Join Date: Mar 2005 Posts: 2 Rep Power: 0	I've found that (at least the first) reason for the segfault is this line: (Toggle Plain Text) movl (%esi), %ecx movl (%esi), %ecx The %esi register contains the parameters for the whole program, and this line is identical to functioning procedures for getting both chars and strings, so I gather there must be some other method of handling ints?

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Programming Forums > Application Development > Assembly

x86 assembly (segfault)